Cross Border Tax Accountant London: What Americans in the Capital Should Expect

A cross border tax accountant in London prepares your US federal return and your UK Self Assessment return on the same facts, files the US reports a UK firm does not, and plans around London's typical pressure points: high UK incomes, property, ISAs and City pay packages. Expect a scoped engagement, a document list for both countries and a timetable built around two tax years.

This guide is for Americans living in London, and for UK nationals in London with US ties such as a green card, US-born children or US investments. It explains what the work involves, which London situations tend to complicate it, and what you should expect from the firm you choose.

What does a cross border tax accountant in London actually do?

A cross border tax accountant in London works across both tax systems, not one. The US side starts from the IRS rule that US citizens abroad are taxed on worldwide income, so a US citizen in London files a federal return each year where the filing threshold is met, and claims the foreign tax credit or the foreign earned income exclusion on that return to avoid being taxed twice. The UK side is the Self Assessment return where one is required, prepared under UK rules for the tax year running from 6 April to 5 April.

Between the two sits the reporting layer. Where the aggregate value of your foreign financial accounts exceeded $10,000 at any time in the calendar year, the IRS requires an FBAR, filed electronically through FinCEN's BSA E-Filing System rather than with the tax return. For most Americans in London, UK current accounts, savings, ISAs and some pensions count towards that total. Form 8938 and Form 8621 may also apply.

The value of one accountant, or one team, doing both is that the numbers on one return drive the relief claimed on the other. UK tax paid in London is usually what reduces US tax, so the two returns have to agree. Our cross border tax accountant London page explains how we organise this work for clients in the capital.

The London situations that make a cross-border year harder

Nothing in UK or US tax law is specific to London. But some circumstances come up far more often in the capital, and each needs both systems to be considered together.

High UK incomes and the £100,000 taper

For the 2026 to 2027 tax year, GOV.UK sets the personal allowance at £12,570, the basic rate at 20% on income from £12,571 to £50,270, the higher rate at 40% from £50,271 to £125,140, and the additional rate at 45% above £125,140. These are the rates for England, Wales and Northern Ireland. The personal allowance is reduced by £1 for every £2 of adjusted net income above £100,000 and reaches zero at £125,140.

Two cross-border points follow. First, UK tax on a London salary in that range is often high enough that the foreign tax credit, rather than the foreign earned income exclusion, is the better route on the US return, although the answer depends on the rest of the year and should be modelled rather than assumed. Second, UK planning that reduces adjusted net income, such as pension contributions, has its own US treatment. The UK saving and the US result need to be looked at together.

Child Benefit and the High Income Child Benefit Charge

The High Income Child Benefit Charge applies where one partner's income is above £60,000. The charge is 1% of the Child Benefit for every £200 of income above that level, so at £80,000 it equals the full benefit, according to GOV.UK's guidance on the charge. It can be paid through PAYE in some cases, but a Self Assessment return is required if you already file one for another reason. For London households with US ties, that often means a return is needed anyway.

Property: buying, letting and selling

London property raises questions on both sides. A US citizen selling a home abroad reports the sale on the US return. The IRS allows up to $250,000 of gain on a main home to be excluded, or $500,000 for a married couple filing jointly, where the ownership and use tests are met: owning and living in the home for at least two of the five years before the sale. UK relief on the same sale is decided under UK rules. The two systems can reach different answers on the same flat, which is why a sale should be reviewed before exchange, not after completion.

Letting a London property while living elsewhere, or letting a US property while living in London, brings rental income into both returns, with each country's rules on allowable expenses and depreciation.

ISAs, UK funds and pensions

An ISA is tax-free in the UK. The US does not recognise the wrapper, and UK funds held in a stocks and shares ISA are often passive foreign investment companies. The IRS requires Form 8621 from PFIC shareholders in specified circumstances. We explain the issue in ISAs, PFICs and US tax. UK workplace pensions and SIPPs have their own treaty treatment and their own reporting.

City pay packages

Bonuses, share awards and deferred compensation are common in London financial and technology roles. When awards vest over several years, and the employee moves between the US and the UK during that period, the income may need to be divided between the two countries, and the US calendar year and the UK tax year will record it at different times. This is detailed work that should be planned in the year of any move, not reconstructed afterwards.

Arriving in London, or leaving it, part-way through a year

Many Americans come to London on a posting of a few years, and many Londoners move to New York or San Francisco. The year of arrival or departure is usually the hardest year of the whole stay. Whether you are UK resident for that tax year is decided by the statutory residence test, which looks at days spent in the UK, work and ties, and not simply at the date you moved. Where the conditions are met, split year treatment can divide the UK tax year into a UK part and an overseas part. We explain both in the statutory residence test explained and split year treatment.

On the US side, a US citizen files on the calendar year throughout, so the US return for the year of the move covers income from both countries. The two returns cover different periods, so the relief claimed on one depends on how the other has been prepared. That is the main reason to get advice before the move, not after the first year-end.

What should you expect from a London cross-border engagement?

Expect a written scope, a document request for both countries, and a timetable that meets both sets of deadlines. The year usually runs like this:

1. Scoping. The firm confirms which returns and reports your year needs: US federal return, any US state return, FBAR, Form 8938 or Form 8621 where relevant, and UK Self Assessment where required.
2. Document collection, once. Information is collected at the same time for both returns, so you are not asked for the same bank statements twice.
3. UK registration check. If you need a Self Assessment return for the 2025 to 2026 tax year and have not filed before, GOV.UK sets 5 October 2026 as the date to tell HMRC.
4. UK return. Paper returns are due by 31 October 2026 and online returns and payment by 31 January 2027.
5. US return and reports. The US return is due 15 April, with an automatic extension to 15 June for Americans abroad and a further extension to 15 October on Form 4868. Interest runs on unpaid US tax from 15 April. The FBAR is due 15 April with an automatic extension to 15 October.
6. Joint review. Both returns are checked against each other before either is filed, so the UK tax claimed as a credit in the US matches the UK return.

What you will be asked for

UK documents	US documents
P60 and P11D from each UK employer	Prior-year US federal and state returns
UK bank, savings and ISA statements with interest and dividends	US brokerage and bank tax statements
Pension statements showing contributions	US retirement account statements
Share award and bonus statements	Maximum account values for the FBAR
Property purchase, letting and sale documents	Copies of prior FBARs and Form 8938 filings

Illustrative example: an American couple in Islington, one on a London salary of around £130,000 with share awards, the other working part-time, with a stocks and shares ISA and a flat bought four years ago that they now plan to sell. In one year they meet the £100,000 taper, the High Income Child Benefit Charge, PFIC reporting for the ISA funds, share awards that may need dividing across two countries, and a property sale that each country will assess separately. None of these is unusual in London. Together, they are a year in which the US and UK returns need to be prepared as one piece of work, and the sale reviewed before contracts are exchanged.

Do you need an accountant with a London office?

No. The IRS and HMRC do not care where your accountant works, and most of the work is done electronically. What matters is that the firm works in both systems, can show credentials on both sides, and reviews the two returns together. A London office helps if you prefer meeting in person or if your affairs involve City pay structures that a local team sees often. Outside the capital, our guide to a cross border tax accountant in Manchester covers the questions that come up most in the North West.

If you are comparing firms, our guide to when to hire cross border tax specialists for US and UK helps you decide whether you need one at all, and the overview of tax obligations for Americans living in the UK sets out the annual routine.

What Americans in London get wrong

Assuming PAYE settles everything. PAYE settles UK tax on a salary. It does not file the US return or the FBAR, and it does not tax US bank interest or dividends, which may need a Self Assessment return.

Treating the ISA as tax-free everywhere. The US taxes income and gains inside it, and the funds inside it can carry PFIC reporting.

Selling property first and asking later. The US exclusion depends on dates and use, and the UK answer on the same sale can differ. Advice before exchange can change the outcome. Advice after completion can only report it.

Using two firms that never speak. A UK firm and a US firm can each be correct and still leave a gap between them. If you use two, agree which one owns the cross-border positions.

Working with us in London

US/UK Cross Border Tax — US CPAs and UK tax advisers working as one team; London, Manchester, New York, San Francisco. For London clients we prepare the UK Self Assessment return and the US return together. Our page for Americans in the UK covers the wider picture. If you are dealing with the end of the remittance basis, see non-dom changes for Americans in the UK. To discuss your own position, get in touch.