Selling your main home in the UK is often entirely free of capital gains tax thanks to Private Residence Relief. It is easy to assume that settles the matter. It does not, because the US runs its own calculation on the same sale — in its own currency, with its own reliefs, on its own timetable.
Two reliefs that do not line up
The US exclusion on a principal residence is capped — $250,000 of gain, or $500,000 for a married couple filing jointly — and considerably lower than full UK relief. Anything above the cap is a taxable gain to the IRS even where the UK takes nothing at all.
The qualifying rules differ too: the US wants two of the last five years of ownership and use, while UK relief follows its own occupation history. A period of letting the property, or of working abroad, can move the two reliefs in opposite directions — which makes the timing of a sale a genuine variable rather than a formality.
Currency: the gain you never saw
Currency is the part that surprises people most. The US computes the gain in dollars, using the exchange rate at purchase and at sale. A property that barely moved in sterling can show a substantial dollar gain purely on currency movement — and that gain is taxable even though in your own currency you made almost nothing.
Repaying the sterling mortgage can produce its own separate, dollar-denominated gain on the same logic: you borrowed dollars-worth of sterling at one rate and repaid at another. It is the least intuitive charge in cross-border property, and it turns up on real returns every year.
What to do before you exchange
Where a sale is coming and there is any flexibility on timing, model it in advance: residence in the year of sale, the two reliefs' overlap, the currency position on both property and mortgage, and — if the property was ever let — the depreciation the IRS will recapture whether or not you claimed it.
And diary the UK side: residential sales with tax due need a standalone CGT return and payment within 60 days of completion, separate from and earlier than Self Assessment. A sale reviewed before exchange routinely saves multiples of the fee; after completion, the options narrow to accounting.
If the property was ever let
Letting the property, even for a short period, changes both calculations and it is the single most commonly overlooked item in a cross-border sale. On the American side, depreciation is recaptured on sale whether or not it was ever claimed, which surprises people who never took the deduction.
That recapture is not covered by the principal residence exclusion. It is calculated separately and taxed separately, so a sale that looks fully sheltered can still produce a bill built entirely from years of letting.
The UK side apportions relief across periods of occupation and periods of letting on its own rules. Our guide to UK rental income and US tax covers the reporting during the years of letting, which is where the records for all of this come from.
The UK return that arrives before Self Assessment
A UK residential property sale with capital gains tax to pay needs its own return, filed and paid within 60 days of completion. It is separate from Self Assessment, it comes far earlier, and missing it carries penalties in its own right.
Where Private Residence Relief covers the whole gain there is generally nothing to file on that route, which is exactly why people who relied on the relief and got it slightly wrong discover the deadline after it has passed.
The gain still appears on the Self Assessment return for the year as well. Filing the standalone return does not replace that, and the two have to agree.
Two calculations on one sale
Setting the two side by side is the quickest way to see why a clean UK outcome does not settle the American one. Each system uses its own currency, its own relief and its own qualifying rules on the same transaction.
| UK treatment | US treatment | |
|---|---|---|
| Main relief | Private Residence Relief, uncapped where it applies in full | Capped exclusion: $250,000 single, $500,000 joint |
| Qualifying rule | Follows occupation history | 2 of the last 5 years of ownership and use |
| Currency | Sterling throughout | Dollars, converted at purchase and at sale |
| Mortgage | Not a separate taxable event | Repayment can create its own dollar gain |
| Letting history | Apportions the relief | Depreciation recaptured whether claimed or not |
Selling up, step by step
Reviewed before exchange, a sale can often be improved. Reviewed after completion, the options narrow to reporting it accurately.
- Fix your residence position for the year of sale on both sides, since it drives everything else.
- Pull the purchase completion statement and the exchange rate on the purchase date.
- Reconstruct any period of letting, including depreciation whether or not it was claimed.
- Calculate the gain twice: once in sterling for HMRC, once in dollars for the IRS.
- Check the sterling mortgage separately for a dollar gain on repayment.
- Diary the 60-day UK deadline from completion where tax is due.
- Report the sale on both annual returns, even where nothing is owed.
An illustrative example
Take an illustrative example: an American couple who bought a London flat a decade ago, lived in it throughout, and sell it having never let it. Private Residence Relief covers the UK gain in full and they file nothing extra to HMRC.
In dollars the picture differs. Their sterling gain is modest, but sterling moved against the dollar across the decade, and the dollar gain is larger than the sterling one. The joint exclusion of $500,000 absorbs most but not all of it.
Their neighbours, who let the flat next door for three of the ten years, face the recapture as well. The UK outcome for both households looks almost identical; the American ones do not.
Common mistakes selling a UK home
The first is treating full UK relief as the end of the matter. The American calculation runs on the same sale with a capped exclusion and a different currency.
The second is forgetting the mortgage. It is the least intuitive charge in cross-border property and it appears on real returns every year.
The third is discovering the 60-day deadline after completion. It runs from completion, not from the end of the tax year, and it does not wait for anyone to work out whether relief applied.
