Do I pay US tax on my UK mortgage?

Potentially, when you repay or refinance it. You don't pay US tax simply for having a UK mortgage, but the act of paying it off (including on a property sale or a remortgage) can trigger a Section 988 currency gain if the exchange rate moved in the dollar's favour since you took out the loan. The gain is taxed as ordinary income at your normal rates, separately from any gain on the property itself.

Does the home-sale exclusion cover the mortgage currency gain?

No. The Section 121 main-home exclusion covers gain on the property; it does not cover the Section 988 currency gain on the mortgage. So you can sell a UK home where the property gain is fully excluded, yet still owe US tax on the currency gain from repaying the sterling mortgage. This is exactly why the trap catches people: they assume an exempt sale means no US tax at all.

What if I lose money on the mortgage exchange rate?

Unfortunately, a currency loss on a personal-residence mortgage is generally treated as a non-deductible personal loss — so you can't claim it. This asymmetry is one of the harshest features of Section 988 for homeowners: a currency gain is taxable, but a currency loss on a personal mortgage typically gives no relief. The treatment can differ for mortgages on investment property.

When does a Section 988 gain actually get triggered?

Commonly on three events: selling the property and paying off the mortgage, refinancing or remortgaging (which is treated as repaying the old loan and taking a new one), and otherwise settling the loan. Each repayment or refinancing is a potential recognition event. Because remortgaging is so common in the UK, Americans can trigger this gain without selling anything — simply by switching to a new fixed-rate deal.

The Foreign Mortgage Currency-Gain Trap (Section 988) for Americans in the UK

Q: What is the Section 988 mortgage currency gain?

It's a US tax on the foreign-exchange gain that can arise when you repay or refinance a non-dollar mortgage. Under Internal Revenue Code Section 988, your sterling mortgage is treated as a dollar liability fixed at the exchange rate when you took it out. If the dollar strengthens against the pound by the time you repay, you settle that dollar debt for fewer dollars — and the IRS treats the difference as a gain, taxed as ordinary income. It has no UK equivalent.

Of all the cross-border property surprises, the Section 988 mortgage currency gain is the most counterintuitive. It can tax you on "profit" you never saw, on a property sale that was otherwise tax-free, simply because the pound moved against the dollar while you held a UK mortgage. Worse, it can be triggered by an ordinary remortgage — no sale required. This guide explains exactly how it works and why it matters for every American with a sterling mortgage.

The short answer

Repaying or refinancing a sterling mortgage can create a separate US tax bill under Section 988 — taxed as ordinary income — even if the underlying property sale is tax-free. US rules treat your UK mortgage as a dollar liability fixed at the exchange rate when you took it out. If the dollar has strengthened against the pound by the time you repay, you settle that dollar debt for fewer dollars, and the IRS taxes the difference as a gain. It has no UK equivalent, the home-sale exclusion doesn't cover it, and a currency loss on a personal mortgage generally gives no deduction.

Key takeaways

Section 988 can tax the currency gain when you repay or refinance a sterling mortgage.
The gain is taxed as ordinary income, separately from any gain on the property.
The Section 121 home-sale exclusion does NOT cover the mortgage currency gain.
A currency loss on a personal-residence mortgage is generally non-deductible.
Remortgaging — not just selling — can trigger it, so it catches people unexpectedly.

Executive summary

Section 988 of the Internal Revenue Code governs how the US taxes foreign-currency transactions, and it treats a foreign-currency debt as a position that can produce gain or loss. For a US citizen with a UK mortgage, the loan's dollar value is fixed when taken out; when repaid, it's re-measured at the current rate. If the dollar strengthened (fewer dollars needed to settle the same sterling debt), the saving is a taxable gain — as ordinary income. The brutal features are three: the home-sale exclusion doesn't shelter it, a loss is usually non-deductible on a personal residence, and refinancing counts as repayment, so an ordinary UK remortgage can trigger it. Because none of this exists in UK tax, there's no UK tax to credit against it — it's a pure US cost that only cross-border awareness catches in advance.

How the gain arises

The mechanics are easier with the standard worked logic (illustrative, not advice):

You take out a £300,000 mortgage when the rate is $1.30/£ — a dollar liability of $390,000.
Years later you repay it when the rate is $1.20/£ — you now need only $360,000 to settle the same £300,000.
You've settled a $390,000 dollar liability for $360,000, so the IRS sees a $30,000 gain — taxed as ordinary income at your normal rates.

The "gain" is purely the exchange-rate movement on the debt. You may not feel any richer — you still repaid £300,000 — but in dollar terms, the debt got cheaper, and US tax follows the dollar.

Why it surprises people: three traps

1. The home-sale exclusion doesn't cover it. Many Americans sell a UK home, apply the Section 121 exclusion, and assume they're done. But §121 covers the property gain, not the mortgage currency gain. The two are separate calculations, and the currency gain can survive a fully-excluded sale.

2. A loss gives no relief. If the pound strengthened instead (you needed more dollars to repay), you have a currency loss — but on a personal-residence mortgage, that loss is generally a non-deductible personal expense. So the rule is one-directional against you: gains taxed, losses ignored. (Investment-property mortgages can be treated differently.)

3. Refinancing counts. A remortgage — switching to a new fixed-rate deal, very common in the UK — is treated for US purposes as repaying the old loan and taking out a new one. That repayment is a recognition event, so you can trigger a §988 gain without selling anything. An American who remortgages every few years could realise these gains repeatedly, unaware.

There's no UK offset

Because the UK has no equivalent to this currency-gain rule, there's no UK tax on the same event to credit against the US charge. Unlike most cross-border property tax — where foreign tax credits or the treaty soften the overlap — the §988 gain stands alone as a pure US cost. That's what makes it so important to anticipate: there's no relief mechanism waiting to absorb it.

How it's managed

You can't change exchange rates, but you can plan around the recognition events:

Know your dollar baseline — record the exchange rate (and dollar value) when you take out or refinance any UK mortgage, so the gain can be computed accurately rather than over-estimated.
Factor §988 into sale and remortgage timing — it's part of the true cost of both, alongside the property gain.
Distinguish personal vs investment property, since the loss treatment differs.
Coordinate with the property sale so the currency gain and any property gain are reported correctly together.

This is genuinely technical, and the calculations depend on accurate historical exchange data — which is why it's an area to model rather than estimate.

Common mistakes we see

Assuming a tax-free property sale means no US tax — the currency gain is separate.
Triggering gains by remortgaging without realising it's a recognition event.
Claiming a currency loss on a personal mortgage that isn't deductible.
Not recording the original dollar value of the mortgage, making the gain hard to compute.
Forgetting there's no UK tax to credit against this US-only charge.