Paid off your British mortgage? America may see a profit you never made

Figures relate to tax year 2025-26 (UK) · 2025 (US)

You borrowed £400,000 in 2014 and cleared the balance in 2026. The bank statement shows pounds in and pounds out. From where you sit, no profit happened anywhere.

America keeps score in dollars, and in dollars something did happen. The loan you took out was worth more than the loan you repaid, and that difference can be taxable income. Most people meet the mortgage gain for the first time in a letter.

Key takeaways

• America treats a sterling mortgage as an asset separate from the house.
• Repaying the loan when the dollar has strengthened can produce taxable income.
• The gain counts as ordinary income, so capital gains rates do not apply.
• A loss on a personal home loan usually gives you no deduction at all.
• Remortgaging can trigger the calculation even though no house changed hands.
• Britain does not tax this, so there is rarely a foreign tax credit to claim.

What is a currency gain on a mortgage?

American tax law looks at anything denominated in a foreign currency as its own transaction. A debt in pounds qualifies, because the amount owed shifts in dollar terms every day.

So your house and your loan travel on separate tracks. The house produces a capital gain. The loan produces a currency result, and the two never meet on the return.

That separation is the whole reason this surprises people. Sellers expect one calculation and get two.

What triggers the calculation?

A repayment triggers it. So does a refinance that replaces the old loan with a new one, because America treats that as settling the original debt. Selling the property and clearing the mortgage from the proceeds triggers it too. The trigger is the discharge of the debt, not the sale of the bricks.

Simply holding the loan changes nothing. Until you repay or replace it, there is nothing to report.

How the mortgage gain is worked out

Take the sterling principal and convert it at the exchange rate on the day you drew the loan down. That gives your dollar borrowing.

Then take the sterling principal you repaid and convert it at the rate on the day of repayment. That gives your dollar repayment.

Repay fewer dollars than you borrowed and the difference is a mortgage gain. Repay more and you have a loss, which usually goes nowhere.

A worked example

The figures below are illustrative and use round numbers to show the mechanics.

James drew a £400,000 mortgage in 2014, when the pound stood near $1.65. In dollars he borrowed about $660,000.

He sold the house in 2026 and cleared a remaining balance of £320,000 with the pound near $1.30. In dollars he repaid roughly $416,000.

The repaid slice of the original loan was worth about $528,000 when he borrowed it. So his mortgage gain is around $112,000, taxed as ordinary income.

James never saw that money. It exists only because the currency moved between the two dates.

Why the gain counts as ordinary income

Currency results on debt fall outside the capital gains rules. They sit with interest and wages instead, which means your marginal rate applies rather than the lower long term rate.

That single point changes the size of the bill considerably. A six figure currency result at ordinary rates costs far more than the same figure taxed as a capital gain.

It also means no holding period helps you. Twelve years of ownership buys you nothing here.

What happens to a loss?

This is where the rules feel unfair. If the pound strengthened and you repaid more dollars than you borrowed, you have an economic loss. But a loss on borrowing for a personal residence is a personal loss, and personal losses generally give no deduction. So the outcome runs one way only.

Loans connected to a rental business can sit differently. Where the borrowing supported a let property, a loss may become usable, and that is worth checking carefully.

Does remortgaging count?

Often yes. Moving to a new lender settles the old debt and creates a new one, which looks like a repayment for American purposes. Staying with the same lender on a new product can also count, depending on how much the terms change. A different rate, a different term and a different principal together look like a new loan.

So a routine two yearly remortgage can quietly crystallise a mortgage gain several times across a single ownership period.

What about overpayments?

Each principal repayment is technically its own settlement of part of the debt. A large lump sum overpayment therefore produces a result in the year you make it. Scheduled capital repayments inside a monthly instalment do the same thing on a much smaller scale. In practice most advisers track the position at the big events and keep a schedule for the rest.

Either way, keep the paperwork. Reconstructing twelve years of amortisation after the fact takes far longer than saving the statements.

Which events produce a result

Does the main home exclusion help?

No. The exclusion for a principal residence applies to the gain on the property, and this result belongs to the loan. So a seller can walk away with no taxable gain on the house and a large charge on the borrowing. The two calculations simply do not talk to each other.

Our guide to selling a UK home on a US return covers the property side of the same transaction.

Is there any credit for UK tax?

Rarely. Britain does not tax the repayment of a loan, so no British tax arises and there is nothing to credit. That leaves the American charge standing alone. Per IRS guidance, foreign currency transactions are measured against the dollar as your functional currency, whatever your day to day life looks like.

Where a British charge does arise on a related disposal, our guide to the foreign tax credit explains the claim.

What about a buy to let mortgage?

The same mechanics apply, but the setting differs. Borrowing attached to a rental business sits inside a trade or business, which changes how a loss behaves. It also interacts with the rental figures you already report. If you hold let property, read this alongside our guide to UK rental income on a US return.

Landlords with several properties should keep a loan schedule per property. Mixing them makes the arithmetic almost impossible to defend later.

What records do you need?

Six documents cover almost every case, and most of them already sit in a drawer somewhere. Gather them once and the mortgage gain calculation takes an hour rather than a week. Missing paperwork is the single biggest reason this work drags on.

1. The mortgage offer showing the principal and the drawdown date.
2. The exchange rate published for that drawdown date.
3. Annual statements showing capital repaid in each year.
4. Redemption statements for every remortgage or product switch.
5. The final redemption figure and the date the lender received it.
6. A note of which property each loan supported, if you own more than one.

What if the pound went the other way?

Then you have a loss, and on a personal home loan it usually disappears. Nothing carries forward and nothing offsets the gain on the house. That asymmetry catches people who borrowed when the pound was weak. They expect the loss to soften a later gain, and it does not.

Knowing the direction early still helps. If the position is a loss, you can stop worrying and file without the extra schedule.

Does it apply to a non-American spouse?

Only to the American side of the ownership. Where a British spouse holds half the property and half the loan, their half sits outside the American system entirely. So the split on the title deed matters more than most couples realise. It can halve the exposure before any planning happens.

Filing jointly changes that, because a joint election pulls the spouse into the American net. Weigh the two positions before you elect.

When does planning actually help?

Before the event, not after. Timing a remortgage, choosing which loan to clear first, and deciding how much to overpay all move the number. Where a sale is coming, we look at the property gain and the mortgage gain together. Sometimes the cheapest route is to repay in a year when other income sits low.

Once the redemption has gone through, the figure is fixed. After that the only work left is reporting it correctly.

What if you borrowed before you became American?

Green card holders meet this often. You took the loan out as a British borrower, years before any American obligation existed, and the drawdown date still sets your starting point.

America does not reset the clock when you join the system. The mortgage gain runs from the original drawdown, not from the day your green card arrived.

So dig out the 2009 mortgage offer. That single document can be worth tens of thousands of dollars in the final figure.

How does it affect the rest of your return?

Ordinary income pushes your total upward, so it can drag other numbers with it. Credits that taper at higher income can shrink, and a bigger figure can lift you into a higher bracket.

It also feeds the calculation for estimated payments. A large mortgage gain in one year often creates an underpayment charge in the same year, unless somebody plans for it.

Our guide to estimated tax payments from the UK explains how to keep that charge away.

Is there a small amount you can ignore?

There is a narrow exception for personal transactions in foreign currency, and it stops at $200 of gain. A mortgage on a family home runs far beyond that. So the exception helps with holiday spending and little else. Nobody clears a property loan inside a $200 window.

Joint borrowers should also check the split before they file. Where two people share a loan, each reports their own share of the result, and a spouse outside the American system reports nothing at all.

Mistakes and penalties we see with mortgage gains

The most common mistake is omission. The return shows the house and ignores the loan, and nobody notices until a review three years later.

The second is using one exchange rate for everything. Averaging the year hides the movement that creates the charge and produces a figure nobody can support.

The third is claiming a personal loss. That claim invites a question and rarely survives it.

Where an earlier year missed the gain, an amended return usually fixes it inside the normal window. Our guide to amending a US tax return sets out the steps and the deadlines.

How US UK Tax Accountants helps

We rebuild the loan history from drawdown to redemption, price each movement on its own date, and separate the property result from the currency result. Then we show you both numbers in one place.

Where earlier returns missed it, we check whether an amendment helps or hurts before filing anything. Our US federal return service covers the work itself.

If you are about to remortgage or sell, get in touch first. The date you pick changes the figure, and only one of those dates is still in your hands.

Last reviewed 24 September 2026. This article is general information and not personal tax advice. Every situation turns on its own facts, so take advice on yours before acting.