US and UK Pensions Across the Border: 401(k)s, IRAs, SIPPs and the Treaty

Why pensions are the treaty's centrepiece

Most cross-border income is handled by the blunt instrument of foreign tax credits. Pensions are different: the US–UK treaty devotes two full articles to them — Article 17 (pensions, social security, annuities) and Article 18 (pension schemes) — and, unusually, key parts survive the saving clause that strips most treaty benefits from US citizens. That makes pensions one of the few areas where the treaty changes outcomes even for Americans, rather than merely confirming what credits would do anyway.

It also makes them the area with the most moving parts. Which country taxes a pension payment depends on where you live, where the scheme is established, whether the payment is periodic or a lump sum, and — for US citizens — which rules pierce the saving clause. Change one variable and the answer changes.

The core treaty architecture

Three rules do most of the work:

• Growth is protected. Under Article 18(1), income building up inside a pension scheme established in one country is generally not taxed by the other until it is paid out. This survives the saving clause — it is why a UK-resident American's 401(k), and a US-taxpayer's SIPP, can grow without annual taxation by the other country.
• Periodic pensions follow the person. Article 17(1) makes pensions and similar remuneration generally taxable in the beneficial owner's residence state. It also contains a provision preserving the tax-exempt character of certain amounts — the clause on which much lump-sum analysis turns.
• Lump sums follow the scheme. Article 17(2) points taxation of a lump-sum payment at the country where the scheme is established, not where the recipient lives.

The periodic-versus-lump-sum distinction is therefore not pension trivia; it is the treaty's central fork. The same pot can face materially different treatment depending on how money leaves it — and what counts as "periodic" against "lump sum" for a modern flexible drawdown account is itself a question of characterisation that deserves care, not assumption.

US pensions held by UK residents

For a UK resident drawing on American retirement savings, the working framework is: periodic 401(k) and IRA payments are generally taxable in the UK as the residence state, while lump sums are pointed back at the US as the scheme state — with a US citizen additionally computing US tax throughout and relying on credits and the surviving treaty provisions to prevent doubling. Roth accounts add a layer: their US-exempt character is generally understood to be respected for UK residents under the treaty's exempt-amount rule, but account history, conversion timing and the manner of withdrawal all matter, and conversions undertaken after UK residence begins raise their own questions.

The practical consequence: the order, shape and timing of withdrawals — periodic against lump, before against after a move, which pot first — is a genuine planning variable with different answers for different people.

UK pensions on US returns

A SIPP or workplace pension held by a US taxpayer is first a reporting exercise and then a treaty one. Treaty protection means the scheme's growth is generally not currently taxed by the US, and employer contributions and relief for personal contributions can be available under Article 18's conditions — typically aimed at people working across the border for limited periods, and conditional enough that eligibility should be confirmed case by case. Whether and where the scheme appears on FBAR and Form 8938, and whether any election or disclosure is appropriate, depends on the arrangement; the costly error is inconsistency from one year to the next.

The tax-free lump sum mismatch

UK schemes allow part of a pension to be taken tax-free in UK terms. Whether that amount is also free of US tax for a US citizen is one of the honestly contested questions in this field: the treaty preserves the exempt character of certain pension amounts, the saving clause exceptions include that provision, and practitioners nonetheless read the combination differently. We will not pretend the answer is settled. What is settled is the sequencing: the analysis must happen before the lump sum is taken, because afterwards the only question left is how to report an outcome you can no longer change.

State Pension and Social Security

Government retirement benefits follow their own rule — Article 17(3) generally assigns cross-border social security payments, including US Social Security paid to UK residents, to the residence country alone, and it survives the saving clause. That, together with the totalization agreement on contributions and combined records, is covered in Social Security and the State Pension.

Honest complexity, managed once

Pensions reward exactly one behaviour: deciding the treaty position for each pot once, before money moves, and then filing both returns to match for as long as the pot pays out. Everything expensive in this area — reclassified lump sums, inconsistent reporting, conversions done in the wrong year — comes from improvising at withdrawal time.