Selling a UK home as a US citizen is taxed twice over by two sets of rules. UK private residence relief can wipe out the gain entirely while the US still taxes whatever exceeds the section 121 exclusion — and the dollar gain is often larger than the pound gain. Cross-border planning decides which bill you pay.
The reliefs look similar. They are not. One is unlimited and depends on how you used the property; the other is capped in dollars and depends on how long you lived there. A sale that is completely tax-free in Britain can still produce a six-figure US gain.
Selling a UK home: the two reliefs side by side
| UK private residence relief | US section 121 exclusion | |
|---|---|---|
| Amount | Unlimited | $250,000, or $500,000 on a joint return |
| Main test | The property was your only or main residence throughout ownership | Owned and lived in as your main home for 2 of the last 5 years |
| Frequency | No limit | Generally unavailable if you excluded another home's gain in the previous 2 years |
| Part-period use | Relief apportioned by months of occupation, plus the final 9 months | All or nothing on the tests, with limited partial relief for certain moves |
| Currency | Pounds | Dollars, translated at the rates on purchase and sale |
| Let or business use | Restricts relief | Non-qualified use and depreciation reduce the exclusion |
HMRC sets out the UK side in Helpsheet HS283, and the IRS explains the exclusion in Topic 701. Nothing in section 121 restricts the exclusion to property in the United States: a house in Surrey qualifies on the same terms as one in Seattle.
The currency gap that creates a phantom gain
The US gain is computed in dollars. You translate the purchase price at the exchange rate on the day you bought and the sale proceeds at the rate on the day you sold, so a move in sterling changes the answer even when the price in pounds has not.
Buy at £600,000 when the pound is worth $1.25 and your basis is $750,000. Sell at £650,000 when the pound is worth $1.40 and the proceeds are $910,000. The pound gain is £50,000; the dollar gain is $160,000. Nothing about the house changed — the currency did.
It works the other way too. A rising dollar can turn a sterling profit into a US loss, which is not deductible on a personal residence. The IRS explains the translation rules on its foreign currency and currency exchange rates page.
The mortgage trap under section 988
A sterling mortgage is a foreign currency debt for US purposes. When you repay or refinance it, the dollar value of what you borrowed is compared with the dollar value of what you repaid, and a difference is a section 988 foreign currency gain or loss.
If the pound has weakened since you drew the loan, you settle a debt that cost more dollars to create than it takes to discharge — and that difference is generally ordinary income, taxed at your marginal rate, with no section 121 exclusion available against it. The mirror-image loss on a personal mortgage is generally not deductible, which makes the rule one-way.
The section 988 calculation is triggered by repaying the loan, not by selling the house. Remortgaging while you stay put can produce the same US income with no sale proceeds to pay it from.
When the US tax actually lands
If your gain exceeds the exclusion, US tax is due at long-term capital gains rates, plus the 3.8% net investment income tax where your income is above the threshold. You would normally look to the foreign tax credit — but if UK private residence relief covered the whole gain, there is no UK tax to credit, as covered in which UK taxes count for the foreign tax credit.
That is the sting. The better the UK relief, the larger the US bill, because relief and credit cancel each other out. Where UK capital gains tax is payable — on a let property or a second home — the credit goes in the passive category on Form 1116, explained in Form 1116 explained.
If you have already left the UK
Selling after you move away brings in the non-resident regime: a UK return is due within 60 days of completion even when no tax is payable, and only the gain since April 2015 is charged for residential property. That is a separate filing from Self Assessment, and it is covered in non-resident capital gains tax.
The US side does not change when you leave. Section 121 still applies if you meet the two-out-of-five-year tests, which is why the sale is often better done within three years of moving out, while the use test is still satisfied.
Planning points before you exchange
- Work out the dollar gain before you accept an offer, not after completion.
- Check whether the sale falls inside the two-out-of-five-year window on the US side.
- Quantify any section 988 gain on repaying the mortgage separately from the property gain.
- Keep the dollar basis: purchase price, stamp duty, legal fees and capital improvements, each at the rate on the day.
- Where spouses are joint owners, check that both qualify for the $500,000 exclusion.
- If UK tax is payable, consider the timing so the credit and the US gain fall in the same year.
Tranzesta runs both calculations before you exchange contracts, including the mortgage, so the US bill is known while you can still do something about it. Book a consultation if a UK sale is on the horizon. Our wider guidance on UK tax for US citizens covers the rest of the picture.
Frequently Asked Questions
Does the US section 121 exclusion apply to a home in the UK?
Yes. Section 121 excludes up to $250,000 of gain on a main home, or $500,000 on a joint return, and nothing in the rule restricts it to property in the United States. You must still have owned and used the property as your main home for at least two of the five years ending on the date of sale.
Do I pay US tax if UK private residence relief covers the whole gain?
Possibly. UK relief removes the UK tax, but it does not remove the US gain. If the dollar gain exceeds your section 121 exclusion, US tax is due on the excess, and because no UK tax was paid there is no foreign tax credit to offset it. This is the most common surprise in a cross-border home sale.
What is a section 988 mortgage gain?
A gain arising because a sterling mortgage is a foreign currency debt for US tax. When the loan is repaid or refinanced, the dollar value borrowed is compared with the dollar value repaid, and if the pound has weakened the difference is generally ordinary income. A corresponding loss on a personal mortgage is generally not deductible.
How do I convert the gain on a UK house into dollars?
Translate the purchase price and costs at the exchange rate on the date each was incurred, and the sale proceeds at the rate on the date of completion. The IRS has no official rate and generally accepts any posted rate used consistently, but the purchase and the sale must each be translated at their own date, not at a single year-end rate.
Do I have to report the sale if no tax is due?
Often yes. On the US side a sale is reported where the gain exceeds the exclusion, where you received a reporting form, or where you choose not to exclude. On the UK side a non-resident must file a capital gains return within 60 days of completion even when private residence relief means nothing is payable.
Is a Dubai property sale treated the same way?
The US rules are identical: the same section 121 exclusion, the same dollar translation and the same section 988 issue on a dirham mortgage. The difference is that the UAE levies no personal capital gains tax, so there is never a foreign tax credit, and the whole gain above the exclusion is taxed by the United States.
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