Americans Selling UK Property: US Tax on Your London Home

An American selling UK property faces US tax even when HMRC takes nothing. UK Private Residence Relief does not exist in the Internal Revenue Code. The US allows only a limited Section 121 exclusion, recalculates your gain in dollars, taxes phantom currency gains on the mortgage, and frequently leaves no foreign tax credit to shelter the balance.

Why does the US tax a London home sale that HMRC treats as tax-free?

Because US citizenship, not residence, is the trigger. The United States taxes its citizens on worldwide income wherever they live and wherever the asset sits. A US passport holder who has lived in Kensington for twenty years, pays UK tax on everything, has no US home, no US bank account and no intention of returning is nonetheless a US taxpayer with a Form 1040 obligation every April.

When that person sells their London home, two tax systems examine the same transaction and reach entirely different conclusions. HMRC applies Private Residence Relief (PRR), and if the property was the seller's only or main residence throughout ownership, the gain is generally relieved in full. The IRS applies Section 121, which is a capped exclusion, not a relief — and the cap is the problem. In a market where prime central London houses have appreciated by seven figures over two decades, the exclusion is a rounding error.

The result is one of the cruellest asymmetries in cross-border tax: a completely legitimate, fully compliant, entirely domestic UK transaction that generates a large and unexpected US federal tax bill, payable in dollars, with no UK tax to credit against it. Your conveyancing solicitor will not mention it. Your UK accountant, correctly applying UK law, will tell you there is nothing to report. Neither is wrong. Both are looking at only half the picture.

The Section 121 exclusion: $250,000 against a London price tag

Section 121 of the Internal Revenue Code permits an individual to exclude up to $250,000 of gain on the sale of a principal residence — $500,000 for a married couple filing jointly, provided both spouses meet the use test and neither has used the exclusion in the preceding two years. To qualify, you must have owned and used the property as your principal residence for at least two of the five years ending on the date of sale.

Three features of this rule cause most of the damage for wealthy sellers:

• It is a cap, not a relief. UK PRR scales with the gain. Section 121 does not. A £400,000 gain may be fully covered; a £2.4 million gain is not remotely covered.
• It applies to foreign property. This is not a drafting oversight in your favour. Section 121 works on a Notting Hill flat exactly as it works on a house in Connecticut — which means the IRS unambiguously expects the sale to appear on your return.
• The married cap can quietly halve. Where one spouse is a non-resident alien and the couple has not made a Section 6013(g) election, joint filing is unavailable and the $500,000 figure collapses to $250,000 against the US spouse's share. Where the property is held solely in the non-US spouse's name, the position changes again — sometimes helpfully, sometimes not.

Beyond the exclusion, the excess gain is taxed at US long-term capital gains rates, and for high earners the 3.8% Net Investment Income Tax applies to the non-excluded portion. That last point matters enormously, and we return to it below, because NIIT is where the foreign tax credit machinery breaks down entirely.

How is the US gain calculated differently from the UK gain?

This is the technical heart of the problem and the point at which most sellers discover they have a liability far larger than the sterling economics suggest.

HMRC computes the gain in sterling: sale proceeds less acquisition cost less enhancement expenditure and incidental costs. Simple. The IRS computes the gain in dollars, and — critically — it does so by translating each leg of the transaction at the exchange rate prevailing on the date of that leg. Your acquisition cost is fixed in dollars at the spot rate on completion day, perhaps in 2008. Your sale proceeds are fixed in dollars at the spot rate on completion day in 2026. Capital improvements are translated at the rate on the date each was paid.

Sterling's long decline against the dollar over the past two decades therefore does something perverse to the arithmetic. If you bought when the pound was strong and sold when it was weak, your dollar gain is smaller than your sterling gain — occasionally a mercy. If the reverse, your dollar gain can materially exceed your sterling gain, and in extreme cases a seller who made a modest sterling profit, or even a sterling loss, reports a taxable dollar gain to the IRS. That is a real tax on an economic gain the seller never experienced in the currency they actually live in.

The Section 988 mortgage trap: taxed on repaying your own loan

If the Section 121 cap is the headline, Section 988 is the ambush. Almost no seller sees it coming, and it is the single most common reason a London sale produces a US bill that bears no relation to the property economics.

Under US tax principles, a foreign-currency-denominated debt is a separate transaction from the asset it financed. When you take out a sterling mortgage, the IRS conceptualises it as borrowing a commodity — pounds — that you must one day return. When you repay or refinance that mortgage, you are treated as settling that obligation, and any movement in the dollar value of the debt between drawdown and repayment produces an exchange gain or loss under Section 988, entirely separate from the gain on the house itself.

The mechanics are unforgiving in one specific direction:

• If sterling has weakened since you borrowed, your debt has become cheaper in dollar terms. You borrowed, say, the dollar equivalent of £1,000,000 at 1.60 — $1,600,000 of value — and you discharge it at 1.25, costing you only $1,250,000. The IRS says you have realised a $350,000 gain.
• That gain is ordinary income, not capital. It is taxed at your marginal US rate — up to the top ordinary bracket — not at preferential long-term capital gains rates. It cannot be sheltered by Section 121, which applies only to gain on the residence.
• The reverse is not symmetrical. If sterling has strengthened and you suffer an economic exchange loss on a personal-use mortgage, the loss is generally treated as personal and non-deductible. Heads the IRS wins; tails you do not.
• Refinancing counts. Many sellers assume the exposure crystallises only at sale. It does not. Every remortgage, product transfer that legally discharges and replaces the old loan, or partial capital repayment can trigger a Section 988 event. Clients who remortgaged repeatedly through a decade of low rates may have unreported gains scattered across multiple back years.

So the London seller can face three distinct US charges from a single completion: capital gains tax on the dollar gain above the exclusion, NIIT on that same gain, and ordinary income tax on a phantom currency gain from repaying a mortgage denominated in the currency they earn, bank and live in. HMRC charges nothing on any of it.

US versus UK: the same sale, two systems

Feature	UK (HMRC)	US (IRS)
Main residence relief	Private Residence Relief — uncapped where conditions are met throughout ownership	Section 121 — capped at $250,000 single / $500,000 joint
Qualifying test	Only or main residence; quality of occupation and intention	Owned and used as principal residence for 2 of the last 5 years
Currency of computation	Sterling throughout	US dollars, each leg translated at its own historic spot rate
Mortgage repayment	Irrelevant to the CGT computation	Potential Section 988 ordinary exchange gain
Headline rate on the gain	Residential property CGT rates of 18% / 24% depending on the band	Long-term capital gains rates of 0% / 15% / 20%
Investment surcharge	None	3.8% Net Investment Income Tax on the non-excluded gain
Annual exemption	CGT annual exempt amount (£3,000 for individuals)	None
Reporting deadline	60-day CGT on UK property return where tax is due; non-residents must report within 60 days even at nil	Form 1040, Schedule D and Form 8949 for the tax year of sale
Tax year	6 April to 5 April	1 January to 31 December

Why doesn't the foreign tax credit fix this?

Most dual filers assume, reasonably, that the US–UK double taxation treaty and the foreign tax credit exist precisely to prevent this outcome. They do a great deal of work — but they fail here for four structural reasons, and understanding why is what separates a defensible position from an expensive surprise.

1. You cannot credit a tax you never paid

The foreign tax credit relieves double taxation by offsetting foreign tax actually paid or accrued. If PRR has relieved the UK gain in full, your UK tax on the disposal is nil. There is nothing to credit. The US charge stands alone, unrelieved, in full. This is the central irony: the better your UK position, the worse your US position. Sellers with partial PRR — a period of letting, a second home, a period of non-occupation — often end up in a better net position than those with perfect UK relief, because the UK tax they pay generates credits.

2. The gain is not the same gain

Even where UK tax is paid, the two computations rarely align. Different base cost, different currency, different allowable expenditure, different rebasing rules. A non-resident selling UK residential property may rebase to April 2015 values for UK purposes; the IRS recognises no such rebasing and looks all the way back to original cost. The UK gain and the US gain can differ by hundreds of thousands, and credit is limited to the US tax attributable to the foreign-source portion of the income.

3. Timing mismatch

The UK tax year ends 5 April; the US year ends 31 December. A sale in, say, February creates UK tax due the following 31 January and US tax due the following 15 April, in different years. Cash-basis credit claimants can find the foreign tax paid in a year in which there is no corresponding US income to offset — producing an unusable credit, a carryforward, and a real cash cost today.

4. The NIIT gap

The 3.8% Net Investment Income Tax sits in a different chapter of the Code from the income tax. The prevailing IRS position is that foreign tax credits under Section 901 cannot be used to offset NIIT. On a $2,000,000 non-excluded gain that is $76,000 of US tax that no amount of UK tax will relieve. Litigation has challenged this on treaty grounds with some taxpayer success, and treaty-based positions are available in appropriate cases — but they require a considered, properly disclosed filing position, not an assumption.

Section 988 gains have no UK counterpart at all

Add to all of this the fact that a Section 988 mortgage gain is a US-only fiction. HMRC does not tax it, so no UK tax exists to credit against it. And because the exchange gain is generally sourced by reference to the taxpayer's residence, a UK-resident American may find the income re-sourced or, worse, treated in a way that produces US tax with no credit relief. Under the treaty, re-sourcing provisions can sometimes assist, but this is technical territory where the analysis must be done in advance.

A worked illustration

Consider a US citizen who has lived in London since 2007. She bought a house in 2008 for £1,200,000 with an £800,000 interest-only mortgage, when sterling stood near 2.00 to the dollar. She sells in 2026 for £3,000,000 at a rate of 1.25 and redeems the mortgage at completion.

• UK position: £1,800,000 sterling gain, fully relieved by PRR. Tax: nil. Nothing to report.
• US property gain: Cost basis of $2,400,000 (£1,200,000 at 2.00). Proceeds of $3,750,000 (£3,000,000 at 1.25). Gain: $1,350,000, before allowable improvements and selling costs.
• Section 121: Excludes up to $250,000 if filing single. Taxable gain: roughly $1,100,000.
• Section 988: The mortgage was borrowed at a dollar value of $1,600,000 and discharged at $1,000,000 — a $600,000 ordinary exchange gain, taxed at marginal rates and outside the exclusion entirely.
• Foreign tax credit: Nil, because no UK tax arose.

She sold a house, discharged her own mortgage, and reinvested in another London property. Her sterling wealth is unchanged in character. Her US tax bill is well into six figures — a liability created almost entirely by exchange rate movements she never chose and never economically realised. This is not an edge case. It is the standard fact pattern for Americans who bought London property in the pre-crisis era and are now downsizing, relocating, or unlocking equity.

What can actually be done about it?

The uncomfortable truth is that once contracts exchange, the planning window has largely closed. Almost every effective lever is pre-transactional. The levers that genuinely move the number include:

• Sequencing the sale against residency and expatriation plans. The timing of a disposal relative to a change in residence, or relative to a properly executed and compliant expatriation, is the single largest variable in the outcome. It must be modelled years ahead, not months.
• Managing the debt separately from the asset. Where a Section 988 gain is inevitable, its timing is often controllable. Repaying, restructuring or refinancing in a year with offsetting ordinary losses or lower marginal rates can save materially. Conversely, repaying a legacy sterling mortgage in a weak-sterling year, purely for convenience, can be an expensive reflex.
• Ownership structure and spousal allocation. Title, beneficial ownership and the Section 6013(g) election interact to determine whether $250,000 or $500,000 of exclusion is available, and whose gain it is. Getting this right before purchase is trivial; changing it before sale is not, and transfers made in contemplation of a disposal invite scrutiny in both jurisdictions.
• Documenting basis in dollars from day one. Every capital improvement, every professional fee, every stamp duty payment should be recorded with its date and the prevailing rate. Undocumented improvements are lost basis, and lost basis is taxable gain.
• Deliberate use of imperfect PRR. Counter-intuitively, a UK position that generates some UK tax can be preferable to one that generates none, because UK tax creates creditable foreign tax. This calculus should be run, not assumed.
• Treaty-based positions, properly disclosed. Where re-sourcing or NIIT relief arguments are available, they must be taken deliberately on the return with adequate disclosure — not discovered under examination.

Our cross-border tax planning team models these outcomes in both currencies and both tax years before a property is listed, so the sale price you negotiate is the one you actually keep.

What if you have already sold — or already remortgaged?

A large proportion of the sellers who come to us have already completed. Some sold years ago on their solicitor's confirmation that the sale was tax-free, and only discovered the US dimension when a bank asked for a W-9 or a subsequent adviser looked at the file. Others have Section 988 exposure from remortgages they never conceived of as taxable events.

This is recoverable, and it is far more recoverable when disclosed voluntarily than when discovered. The IRS Streamlined Filing Compliance Procedures exist for taxpayers whose failure to report was non-wilful, and for a qualifying seller they can bring several years of returns and information reporting into order without the punitive penalty regime that applies to those who wait to be found. Where foreign accounts held the proceeds, FBAR exposure runs in parallel and should be assessed at the same time.

The critical point is sequencing. A voluntary, properly constructed disclosure that presents the currency computations, the basis evidence and the treaty positions coherently is a fundamentally different conversation with the IRS than a correspondence audit triggered by third-party data. UK conveyancing records, land registry data and automatic exchange of information mean the transaction is far more visible than most sellers assume.

Who is most exposed?

In our experience the highest-risk profiles are consistent:

• Americans who bought London property between 2003 and 2010, when sterling was materially stronger, and are selling now.
• Accidental Americans and dual nationals who have never filed a Form 1040 and do not regard themselves as US taxpayers.
• Long-term UK residents with large interest-only sterling mortgages taken out at historically strong sterling rates.
• Couples with one US and one non-US spouse, where ownership was arranged for UK reasons without regard to the US exclusion cap.
• Executives relocating back to the US who sell the London home on departure — often in the same year they resume US residence, compounding the charge.

Each of these profiles benefits from a co-ordinated review across both systems. Our high-net-worth practice works the UK and US computations side by side, in the same room, on the same file — which is the only way the mismatches surface before they cost money. Where the property forms part of a broader estate plan, our trusts and estate planning specialists model the disposal against succession objectives, because the vehicle that saves capital gains tax often creates an estate tax problem, and vice versa.

The principle worth remembering

UK Private Residence Relief is a relief from a UK tax. It is not a statement about the property, the gain, or your global tax position. It stops at Dover. Every American who owns UK residential property should assume, until advised otherwise by someone qualified in both systems, that a sale is a taxable US event — and that the number the IRS cares about is denominated in a currency they may not have thought about since the day they signed the mortgage.

The sellers who lose money on this do not lose it because the rules are unfair. They lose it because nobody told them the rules applied, and by the time they found out the transaction was closed and the levers were gone.

Speak to us before you list

If you are an American contemplating the sale of a UK home — or you have already sold and the US position was never addressed — a confidential conversation now is worth considerably more than a remedial one later. Jungle Tax advises founders, executives, families and private clients on exactly this intersection, and we routinely reduce or eliminate exposures that clients had been told were unavoidable. Explore our private client tax services, or contact us for a discreet, no-obligation consultation to model your sale in both currencies and both tax systems before contracts exchange.