The US side: worldwide gains and the main home exclusion
If you are a US citizen, green card holder or otherwise US tax resident, the US taxes your worldwide gains, so a UK house sale falls within US tax wherever you live. The Section 121 main home exclusion can shelter up to $250,000 of gain, or $500,000 for a married couple filing jointly, where you owned the home and used it as your main residence for at least two of the five years before the sale; gain above that is taxable and must be reported. Horizon is a UK Chartered Tax Adviser practice, so the US filing itself is handled by our US partners (Enrolled Agents and CPAs), whom we coordinate for you.
The UK side: non-resident CGT and the 60-day return
The UK looks at the same sale separately. If you are non-resident when you complete, you are normally taxed only on growth since 5 April 2015, and Private Residence Relief plus the automatic final nine months often reduces the UK gain to nil, although a tax year abroad only counts as occupation if you, your spouse or civil partner spend at least 90 days in the home. Either way, non-residents must report the disposal to HMRC within 60 days of completion, even when no tax is due. See our 60-day non-resident CGT rules and the sell versus rent decision for the UK detail.
The currency trap: a dollar gain from a nil sterling gain
Every figure on a US return must be expressed in dollars, translated at the exchange rate prevailing when each amount arose. Your purchase price is translated at the rate on the day you bought, your proceeds at the rate on the day you sold. If the pound strengthened against the dollar in between, you can show a taxable dollar gain even where the sterling gain is nil; the Section 121 exclusion can still shelter that gain if you qualify.