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.