US citizens living in the UK face a uniquely complex situation when they sell assets — from UK property to shares and investments. They must navigate both UK Capital Gains Tax (CGT) and US federal capital gains tax, applying the rules of two different systems to the same transaction. This guide explains how both systems work, how the Foreign Tax Credit prevents double taxation, and the key planning opportunities available to US expats facing significant capital gains in the UK.
UK Capital Gains Tax: The Basics
UK Capital Gains Tax applies to gains made by UK residents on the disposal of chargeable assets. Key rates for 2025/26:
- Residential property: 18% (basic rate taxpayers) or 24% (higher/additional rate taxpayers)
- Other assets (shares, business assets, etc.): 18% (basic rate) or 24% (higher/additional rate) — rates were increased in the October 2024 Autumn Budget from 10%/20%
- Business Asset Disposal Relief (BADR): 10% on qualifying business disposals up to a lifetime limit (reduced to £1 million)
- Annual Exempt Amount: £3,000 for 2025/26 (reduced from previous years)
UK CGT is calculated on UK Self Assessment and reported on the SA108 supplementary pages (Capital Gains Summary). For residential property disposals, a separate 60-day CGT return must be filed and any tax paid within 60 days of completion.
US Capital Gains Tax on UK Asset Sales
US citizens pay US capital gains tax on worldwide gains, including gains on UK assets. US CGT rates for 2026:
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- Long-term capital gains (assets held more than one year): 0%, 15%, or 20% depending on taxable income thresholds
- Short-term capital gains (assets held one year or less): taxed at ordinary income rates (up to 37%)
- Net Investment Income Tax (NIIT): Additional 3.8% on net investment income (including capital gains) for taxpayers with modified AGI above $200,000 (single) / $250,000 (married filing jointly)
- Depreciation recapture: 25% rate on gains attributable to prior depreciation deductions on real property
How the Foreign Tax Credit Prevents Double Taxation
The Foreign Tax Credit (Form 1116) is the primary mechanism for preventing double taxation. UK CGT paid on the sale of a UK asset is a creditable foreign tax that can offset the US capital gains tax on the same gain.
Key rules:
- UK CGT is a creditable tax — it is a compulsory levy on gain, imposed at a level similar in structure to US capital gains tax
- The FTC on capital gains goes into the “passive income” basket for Form 1116 limitation purposes
- If UK CGT rates exceed US capital gains rates, you may have excess foreign tax credits — these can be carried back one year and forward ten years
- For the NIIT (Net Investment Income Tax), the FTC is more complex — there is no direct mechanism to credit foreign taxes against the NIIT. This 3.8% tax may represent an absolute additional cost for higher-income US expats with UK capital gains.
Principal Private Residence Relief: UK vs US Rules
The most common capital gains situation for US expats in the UK involves selling their UK home — whether a former primary residence or a current one. The two countries approach this very differently:
- UK Principal Private Residence (PPR) relief: Full relief from UK CGT for the period the property was your only or main residence. The final 9 months of ownership also qualify regardless of use. With lettings relief removed (in most cases since 2020), only periods of actual occupation qualify for full relief.
- US Section 121 Exclusion: Up to $250,000 of gain ($500,000 for married filing jointly) is excluded from US capital gains tax if you used the property as your principal residence for at least 2 of the 5 years before sale. This is a dollar cap, not a period-based proration.
A US couple selling a UK home that was their principal residence for the entire period of ownership will typically owe no UK CGT (PPR relief) and be eligible for up to $500,000 of US gain exclusion under Section 121 — resulting in little or no tax in either country, subject to the gain amount and specific qualifying period.
However, complications arise if the property was rented for part of the ownership period, if the property was never the US principal residence (only UK principal residence), or if the gain exceeds the US exclusion limit. In these cases, careful calculation of the UK and US tax positions is required.
Currency Gains: The Hidden US Tax on UK Property Sales
Even if you qualify for full UK PPR relief and the US Section 121 exclusion on the property gain, there may be a separate currency gain on the mortgage. This is one of the most frequently overlooked US tax issues for US expats selling UK property:
If you purchased your UK home with a GBP-denominated mortgage and sterling has appreciated against the dollar since you took out the mortgage, the dollar value of your outstanding mortgage has increased. When you repay the mortgage, you are repaying more dollars than you originally borrowed in dollar terms — and that difference is a foreign currency gain taxable in the US under IRC Section 988.
Example: A £300,000 mortgage taken out when GBP/USD was 1.25 ($375,000 in USD terms). When the property is sold and the mortgage repaid, GBP/USD is 1.40 — the mortgage is now worth $420,000 in USD terms. The $45,000 difference is a Section 988 currency gain, taxable as ordinary income in the US in the year the mortgage is repaid. There is no UK equivalent tax on this currency gain.
UK Shares and Investments
US expats selling UK shares or investment fund units face the full UK CGT calculation (above the £3,000 annual exempt amount) plus US capital gains tax. Key considerations:
- UK shares in operating companies are generally not PFICs — so the standard capital gains rules apply
- UK investment trust shares listed on the LSE are not PFICs either (they are corporations, but if the MTM election was not made, the Section 1291 excess distribution regime may apply if they are PFICs)
- The UK “bed and breakfasting” rules (30-day rule, same-day rule, pooling) determine cost basis for UK CGT. The US uses first-in-first-out (FIFO) or specific identification depending on election. The cost basis for US purposes will often differ from the UK pooling cost.
- UK ISA gains are exempt from UK CGT. For US purposes, ISA gains are fully taxable. The divergence creates a situation where a US expat has a US tax liability with no UK tax against which to credit it.
Non-Resident CGT on UK Property
US citizens who are not UK residents but own UK property are subject to UK Non-Resident CGT (NRCGT) on disposals of UK residential property (since April 2015) and commercial property (since April 2019). NRCGT applies only to gains accruing from the relevant April 2015 / April 2019 base date for pre-existing owners. US expats who return to the US but retain UK property must be aware of NRCGT obligations when they sell.
Frequently Asked Questions
If I pay UK CGT on a property sale, do I still owe US tax?
Potentially yes, but UK CGT is creditable against US capital gains tax. In many cases where UK CGT rates match or exceed US capital gains rates, the foreign tax credit fully offsets the US liability. However, the 3.8% NIIT is not directly creditable, so higher-income taxpayers may owe a residual US amount. Each situation depends on the specific UK and US rate comparison and income levels.
My UK property was my main home. Do I owe any US tax on the sale gain?
If the gain is within the Section 121 exclusion ($250,000 single / $500,000 married) and you meet the two-year use requirement, the property gain is excluded from US tax. However, any currency gain on a GBP mortgage may still be taxable as ordinary income. Full PPR relief from UK CGT means there is no UK tax to credit, so the Section 988 currency gain represents a net US cost.
What is the UK CGT reporting deadline for property?
UK residential property disposals must be reported and any CGT paid within 60 days of completion using HMRC’s online CGT reporting service. Commercial property has a 60-day reporting requirement for non-residents; UK residents report on Self Assessment. The US capital gains are reported on Schedule D of Form 1040 for the full tax year in which the disposal occurred.
I inherited UK shares. How are they taxed if I sell them?
For UK CGT, inherited assets receive a base cost “uplift” to the market value at the date of the deceased’s death. For US tax, the US stepped-up basis rules of IRC Section 1014 also apply to inherited foreign assets — the US cost basis is the fair market value in USD at the date of death. If GBP/USD has changed since the date of death, the UK and US cost bases will differ even before the sale.
Expert Cross-Border CGT Planning
Capital gains involving UK assets and US obligations require simultaneous calculation in both tax systems with careful foreign tax credit planning. Whether you are selling a UK home, investment portfolio, or business, our cross-border tax specialists can model both the UK and US outcomes, maximise available reliefs and exclusions, and prepare both the UK CGT return and the US Schedule D correctly.
Contact us today for cross-border CGT planning and compliance advice.