Under the US-UK tax treaty, a SIPP is treated much closer to how it works in the UK: the IRS defers tax on dividends, interest, and capital gains accumulating inside the plan until money is actually paid out. Without that treaty position, the IRS falls back on rules that would either tax the plan’s income every year as a foreign grantor trust or hit each underlying fund with punitive PFIC treatment. The deferral is not automatic, though. You have to claim it on your return, keep the SIPP within the scope of the treaty, and continue filing the foreign-account paperwork that the treaty does not remove.
What the IRS Does If You Do Not Claim the Treaty
The US taxes citizens and residents on worldwide income, and a SIPP gets no automatic pass for being tax-advantaged in the UK. Left alone, the IRS sees one of two things.
The first is foreign grantor trust classification. You are treated as the owner of the trust, and every dividend, interest payment, and realized gain inside the SIPP gets added to your US taxable income each year. The UK-side deferral effectively disappears for US purposes. Foreign trust reporting under Section 6048 also engages, with penalties that start at the greater of $10,000 or 35% of the gross reportable amount and grow by another $10,000 for each 30-day period the failure continues after an IRS notice.1Office of the Law Revision Counsel. 26 USC 6048 – Information With Respect to Certain Foreign Trusts2Office of the Law Revision Counsel. 26 USC 6677 – Failure to File Information With Respect to Certain Foreign Trusts
The second is worse. If the SIPP holds UK mutual funds or other pooled investments, each fund can be a Passive Foreign Investment Company. PFIC treatment taxes excess distributions at the highest marginal rate and adds interest charges calculated as if tax should have been paid each year. Each PFIC also carries its own Form 8621 filing.
The treaty is what makes holding a SIPP as a US taxpayer workable. In practical terms, claiming its protections is not optional.
How the Treaty Defers Tax on SIPP Growth
Article 18(1) of the US-UK treaty provides that income earned inside a UK pension scheme is taxable to the individual only when it is paid out or transferred to another plan.3Treasury. Convention Between the Government of the United States of America and the Government of the United Kingdom – Article 18 While assets stay inside the SIPP, the US cannot tax the dividends, interest, or capital gains building up there.
The deferral covers all types of investment return, not particular asset classes. UK equities, bonds, and property funds inside the wrapper are all protected because the treaty applies to income and gains earned by the pension scheme as a whole. Your SIPP compounds without US tax drag, which is the same result Congress produces domestically for a 401(k) or IRA.
To use the deferral, you claim it on Form 8833 with your tax return, identifying the treaty-based position and the article you are relying on.4Internal Revenue Service. Claiming Tax Treaty Benefits Skip the disclosure and the IRS can revert to the default grantor-trust or PFIC treatment. The SIPP also has to qualify as a “pension scheme” under the treaty, meaning regulated and tax-favored under UK law, which a properly established SIPP meets.
Contributions While You Work in the UK
Article 18(5) offers a narrower benefit for contributions. If you are a US citizen living and working in the UK for a UK employer, you can deduct or exclude your SIPP contributions on your US return during that UK employment, and employer contributions are not taxed to you in the US either.5Treasury. Convention Between the Government of the United States of America and the Government of the United Kingdom – Article 18(5) The benefit only extends to contributions that qualify for UK tax relief and is subject to the same limits that apply to comparable US plans.
Once you leave the UK and the UK employment, new contributions generally lose the US deduction. For most SIPP holders who have already moved to the US, the contribution deduction is backward-looking; the ongoing value of the treaty is the accumulation deferral under Article 18(1).
If you put money into the SIPP from income that had already been taxed in the US, those after-tax contributions can create basis in the plan. When distributions start, the portion attributable to previously taxed contributions should not be taxed again, using the same recovery approach the IRS applies to after-tax contributions in US retirement accounts under Section 72. Tracking that basis takes careful records going back to the contribution years.
How SIPP Withdrawals Are Taxed
When distributions begin, the treaty stops sheltering and starts allocating taxing rights between the two countries. The rules split by how you take the money.
Regular Periodic Withdrawals
Article 17(1)(a) says pension payments beneficially owned by a resident of one country are taxable only in that country.6Treasury. Convention Between the Government of the United States of America and the Government of the United Kingdom – Article 17 If you live in the US when you begin drawing your SIPP, regular withdrawals are taxable in the US as ordinary income at your marginal rate, which runs from 10% to 37% in 2026 depending on total income.
The UK may still withhold tax at source on the payment. When it does, you claim a Foreign Tax Credit on Form 1116 to offset the UK tax against US liability and avoid double taxation.7Internal Revenue Service. Foreign Tax Credit The credit is capped at the lesser of the UK tax actually paid or the US tax on that foreign-source income, but the UK withholding rate on SIPP payments is usually lower than the US rate, so the credit tends to be fully usable.
The 25% Tax-Free Lump Sum
UK rules let you take up to 25% of the SIPP as a tax-free lump sum, subject to a cap of £268,275 under current rules.8GOV.UK. Find Out the Rules About Individual Lump Sum Allowances Whether the US honors that tax-free treatment is one of the most contested questions in cross-border pension planning.
The argument for a US exemption relies on Article 17(1)(b), which provides that a pension amount exempt from tax in the country where the scheme is established should also be exempt in the recipient’s country of residence.9UK Legislation. The Double Taxation Relief (Taxes on Income) (The United States of America) Order 2002 – Article 17(1)(b) Because the UK exempts the 25% lump sum, the treaty language suggests the US should too. The IRS has never formally agreed. The conservative position among US tax professionals is to treat the 25% lump sum as fully taxable on the US return at ordinary income rates. Taxpayers who want to rely on Article 17(1)(b) to exclude it disclose the position on Form 8833 and accept the audit risk. On a large SIPP, the tax at stake is significant, so the size of the fund should drive how much professional advice is worth getting.
Full Lump-Sum Distributions
Article 17(2) covers lump-sum payments from a pension scheme, providing that they may be taxable only in the country where the scheme is established. Read literally, a true lump-sum distribution from a UK SIPP could be taxable only in the UK.
The friction is in what counts as a “lump-sum payment.” The IRS reads it narrowly, treating only a complete withdrawal of the entire remaining balance as qualifying. A large partial withdrawal, even one that empties most of the account, may not qualify. Advisors generally treat Article 17(2) as a high-risk position unless the SIPP is being fully closed out. If you do take a full distribution and claim it, disclose on Form 8833 and keep documentation showing the account was fully liquidated.
The 3.8% Net Investment Income Tax
Distributions from US qualified plans like 401(k)s, IRAs, and 403(b)s are specifically excluded from the 3.8% Net Investment Income Tax under Section 1411. Foreign pension distributions do not get the same carve-out. The Form 8960 instructions state that distributions from a foreign retirement plan paid as an annuity and including investment income are not excluded from net investment income.10IRS.gov. 2025 Instructions for Form 8960 – Net Investment Income Tax
If your modified AGI exceeds $200,000 single or $250,000 married filing jointly, SIPP distributions will likely take an extra 3.8% on top of ordinary income tax. On a $100,000 distribution, that is $3,800 that a comparable US retirement withdrawal would not trigger. It is worth building this into your withdrawal timing, especially in years when other income already pushes you over the threshold.
US Reporting the Treaty Does Not Eliminate
Treaty relief takes care of the tax on growth. It does not take care of the paperwork. A US taxpayer with a SIPP has multiple annual filings, and the penalties for missing them are large enough to dwarf the tax benefit itself.
Form 8833 Treaty Position Disclosure
File Form 8833 with your return each year you claim treaty deferral on the SIPP’s growth.11Internal Revenue Service. Form 8833 (Rev. December 2022) Treaty-Based Return Position Disclosure The form identifies the treaty article (Article 18 for deferral, Article 17 for distributions) and explains why the treaty overrides the normal Code treatment. The penalty for failing to disclose a treaty-based position is $1,000 per failure for individuals.12Office of the Law Revision Counsel. 26 USC 6712 – Failure to Disclose Treaty-Based Return Positions The $10,000 figure sometimes cited applies only to C corporations.
FBAR (FinCEN Form 114)
If your combined foreign financial accounts, including the SIPP, exceed $10,000 in aggregate at any point during the year, you file the Report of Foreign Bank and Financial Accounts.13Financial Crimes Enforcement Network. Report Foreign Bank and Financial Accounts The FBAR goes to FinCEN, not the IRS, by April 15 with an automatic extension to October 15. Report the maximum account value during the year in US dollars.
FBAR penalties are among the harshest in the US tax system. Non-willful violations can reach around $16,500 per account per year, adjusted annually for inflation. Willful violations can reach the greater of roughly $165,000 or 50% of the account balance per year. For a SIPP worth several hundred thousand pounds, the exposure from a single missed FBAR can exceed the balance of the account.
Form 8938 (FATCA Reporting)
Separately from the FBAR, you may need to report the SIPP on Form 8938 if the total value of your specified foreign financial assets exceeds the thresholds. For US residents:
- Single or married filing separately: value exceeds $50,000 on the last day of the year, or $75,000 at any point during the year.
- Married filing jointly: value exceeds $100,000 on the last day of the year, or $150,000 at any point during the year.
Higher thresholds apply if you live outside the US: $200,000/$300,000 for single filers and $400,000/$600,000 for joint filers.14Internal Revenue Service. Summary of FATCA Reporting for US Taxpayers Form 8938 goes with your tax return, unlike the FBAR.15Internal Revenue Service. Comparison of Form 8938 and FBAR Requirements
Forms 3520 and 3520-A: Relief Under Rev. Proc. 2020-17
Before 2020, SIPP holders also faced Forms 3520 and 3520-A as foreign trust returns, with the Section 6677 penalties described above hanging over every year. Revenue Procedure 2020-17 eliminated these filings for eligible individuals who own tax-favored foreign retirement trusts, including SIPPs, and the relief applies retroactively to prior years as well.16Internal Revenue Service. Rev. Proc. 2020-1717Internal Revenue Service. Instructions for Form 3520 – Introductory Material
To qualify, the SIPP must be established under UK law to provide pension or retirement benefits, be tax-favored in the UK, have annual reporting available to UK tax authorities, and have contribution limits within the revenue procedure’s thresholds (an annual limit of $50,000 or less, or a lifetime limit of $1,000,000 or less, at the applicable exchange rate).18Internal Revenue Service. Rev. Proc. 2020-17 – Section 5.03 A standard SIPP with current UK contribution limits generally meets the test. The relief does not cover FBAR or Form 8938, only the foreign trust forms.
Form 8621 Relief for PFIC Holdings
If the SIPP holds pooled investments that would be PFICs, Treasury Regulation 1.1298-1(c)(4) provides an important exception. A member or beneficiary of a plan treated as a foreign pension fund under a US income tax treaty does not have to file Form 8621 for PFIC interests held inside the plan, as long as the treaty defers tax on the fund’s income until it is paid to the individual.19GovInfo. 26 CFR 1.1298-1 – Section (c)(4) Article 18(1) of the US-UK treaty meets that condition, so SIPP holders who properly claim treaty benefits are exempt from the annual PFIC filings that would otherwise apply to each fund.
The Form 8621 instructions confirm the exception for shareholders in treaty-recognized pension arrangements.20Internal Revenue Service. Instructions for Form 8621 Without it, a SIPP holding five or six UK funds would need five or six separate Form 8621 filings each year, each carrying its own penalty risk.
State Tax Does Not Always Follow the Treaty
Federal treaty protection does not guarantee state protection. The IRS itself notes that some US states do not honor treaty provisions.21Internal Revenue Service. United States Income Tax Treaties – A to Z In a state that ignores the deferral, your SIPP’s annual investment income could be subject to state income tax even though it is deferred federally. State rates range from zero to more than 13%.
The impact depends on where you live. States with no income tax remove the issue. States that conform fully to federal treatment usually respect treaty positions. The risk sits with states that decouple from federal treaty provisions or have unclear guidance on foreign pension deferral. In a high-tax state, confirm with a tax professional before assuming the SIPP’s growth is fully sheltered.
UK Inheritance Tax Change Effective April 2027
How SIPPs are treated at death is changing on April 6, 2027. Unused SIPP funds currently pass largely outside the scope of UK Inheritance Tax, which makes them highly tax-efficient for wealth transfer. From April 2027, unused pension funds and death benefits will be included in the deceased member’s estate for IHT.22GOV.UK. Inheritance Tax: Unused Pension Funds and Death Benefits
Transfers to a surviving spouse or civil partner and to registered charities remain exempt under existing IHT principles. Death-in-service benefits and dependant’s pensions from defined benefit arrangements are also outside the change. For a US-resident SIPP holder whose beneficiary is, say, an adult child, the SIPP could face UK IHT at 40% above the nil-rate band alongside US estate or income tax on the inherited funds. Interaction with the US-UK Estate Tax Treaty is an area where cross-border estate planning done in advance can prevent a combined tax hit that consumes a large share of the balance.