US Personal Tax Services for US Law Firm Partners in London

A US-citizen equity partner in the London office of a US law firm files in both countries every year. The IRS taxes the full K-1 share and HMRC taxes the worldwide profit share as a UK resident. Add state nonresident returns, and foreign tax credits have to be sourced carefully so the same London profits are not taxed twice.

For this partner, US personal tax services means more than filling in a Form 1040. It means rebuilding one set of profits under two tax systems, with different tax years, different rules on where income comes from, and different payment dates. At Jungle Tax we prepare these returns for US partners in London, and we also bring the missed years up to date. This guide goes through each item in the order we deal with it in a typical engagement.

Why is a US law firm partner in London taxed by two systems at once?

The United States taxes its citizens on worldwide income wherever they live. The United Kingdom taxes its residents on worldwide income. As a US citizen living in London under the statutory residence test, you fall fully into both systems. The firm itself is usually a US general partnership or limited liability partnership. Both countries normally treat it as transparent, so neither taxes the firm as an entity. Each of them taxes your share of its profit, whether or not the cash has been paid out to you.

Double taxation is avoided mainly by foreign tax credits, supported by the US-UK income tax treaty and the US-UK social security (totalisation) agreement. None of this relief is automatic. Each piece must be claimed on the right form, in the right year, using figures that agree with each other across both returns. That is where most errors happen.

Your US filing stack: what the Schedule K-1 and K-3 actually tell you

The firm files Form 1065 and sends each partner a Schedule K-1, usually with a Schedule K-3 for international items. Firms with hundreds of partners often issue these late in the season, after extending their own return. For a London partner, the boxes that matter most are:

• Box 1, ordinary business income: your distributive share of the firm's trading profit. This is the core figure that both countries tax.
• Box 4, guaranteed payments: fixed amounts paid for services or capital, whatever the firm's profit. Some firms use these for non-equity or hybrid partners.
• Box 13, other deductions: often includes unreimbursed partnership expenses or items passed through from the firm's investments.
• Box 14, self-employment earnings: the figure Schedule SE would use if you had no totalisation exemption.
• Box 16 and the Schedule K-3: the foreign transactions detail. It shows how much of your share is foreign source, which foreign tax credit category it falls in, and any foreign taxes paid at firm level.

The Schedule K-3 is the document most often ignored, and it matters most. Parts II and III show how the firm has sourced its income between the US and the rest of the world. That split directly limits the UK tax you can credit on your US return.

The sourcing trap: why a London partner's profit share can be partly US source

Many partners assume that because they live and work in London, all of their profit share is foreign-source income for foreign tax credit purposes. Often it is not. Under the US partnership rules, the character and source of a distributive share are generally decided at partnership level. What counts is where the firm's income-producing services were performed, not where you personally sat. If most of the firm's fee income comes from lawyers working in the United States, a large part of every partner's share may be US source, including the share of a partner who never worked in America that year. Firms apply different methods, so read your K-3 rather than assuming the answer.

The effect is real. HMRC taxes your whole worldwide share at UK rates. The US foreign tax credit limitation only lets you credit UK tax against US tax on foreign-source income. If the K-3 treats part of your share as US source, some UK tax may be left uncredited on the US side while the US still taxes that slice. For US citizens resident in the UK, the treaty contains special relief rules, including re-sourcing provisions, to deal with exactly this overlap. Applying them needs careful preparation and usually a separate treaty-resourced basket on Form 1116. We model it every year rather than accept whatever a software default produces.

Guaranteed payments for services are treated differently. They are generally sourced where you perform the services, so a guaranteed payment for work done in London is foreign source. That is one reason the mix between guaranteed payments and distributive share can affect your net tax position.

How does HMRC tax a UK-resident partner of a US law firm?

HMRC's starting point is clear: UK-resident partners are taxed on their share of the partnership's worldwide profits. It makes no difference that the firm is organised in a US state, run from New York or elsewhere, or that most of its fee income is earned in the United States. As a UK resident, your whole profit share is taxed as trading income, together with Class 4 National Insurance contributions.

The SA800 partnership return and the UK "mirror" computation

A partnership that trades in the UK through a London office must file a UK Partnership Tax Return (SA800). The firm's nominated partner, usually supported by the firm's UK tax advisers, files it. For a firm managed and controlled outside the UK, HMRC's guidance says the SA800 returns the UK branch profit only. UK-resident partners, however, must still report their worldwide share on their own returns. HMRC helpsheet HS380 explains the two-statement approach: one statement of worldwide profit for resident partners and one of UK profit for non-residents.

In practice, the firm (or its UK advisers) prepares a "mirror" computation for its UK-resident partners. This recasts the firm's US-basis accounts into UK taxable profit. The adjustments are significant:

• UK rules on accounting profit (generally UK GAAP or IFRS principles) replace US tax accounting, including treatment of work in progress and accruals.
• UK capital allowances replace US depreciation.
• Expenses allowed in the US may be disallowed in the UK, and the reverse. Client entertaining and some partner-level costs are common examples.
• Profits are translated into sterling, using a rate that has to be applied consistently each year.

You then enter your share on the partnership (full) pages of your SA100 Self Assessment return. The UK taxable figure will almost never equal your K-1 box 1 in dollars, and it should not. Our job is to reconcile the two so that each return can be defended by itself.

Basis period reform and the calendar-year firm

Most US law firms have a 31 December year end. Before reform, UK partners were taxed on the accounts period ending in the tax year, which gave the well-known overlap profits. From 2024/25 the UK moved to the tax year basis. Partners are now taxed on profits arising between 6 April and 5 April, apportioned from the firm's accounting periods. 2023/24 was the transition year. Transition profits arising from the change are by default spread over five years, 2023/24 to 2027/28, unless the partner elects to accelerate them.

For a calendar-year firm, each UK return now combines roughly three-quarters of one year's profit with one quarter of the next. The later year's accounts are often not final by 31 January, so provisional figures must be used and amended later. That affects the UK tax figure you are crediting on the US return, which in turn affects the Form 1116 reconciliation. If you joined or left the partnership during the transition window, or still have transition profits being spread, the tie-out needs even more care.

What about the remittance basis?

From 6 April 2025 the UK abolished the remittance basis and replaced it with a four-year foreign income and gains (FIG) regime for people arriving after ten years of non-residence. For a London-based partner, it rarely shelters much. Profits from work done in the London office are UK profits whatever regime applies. Any relief on overseas trading profits depends on your arrival date and on how the firm's profits are divided between UK and non-UK activity. Newly arrived partners should have this reviewed in their first year, but should not rely on it.

State composite and nonresident returns

Moving to London does not always end your connection with a US state. Two separate state issues apply.

Source taxation by the firm's states. States where the firm has offices generally tax nonresident partners on the share of firm income apportioned to that state, usually by a formula based on receipts, payroll or property. Many firms file a composite or group return for nonresident partners and pay the tax on their behalf. That can satisfy your filing obligation in those states. Where you are not in the composite, or where joining it is a poor choice, you file individual nonresident returns. Tax paid by the firm for you appears on the K-1 or on state supplementary schedules, and it must be treated consistently on your federal return.

Residence in your former home state. Some states, particularly those that apply domicile concepts, can keep treating a departed partner as a resident, taxable on worldwide income, unless a clear break is shown. Keeping a home there, voter registration, a driver's licence or a declared intention to return can all sustain residence. States generally give no credit for UK tax. A partner who is still a resident of a high-tax state can therefore face a real third layer of tax on London profits.

From the UK side, HMRC generally allows credit for US state income taxes as well as federal tax, where those taxes fall on income the UK is also taxing. The amounts, the source of the profit and the treaty ordering rules all need to line up. State tax is a reconciliation item on both returns, not a footnote.

Foreign tax credits: making UK tax work on Form 1116

For most London partners, the foreign tax credit on Form 1116 is the main relief. Because UK income tax rates on high earners are generally higher than US federal rates, a properly prepared claim often reduces US federal tax on foreign-source profit to nil. Getting there needs attention to several points:

• Category. A partner's trading profit is general category income, not passive. Keep it apart from your investment income.
• Timing. The UK tax year does not match the US calendar year, and UK tax is paid through payments on account and balancing payments. Most partners do best by electing to claim credits on the accrued basis, which matches UK tax to the US year in which the income arises. The election is generally irrevocable once made, so it should be a deliberate decision.
• Currency. Accrued taxes are generally translated at the average rate for the year. Taxes paid more than two years after the year end, or in arrears, may need the rate on the payment date. Amended UK figures from basis period true-ups create US adjustments.
• Carryovers. Excess UK credits can generally be carried back one year and forward ten years. A London partner builds up substantial carryovers. They are valuable if you later move back to the US, but only if they are tracked on every return, including the ones filed late.
• Social security contributions. UK Class 4 National Insurance on income covered by the totalisation agreement is generally not a creditable foreign tax for US purposes. Leave it out of the Form 1116 figures.

Foreign earned income exclusion or foreign tax credit?

A law firm partner's share of profit from personal services is generally earned income. Because capital is not a material income-producing factor in a law practice, the share can qualify for the foreign earned income exclusion on Form 2555. The 2026 exclusion limit is about $132,900 (figure to verify each year). For a partner earning seven figures, however, the exclusion covers only a small slice. Claiming it also reduces the foreign tax credit available on the excluded portion and can push the remaining income into higher brackets. For most equity partners, relying on the foreign tax credit alone is the better long-term position. An election to use the exclusion, once revoked, generally cannot be made again for five years without IRS consent, so the choice should be modelled rather than made by default.

Self-employment tax and the totalisation certificate

An equity partner's distributive share is normally subject to US self-employment tax. The Social Security part stops at the annual wage base, but the Medicare part is uncapped, plus the 0.9% Additional Medicare Tax above the thresholds. For a partner with a large profit share, that alone is a significant sum each year.

The US-UK totalisation agreement stops you paying into both systems. For self-employed people, the general rule is that you are covered only by the social security system of your country of residence. A UK-resident partner therefore pays UK Class 4 NICs and should be exempt from US self-employment tax. That exemption has to be documented. You obtain a certificate of coverage from HMRC confirming UK coverage, and attach a statement to your US return claiming the exemption as the Schedule SE instructions direct. The IRS's totalization agreements guidance sets out the certificate requirement.

Without the certificate, many returns either overpay US self-employment tax or claim an exemption that cannot be supported. If earlier years were filed with self-employment tax paid in error, amended returns can often recover it within the refund window. If the exemption was claimed without a certificate, one can usually be obtained after the event.

The net investment income tax does not normally apply to the active trading profit of a partner who materially participates in the firm. It can apply to investment income passed through on the K-1, and to your own UK investment income. The IRS position is that foreign tax credits do not offset it, so it needs separate planning.

Tax equalisation and partner tax distributions: the reconciliation item

US firms usually make tax distributions to partners during the year: cash advances of profit, sized to help partners meet estimated tax at assumed rates. Some firms with international partners also run tax equalisation or cross-border policies. These can adjust partner allocations or make balancing payments so that partners in higher-tax jurisdictions such as London are not worse off than US-based peers, or so that the firm's overall tax burden is shared on an agreed basis.

For the preparer, these arrangements are a reconciliation exercise rather than extra income:

• A tax distribution is a draw against your share of profit. It is not separately taxable, but it reduces your capital account and must be tracked for basis.
• An equalisation adjustment may change your allocated profit, and so your K-1 and your UK share. It may also be paid as a separate payment with its own character. The partnership agreement and the firm's policy document decide which.
• Where the firm pays UK tax or state composite tax on your behalf, that payment is usually charged to your capital account. It must be shown as tax paid by you, not as a firm expense, on both returns.
• The UK profit share, the K-1, the cash statements and the capital account roll-forward should tie together each year. If they do not, one of the returns is probably wrong.

We ask for the firm's partner tax pack, capital account statement and any equalisation calculation every year for exactly this reason.

US vs UK: how the obligations compare

Quarterly estimated payments vs UK payments on account

The two payment calendars do not match, and a partner who ignores the gap gets penalties or poor cash flow. In the US, estimated tax is due in four instalments, normally 15 April, 15 June, 15 September and 15 January (see Form 1040-ES). To avoid the underpayment penalty, a high earner generally has to pay 110% of the prior year's tax or 90% of the current year's tax. In the UK, you make two payments on account, on 31 January and 31 July, each normally half of the prior year's liability, with a balancing payment the following 31 January.

Where foreign tax credits bring US federal tax on London profits to nil, federal estimates may be small. They are rarely zero, because US-source slices of the profit share, pass-through investment income and your own investments can all leave residual US tax. State estimates or firm composite payments run separately. Your US estimates should be based on a projection that allows for the UK credit position and the K-3 sourcing, not simply last year's figures. On the UK side, a large rise in profit share can mean a "catch-up" balancing payment plus a higher payment on account falling on the same 31 January, so plan cash for it.

A preparation workflow that holds up in both countries

1. Confirm residence and status. UK statutory residence for the tax year, any split-year treatment, and continuing US state residence exposure.
2. Collect the firm pack. K-1, K-3, state composite schedules, capital account statement, tax distribution records and any equalisation calculation.
3. Obtain the UK profit figures. The worldwide profit share from the firm's UK computation, apportioned to the UK tax year, with provisional figures identified.
4. Prepare the UK return first, or at least project it, because the UK liability drives the US foreign tax credit.
5. Model the Form 1116 position using the K-3 sourcing, the accrual election, currency conversion, treaty re-sourcing and carryovers.
6. Apply the totalisation exemption with the certificate of coverage on file.
7. Prepare state returns or confirm composite coverage, and document any break in state residence.
8. Complete information returns: the FBAR (FinCEN 114) and Form 8938 for UK bank, savings, pension and investment accounts.
9. Reconcile and set payments. Tie out the K-1, UK share and capital account, then set US estimates and confirm UK payments on account.

Catching up missed years as a law firm partner

Partners are busy, firm packs are late and some UK-based partners are not told clearly that the US return is still theirs to file. We regularly meet partners with one or more of these gaps:

• US returns filed on time but without Forms 1116, so foreign tax credit carryovers were never built up;
• US self-employment tax paid in error, or an exemption claimed with no certificate on file;
• no FBAR or Form 8938 for UK current accounts, savings, ISAs or the partner's UK pension arrangements;
• state nonresident returns missed where the partner was not in the firm's composite;
• UK returns filed with the UK branch figure only, instead of the worldwide share a resident partner must report.

For US returns not filed at all, the IRS Streamlined Foreign Offshore Procedures are often the right route for a non-wilful partner living abroad. You file the last three years of returns and six years of FBARs with a certification of non-wilful conduct. Where you meet the non-residency test, no miscellaneous offshore penalty applies. Because UK tax usually exceeds US tax on the same profits, the tax actually due on a streamlined submission is often modest. The value is in regularising the position and restoring carryovers. Where the returns were filed but information returns were missed, the delinquent FBAR or delinquent information return procedures may apply instead. Our FBAR penalty calculator shows the exposure these routes are designed to remove. If the UK return understated worldwide profits, HMRC's disclosure routes are generally better than waiting for an enquiry. We prepare both sides together so the corrected figures agree.

What to have ready before your first meeting

• K-1s and K-3s for every open year, plus state composite or withholding statements;
• the firm's UK partner tax pack and your SA100 returns;
• capital account statements and tax distribution or equalisation records;
• HMRC certificate of coverage, if you have one;
• a list of UK and US financial accounts, pensions and investments, with year-end balances;
• any prior-year US returns, including foreign tax credit carryover schedules.

Partners with broader cross-border questions, such as a planned move back to the US or the timing of a partnership exit, can also look at our cross-border tax planning page and our wider US-UK tax accountants service. Our engagement stays focused on accurate return preparation and compliance.

Speak to a US-UK specialist

An equity partnership in the London office of a US firm is a valuable position, and it deserves returns that agree across the K-1, the UK partnership computation and every state filing. Jungle Tax prepares annual US and UK returns for law firm partners, reconciles tax distributions and equalisation, and brings missed years and unfiled FBARs up to date quietly. For a confidential review of your position, contact our cross-border team.