Does a UK SIPP have to be reported on the FBAR?

There is no official rule naming SIPPs. The FBAR test is whether you have a financial interest in, or signature or other authority over, a foreign financial account. Because a SIPP is normally a designated account with a UK provider, with a determinable balance and member-directed investments, the position we take in the returns we prepare is to report it. The FBAR retirement plan exception covers US plans and IRAs, not UK registered schemes.

Are employer contributions to my UK SIPP taxable in the United States?

Under the Internal Revenue Code alone they generally would be, because a UK registered scheme is not a qualified plan. Article 18(5) of the US-UK treaty can keep them out of US income where you are a US citizen resident in the UK, the employment is exercised in the UK, the cost is borne by a UK employer or UK permanent establishment, and the contributions qualify for UK tax relief. The exclusion is capped by reference to a generally corresponding US scheme.

Do I need to file Form 8833 to claim Article 18(5)?

Excluding an employer contribution that the Code would treat as compensation is a treaty-based return position under section 6114, so Form 8833 is the natural disclosure. Some preparers rely on a regulatory exception and omit it. On a catch-up filing we file it, because it costs nothing, creates a clear record of what was claimed and why, and avoids the separate penalty exposure that non-disclosure carries under section 6712.

Is the growth inside my SIPP taxable in the US each year?

Article 18(1) provides that income earned by a pension scheme is taxed as the member's income only when, and to the extent that, it is paid to or for the member's benefit, and that paragraph is excepted from the saving clause, so it reaches US citizens. On that basis the dividends, interest and gains arising inside the wrapper are not current US income, and a scheme-to-scheme transfer is not a triggering payment.

Do I have to file Form 8621 for the funds inside my SIPP?

Generally no. Regulations section 1.1298-1(c)(4) excepts PFIC interests held through an arrangement treated as a foreign pension fund under a treaty, where the treaty taxes the fund's income to the member only when it is paid out. That description matches Article 18(1). The exception depends on the treaty characterisation holding, so if the pension analysis fails the Form 8621 obligation returns with it.

How many years do I have to go back to fix missed SIPP reporting?

It depends on the route. A streamlined foreign offshore submission requires delinquent or amended returns for the most recent three years for which the due date has passed, delinquent FBARs for the most recent six years, and a signed Form 14653. If only the FBARs were missed and the income was correctly reported, the fix is late FBARs filed through the BSA E-Filing System with a reason for late filing.

Missed Reporting UK SIPP Employer Contributions: US Fix

Missed Reporting UK SIPP Employer Contributions is one of the most common and most fixable gaps we find when a US person living in the UK brings several years of filings up to date: the self-invested personal pension has quietly received company money for years, and it has never appeared on a FinCEN Form 114, never appeared on a Form 8938, and never been addressed anywhere on the Form 1040. The short answer is that the missed reporting is usually an information-return problem rather than a tax problem, because Article 18 of the US-UK income tax treaty is capable of keeping employer contributions and internal growth out of current US income, but the relief is not automatic, it is conditional, and it has to be claimed on each year you go back and file.

That distinction matters enormously to the outcome. A case where the treaty position holds is a catch-up filing exercise with little or no additional US tax, remediated through late FBARs and either amended or delinquent returns, or through the Streamlined Foreign Offshore Procedures. A case where the treaty position does not hold, because the employment condition fails or the scheme falls outside what the competent authorities agreed generally corresponds to a US employer plan, can produce years of additional US compensation income with no cash to pay it. The failure mode we see most often is a taxpayer who assumed the first outcome and never tested the conditions.

What does Missed Reporting UK SIPP Employer Contributions actually mean in a US filing?

A self-invested personal pension is a UK registered pension scheme in which the member directs the underlying investments, held with a UK SIPP operator. HMRC's Pensions Tax Manual describes the main scheme families at https://www.gov.uk/hmrc-internal-manuals/pensions-tax-manual/ptm022000, distinguishing occupational pension schemes set up by a sponsoring employer from personal pension schemes normally established by a financial institution. A SIPP sits in the second family, even when the only money going into it comes from a company. That structural point drives almost everything that follows on the US side.

When a US person in the UK has a SIPP that has received employer contributions, four separate US obligations can be in play in every single year, and they fail independently of each other:

FinCEN Form 114, the FBAR, filed with FinCEN rather than with the IRS, if the member has a financial interest in or signature or other authority over foreign financial accounts whose aggregate maximum value exceeded 10,000 US dollars at any point in the calendar year.
Form 8938, filed with the Form 1040 under FATCA, if the taxpayer is over the applicable specified foreign financial asset threshold.
The income tax question on the Form 1040 itself: whether the employer contributions were current compensation, and whether income and gains arising inside the scheme were current income of the member.
Form 8833, the treaty-based return position disclosure required by section 6114, where the return relies on the treaty to displace what the Internal Revenue Code would otherwise require.

A client can have filed a technically clean Form 1040 for six years and still have six years of missed FBARs, and the reverse also happens. In the returns we prepare, we test each of the four in each year rather than reasoning from a single conclusion about the pension.

Is a UK SIPP a reportable financial account on FinCEN Form 114?

There is no official IRS or FinCEN statement that says a UK SIPP is always an FBAR-reportable account, and anyone who tells you otherwise is compressing a genuinely uncertain area. The FBAR rules work from a definition of foreign financial account, not from a list of pension products. The IRS FBAR page at https://www.irs.gov/businesses/small-businesses-self-employed/report-of-foreign-bank-and-financial-accounts-fbar sets out who must file and confirms that whether an account produced taxable income has no effect on whether it is a foreign financial account. The underlying regulation at 31 CFR 1010.350 defines bank accounts, securities accounts and a category of other financial accounts that captures, among other things, accounts with a person in the business of accepting deposits as a financial agency and interests in pooled funds issuing shares to the public.

The practical drivers we work through for each SIPP are these:

Whether the member is the named beneficial owner of a designated, individually identified account held with a UK institution, which is the normal SIPP arrangement.
Whether the account has a determinable maximum value during the calendar year, evidenced by periodic statements, as opposed to an unquantified promise of a future benefit.
Whether the member can direct the investments held in the account, which is the defining feature of a SIPP and which points towards the member having authority over it.
Whether the underlying holdings are themselves separate foreign financial accounts, which matters where the SIPP wrapper holds a platform cash account and a dealing account at a different institution.
Whether any FBAR exception applies. It is worth noting that the retirement plan exception in the FBAR regulations is written around US plans described in Internal Revenue Code sections 401(a), 403(a) and 403(b) and individual retirement arrangements. A UK registered pension scheme is not within it.

The position we take in the returns we prepare is to report a UK SIPP on the FBAR where the member has a designated account with a determinable balance and the ability to direct investments, which covers the overwhelming majority of SIPPs, and to document the reasoning in the file. We do not treat a defined benefit promise the same way, because there is no account and no determinable balance. Where a fact pattern is genuinely marginal, reporting costs nothing and removes the risk; not reporting saves nothing and creates a six-year exposure. The FBAR is due 15 April with an automatic extension to 15 October and is filed electronically through FinCEN's BSA E-Filing System at https://bsaefiling.fincen.gov.

How does Form 8938 treat an interest in a UK SIPP, and what is the maximum value?

Form 8938 is cleaner, because the instructions deal with foreign pensions expressly. An interest in a foreign pension plan is a specified foreign financial asset. The instructions at https://www.irs.gov/instructions/i8938 direct you to report the interest in Part VI as an other foreign asset, and they are explicit that you do not separately report the individual assets held inside the plan. That is a meaningful simplification: you are reporting one line for the SIPP, not a schedule of every fund inside it.

The maximum value rule for a pension interest is different from the rule for an ordinary account. For an interest in a foreign pension plan the maximum value is the fair market value of your beneficial interest in the assets of the plan as of the last day of the tax year. For a SIPP that is normally straightforward, because the provider issues a year-end valuation. The instructions also provide a fallback: if you cannot determine fair market value from readily accessible information, you use the fair market value of distributions received during the year, and if there were no distributions and you have no way of knowing the value, you use zero. We use that fallback very sparingly for a SIPP, because a SIPP almost always has a readily available year-end statement, and a zero on a form that is later shown to have been valued at seven figures is an uncomfortable place to be.

The thresholds differ from the FBAR, and the IRS comparison table at https://www.irs.gov/businesses/comparison-of-form-8938-and-fbar-requirements sets them out. For a specified individual living outside the United States, over 200,000 US dollars on the last day of the year or over 300,000 at any time for an unmarried filer, and over 400,000 or 600,000 respectively for a married couple filing jointly. The penalty structure is why the form matters. Failure to file a complete and correct Form 8938 carries a 10,000 US dollar penalty, with a further 10,000 for each 30-day period of continued failure after notice up to an additional 50,000, and a 40 per cent accuracy-related penalty can apply to an underpayment attributable to an undisclosed specified foreign financial asset.

Are employer contributions to a SIPP taxable US compensation without the treaty?

Start with the Code, because that is where the IRS starts. A UK registered pension scheme is not a qualified plan under section 401(a). Absent a treaty, the exclusions and deferrals that make an employer contribution to a US qualified plan invisible to the employee simply do not apply, and the general expectation is that an employer contribution to a non-qualified foreign arrangement is compensation for services which is includible when it is paid or when the employee's rights are no longer subject to a substantial risk of forfeiture. The IRS page on foreign pensions at https://www.irs.gov/businesses/the-taxation-of-foreign-pension-and-annuity-distributions confirms the basic framing that foreign pensions are not characterised as qualified plans and that the saving clause allows the United States to tax its citizens on worldwide income.

That is the exposure sitting behind an unaddressed SIPP. A UK company director on a modest salary who has been taking meaningful employer pension contributions each year, entirely properly and entirely tax-efficiently for UK purposes, may be carrying several years of unreported US compensation income unless a treaty position is taken and holds. The mismatch is worse than it looks, because there is no cash in hand to pay that US tax, and the UK tax paid on the small salary that was drawn instead is the wrong pool of foreign tax in the wrong amount to credit against it.

What does Article 18(5) of the US-UK treaty do for employer contributions?

Article 18(5) of the Convention, which you can read in the official text at https://home.treasury.gov/system/files/131/Treaty-UK-7-24-2001.pdf, is written specifically for this situation. Subparagraph (a) provides that where a citizen of the United States who is a resident of the United Kingdom exercises an employment in the United Kingdom the income from which is taxable in the United Kingdom and is borne by an employer who is a resident of the United Kingdom or by a permanent establishment situated in the United Kingdom, and the individual is a member or beneficiary of, or participant in, a pension scheme established in the United Kingdom, then contributions paid by or on behalf of that individual are deductible or excludable in computing his US taxable income, and any benefits accrued under the scheme, or contributions made by or on behalf of the individual's employer, during that period and attributable to the employment, are not treated as part of the employee's taxable income in the United States.

The provision closes with a condition that is easy to miss: it applies only to the extent that the contributions or benefits qualify for tax relief in the United Kingdom. That is the hinge on which most owner-managed cases turn, and we come back to it below.

The reason Article 18(5) works at all for a US citizen is the 2002 Protocol. Article I of the Protocol at https://home.treasury.gov/system/files/131/Treaty-UK-Protocol-7-19-2002.pdf deleted and replaced Article 1(5) of the Convention so that the saving clause in Article 1(4), under which the United States may tax its citizens as if the Convention had not come into effect, does not affect the benefits conferred under paragraphs 1 and 5 of Article 18. Without that amendment the relief would be worthless to precisely the people it was drafted for. When we document a treaty position on a catch-up filing, we cite the Protocol, not just the Convention, because the saving clause carve-out is the part an examiner will want to see addressed.

The conditions we test, year by year, before relying on Article 18(5) are:

US citizenship. Paragraph 5 is drafted for a citizen of the United States. A green card holder resident in the UK is in a different analytical position and needs the question worked through separately.
UK residence under the treaty residence article for the year in question, which is not the same test as UK statutory residence alone where there is a dual-residence tie-breaker in play.
An employment exercised in the United Kingdom, the income from which is taxable in the United Kingdom.
Remuneration borne by a UK-resident employer or by a UK permanent establishment. Contributions borne by a US parent or a third-country entity do not sit comfortably here.
Contributions attributable to that employment, made during the period of that employment.
UK tax relief actually obtained on the contributions or benefits, because the relief is available only to that extent.
A scheme the US competent authority has agreed generally corresponds to a US scheme, under subparagraph (d).

The generally corresponding limit: what Article 18(5)(b) caps

Two limits sit on top of the relief. Subparagraph (b) provides that the reliefs available under paragraph 5 shall not exceed the reliefs that would be allowed by the United States to its residents for contributions to, or benefits accrued under, a generally corresponding pension scheme established in the United States. The Treasury technical explanation to the Protocol puts it as a lesser-of test: the benefit is the lesser of the relief allowed under the UK scheme and the relief that would be allowed for a generally corresponding US scheme. In practice this means a very large employer contribution can outrun the treaty shelter, and the excess has to be brought into US income. That is a real constraint for senior executives and owner-managers making large one-off or carry-forward-funded contributions, and it is the calculation most commonly skipped.

Subparagraph (c) is the provision nobody expects. It provides that contributions made to, or benefits accrued under, a UK scheme are treated as contributions or benefits under a generally corresponding US scheme for the purpose of determining the individual's eligibility to participate in, and receive tax benefits with respect to, a US scheme. The technical explanation gives the example directly: UK pension contributions may be counted in determining whether the individual has exceeded annual contribution limits on the US side. A client who is funding a SIPP heavily in the UK and also expects to make US-side retirement contributions needs that interaction modelled, not assumed.

Subparagraph (d) requires competent authority agreement that the UK scheme generally corresponds to a US scheme. The Exchange of Notes at https://home.treasury.gov/system/files/131/Treaty-UK-Notes-7-24-2001.pdf supplies it, stating that the pension schemes listed for one Contracting State in connection with Article 3(1)(o) generally correspond to those listed for the other. The UK list covers employment-related arrangements approved as retirement benefit schemes for the purposes of Chapter I of Part XIV of the Income and Corporation Taxes Act 1988, and personal pension schemes approved under Chapter IV of Part XIV of that Act, together with any identical or substantially similar schemes established under legislation introduced after the Convention was signed, which is how the Finance Act 2004 registered pension scheme regime is carried into the list.

There is a genuine drafting tension here that competent commentary should not paper over. The technical explanation, discussing subparagraph (d), reasons that because paragraph 5 applies only to persons employed by a UK employer or UK permanent establishment, the relevant UK plans are those that correspond to employer plans in the United States, and accordingly it applies with respect to retirement benefit schemes for the purposes of Chapter I of Part XIV of ICTA 1988, which is the occupational scheme limb. A SIPP is a personal pension scheme, the Chapter IV limb. We do not read that as excluding a SIPP that receives employer contributions in the course of a UK employment, and it is not how the provision is generally applied, but it is the point on which a paragraph 5 position for a SIPP is weakest. On a large multi-year catch-up we document the employment link, the UK employer bearing the cost, the UK relief obtained and the registered status of the scheme, so that the position is evidenced rather than assumed.

Do you have to file Form 8833 for the Article 18(5) position?

Form 8833 is the treaty-based return position disclosure required by section 6114, described at https://www.irs.gov/forms-pubs/about-form-8833. The mechanical test is whether the return takes a position that a treaty overrules or modifies a Code provision and thereby reduces the tax otherwise due. Excluding from income an employer contribution that the Code would otherwise treat as compensation is exactly that kind of position. There are regulatory exceptions to disclosure, and some practitioners take the view that certain Article 18 positions fall inside one of them, which is why you will find UK pension returns prepared both ways.

Our practice is to file the Form 8833 and to state the position properly: the treaty and article relied on, the Code provisions displaced, and a concise explanation of the facts that satisfy the paragraph 5 conditions. On a remediation file this is not a close call. A disclosure costs nothing, a failure to disclose carries its own penalty exposure under section 6712, and a taxpayer who is already filing several years late benefits from a record showing exactly what was claimed and why. It also forces the preparer to articulate the position in writing, which is the fastest way to discover that one of the conditions does not actually hold.

Why growth inside the SIPP is generally not currently taxed under the treaty

Article 18(1) does separate work from paragraph 5 and is worth understanding on its own terms. It provides that where an individual resident in one State is a member or beneficiary of, or participant in, a pension scheme established in the other State, income earned by the pension scheme may be taxed as income of that individual only when, and to the extent that, it is paid to or for the benefit of that individual from the scheme, and not where it is transferred to another pension scheme. Article 1(5)(a), both in the original Convention and as replaced by the Protocol, excepts paragraph 1 from the saving clause. The technical explanation states the consequence plainly: a US citizen resident in the United Kingdom will not be subject to US tax on the earnings and accretions of a UK pension fund with respect to that US citizen.

So the dividends, interest and realised gains arising inside the SIPP wrapper are not, on the strength of Article 18(1), current US income. Two practical consequences follow. First, the missed years are usually much less expensive than clients fear, because the compounding growth inside the wrapper is not being taxed annually. Second, a scheme-to-scheme transfer, for example consolidating several old UK pots into a single SIPP, is expressly outside the paid-to-the-individual trigger and is not a US-taxable event on the strength of that paragraph. We still document a transfer carefully, because a transfer year is exactly the kind of year in which an FBAR maximum value spikes and a previously sub-threshold Form 8938 position flips.

Does PFIC reporting apply to funds held inside the SIPP wrapper?

This is the question that causes the most unnecessary alarm, and it has a clean answer. A SIPP typically holds UK-domiciled open-ended investment companies, unit trusts and exchange traded funds, every one of which is a passive foreign investment company from a US perspective. Held in a taxable UK brokerage account, that is a Form 8621 problem of real magnitude. Held inside a qualifying pension wrapper, it generally is not.

The relief is in the regulations, not just in practice. Regulations section 1.1298-1(c)(4) provides an exception for PFIC stock held through certain foreign pension funds: a shareholder who is a member or beneficiary of, or participant in, an arrangement treated as a foreign pension fund under an income tax treaty to which the United States is a party, and which owns directly or indirectly an interest in a PFIC, is not required under section 1298(f) to file Form 8621 with respect to that PFIC interest if, under the applicable treaty, the income earned by the fund may be taxed as the income of the shareholder only when and to the extent it is paid to or for the benefit of the shareholder. That last clause is Article 18(1) almost word for word, which is why the treaty position and the PFIC position stand or fall together. The Form 8621 instructions at https://www.irs.gov/instructions/i8621 carry the same exception.

The consequence is worth stating as a rule: the pension wrapper is what protects the funds, and the treaty is what protects the wrapper. If the Article 18(1) analysis fails for a particular arrangement, the PFIC exception fails with it, and a decade of unreported fund holdings becomes the dominant problem in the file rather than the pension contributions. That is why we resolve the treaty characterisation before we scope a remediation, not after.

When the US person controls the employer: whose contribution is it, and is it still a SIPP?

The commonest version of this fact pattern is not a large corporate at all. It is a US citizen who owns a UK limited company, pays a small salary, and takes the bulk of the reward as company pension contributions into a SIPP. Article 18(5) is drafted around employer contributions, and nothing in it says the employer cannot be a company the individual controls. But related-party control changes the evidential burden in three specific ways.

The wholly and exclusively test. UK relief for an employer contribution is given by deduction in computing the employer's profits, and HMRC's guidance at https://www.gov.uk/hmrc-internal-manuals/pensions-tax-manual/ptm043100 confirms that it must be incurred wholly and exclusively for the purposes of the trade and that relief is given for the period in which the contribution is actually paid. If HMRC were to restrict relief on a contribution disproportionate to the director's duties, Article 18(5) narrows with it, because the relief applies only to the extent the contributions qualify for tax relief in the United Kingdom. The US position is directly coupled to the UK outcome.
Substance over label. Where the owner-manager also directs the contribution, sets its amount and is the sole beneficiary, the argument that it is genuinely an employer contribution rather than a redirection of the owner's own remuneration has to be supportable on the documents: the board minute, the payroll records, the scheme's own record of the contributing party and the company accounts.
UK allowances still bite. The employer contribution counts towards the member's annual allowance, which gov.uk states at https://www.gov.uk/tax-on-your-private-pension/annual-allowance as 60,000 pounds, with unused allowance carried forward from the previous three tax years and a taper where threshold income exceeds 200,000 pounds and adjusted income exceeds 260,000 pounds. An annual allowance charge is UK tax on the member, and it is a fact we need before we can say the contribution qualified for UK relief in full.

There is also a structural boundary question. Owner-managers who want to hold commercial property, lend back to the trading company, or pool several family members' pensions frequently end up in a small self-administered scheme rather than a SIPP. A SSAS is an occupational scheme established by the sponsoring employer, which lines up more naturally with the technical explanation's reading of subparagraph (d), while a SIPP is a personal pension scheme established by a financial institution. Clients routinely describe the two interchangeably, and the paperwork is the only reliable guide. Before we characterise a contribution we confirm from the scheme documentation which one it actually is, because the answer can change the FBAR analysis, the Form 8938 valuation approach and the strength of the paragraph 5 position at the same time.

A worked example: four catch-up years on a director's SIPP

The following figures are illustrative only and are used to show the mechanics, not to predict an outcome. Assume a US citizen who has been resident in the UK throughout, who owns and is employed by a UK trading company, and who is paid a small salary with the company making an annual contribution into her SIPP. Assume the employer contributions are 40,000 pounds a year for four years, that full UK corporation tax relief was obtained in each year with no annual allowance charge, and that the SIPP value grows from around 210,000 pounds to around 430,000 pounds over the period. Assume that an exchange rate is applied to each year, using the Treasury reporting rate for the FBAR and an appropriate published rate for the Form 1040; those rates are assumptions in this illustration, not figures we are quoting for a future year.

The reporting picture is then this. Four missed FBARs, because the SIPP alone exceeded the 10,000 dollar aggregate threshold in every year and so did a UK current account alongside it. Form 8938 is in play once the year-end sterling value converts to more than 200,000 dollars for an unmarried filer living abroad, which on these assumptions is every year, reported as a single Part VI line at the year-end fair market value of her beneficial interest. No Form 8621 for the underlying UK funds, on the strength of the regulatory pension exception. On the income side, four years of employer contributions excluded under Article 18(5), each supported by a Form 8833, and no current tax on the growth inside the wrapper under Article 18(1). The additional US tax across the four years, on these assumptions, is nil or close to it. The exposure that was avoided was never a tax bill; it was four years of information return penalty risk and a PFIC position that would have unravelled if the treaty analysis had been wrong.

How do you remediate missed SIPP reporting across several years?

There are three routes, and the choice is driven by whether the omission was non-willful, whether there is unpaid US tax, and whether original returns were filed at all. The IRS sets out the current menu at https://www.irs.gov/individuals/international-taxpayers/options-available-for-us-taxpayers-with-undisclosed-foreign-financial-assets. One currency point matters here: the IRS no longer maintains a separate named route for delinquent FBARs, so anyone still describing a Delinquent FBAR Submission Procedure as a live IRS programme is working from stale guidance. Late FBARs are filed through FinCEN's BSA E-Filing System with a reason for late filing selected, or they are filed as part of a streamlined submission.

Late FBARs only, where the income was correctly reported and the tax paid. File the missing FinCEN Form 114 reports electronically through the BSA E-Filing System, selecting a late filing reason and giving a clear, accurate explanation. The IRS guidance is to file as soon as possible to keep potential penalties to a minimum, and penalties may not be asserted where there is reasonable cause and the income was properly reported.
Amended or delinquent Forms 1040, where the returns need the treaty position, the Form 8938 and the Form 8833 added. This is the route where the pension was the only defect and the taxpayer is otherwise compliant.
The Streamlined Foreign Offshore Procedures, where the non-compliance was non-willful and the non-residency requirement is met. Details are at https://www.irs.gov/individuals/international-taxpayers/streamlined-filing-compliance-procedures and https://www.irs.gov/individuals/international-taxpayers/us-taxpayers-residing-outside-the-united-states.

The streamlined foreign offshore route is the one most often appropriate here, and its mechanics are prescriptive. For a US citizen, the non-residency requirement is met if, in one or more of the most recent three years for which the return due date has passed, the individual did not have a US abode and was physically outside the United States for at least 330 full days. The submission comprises delinquent or amended returns for the most recent three years for which the due date has passed, delinquent FBARs for the most recent six years for which the FBAR due date has passed, and a signed Form 14653 certifying that the failures were non-willful. The returns must be marked Streamlined Foreign Offshore in red at the top, and the delinquent FBARs are filed electronically selecting Other as the reason for late filing and entering Streamlined Filing Compliance Procedures in the explanation box. Taxpayers who properly complete the foreign offshore route are not subject to failure-to-file and failure-to-pay penalties, accuracy-related penalties, information return penalties or FBAR penalties. We set out how we run these engagements at https://us-uktax.com/streamlined-foreign-offshore-procedures.

Sequencing the treaty position and the disclosure across the catch-up years

The second thing nobody covers properly is sequencing. A multi-year SIPP catch-up is not six independent returns; it is one position stated six times, and inconsistency between the years is what draws attention. The three-year and six-year windows are also different lengths, which means a streamlined submission will always contain FBAR years for which there is no accompanying return, and the treaty story has to be coherent across the longer window even though the returns only cover part of it.

The order we work in is deliberate:

Fix the characterisation first. Confirm from the scheme documents whether the arrangement is a personal pension scheme or an occupational scheme, who legally made each contribution, and whether the member holds a designated account. Everything downstream depends on this, and it must not change between years.
Build the year-by-year fact table before drafting anything: treaty residence, the UK employment, the employer bearing the cost, the amount of each employer and member contribution, whether UK relief was obtained in full, any annual allowance charge, and the year-end and maximum scheme values.
Identify the years in which a condition genuinely fails. A year of US secondment, a year with no UK employment, a year in which contributions were borne by a non-UK entity, or a year in which an annual allowance charge restricted UK relief are all years where the Article 18(5) position is different. Say so in those years rather than smoothing them over. A file that claims the same relief in a year the taxpayer was not employed in the UK is far more damaging than a file that concedes one year.
Keep the Form 8833 wording identical across the years where the facts are identical, and varied only where the facts genuinely varied. An examiner comparing four disclosures will notice both consistency and unexplained drift.
Make the Form 14653 narrative match the returns. The certification explains why the reporting was missed; it should describe the SIPP the same way the Form 8938 and the Form 8833 describe it, and it must not assert a treaty conclusion the returns do not actually take.
File the FBAR years that sit outside the return window on the same understanding, so that a year five or year six FBAR does not imply a scheme value or an account structure inconsistent with the returns that accompany the submission.

The failure points we see most often

Treating the treaty as automatic. Article 18(5) is conditional on the employment, on a UK-borne cost and on UK relief actually being obtained. Nobody checks the last condition until we ask for the annual allowance position.
Reporting the underlying funds rather than the pension interest on Form 8938, which produces a long and unnecessary Part VI and often contradicts the Form 8621 position taken elsewhere in the return.
Assuming the PFIC exception applies without confirming the treaty characterisation on which it depends.
Omitting Form 8833 on the theory that an exception applies, then having no contemporaneous record of what was claimed when the years are reopened.
Missing the FBAR in a transfer year, where a consolidation pushed the maximum value far above the usual level even though the year-end value looked ordinary.
Describing a SSAS as a SIPP, or the reverse, and running the wrong analysis for years.
Using the Form 8938 zero-value fallback for a scheme that issues an annual statement.

Where this sits in the wider filing

A missed SIPP is rarely the only thing in the file. The same client usually has UK bank and investment accounts, often a UK company that brings a Form 5471 into play, and frequently a taxable UK fund position that does need Form 8621 treatment. The pension is the piece that most often determines whether the rest of the remediation is straightforward or expensive, because it sets the PFIC exposure and the size of the information return risk. Our cross-border compliance work is set out at https://us-uktax.com/us-tax-services and https://us-uktax.com/cross-border-tax-planning, and if you are carrying unaddressed SIPP years and want the Article 18 conditions tested before you commit to a remediation route, you can reach us at https://us-uktax.com/contact.