Section 988: The Hidden US Tax Trap on Your UK Mortgage

You can sell your UK home at a genuine loss and still owe US tax — because of your mortgage, not the property. Here’s the currency-gain rule almost no American buyer knows about until it’s too late to plan around.

You Can Sell Your UK Home at a Loss and Still Owe US Tax

Of everything that catches American owners of UK property off guard on their US tax return, this is consistently the one specialist accountants say surprises people most. It has nothing to do with whether your property gained or lost value. It has nothing to do with rental income. It's about your mortgage — specifically, what happens to the loan balance when it's denominated in a currency other than dollars.

The mechanism is called Section 988 of the US tax code, and understanding it before you take out a UK mortgage, not after you pay it off, is genuinely worth the effort.

Why a Mortgage Counts as a Separate Transaction at All

This feels unintuitive to most Americans, because a US mortgage is never treated as its own taxable event — you borrow dollars, you repay dollars, there's nothing to measure a "gain" against. A foreign-currency loan is different under US tax rules specifically because the value of the debt itself, measured in dollars, moves independently of the property.

Here's the mechanic: when you borrow £500,000 to buy your UK home, that loan has a dollar value on the day you take it out, based on the exchange rate that day. When you eventually repay or refinance that loan, the same £500,000 principal has a different dollar value if the exchange rate has moved. If the pound has weakened against the dollar in the meantime — meaning it now takes fewer dollars to buy the same number of pounds — the IRS treats this as though you effectively repaid the loan more cheaply than you borrowed it, in dollar terms. That difference is your Section 988 gain, and it's taxable.

A Worked Example

Say you take out a £500,000 mortgage when GBP/USD sits at 1.30, meaning the loan is worth roughly $650,000 in dollar terms on day one. Some years later, you refinance or repay the mortgage in full, and by that point GBP/USD has fallen to 1.20 — the pound has weakened against the dollar. The same £500,000 principal is now worth roughly $600,000.

In dollar terms, you effectively extinguished a $650,000 obligation using only $600,000 worth of currency. That $50,000 difference is treated as a Section 988 gain — reportable, and taxed as ordinary income, which is typically a materially worse tax rate than long-term capital gains. This happens regardless of what the property itself did — it could have appreciated, stayed flat, or genuinely lost value, and the currency gain on the mortgage is calculated completely independently.

The Trap Within the Trap: This Isn't Sheltered by the Home Sale Exclusion

Most Americans who've owned a primary residence are at least vaguely aware of the Section 121 exclusion — the rule that shelters up to $250,000 (or $500,000 for a married couple filing jointly) of genuine capital gain on the sale of a main home from US tax. The natural assumption is that this same protection covers everything related to selling your UK home, including any currency effects.

It doesn't. Section 121 applies to the gain on the property itself, calculated as sale price minus purchase price minus qualifying improvements. The Section 988 currency gain on your mortgage is a legally and mechanically separate calculation, and the home-sale exclusion has no bearing on it whatsoever. You can fully shelter a genuine property gain under Section 121 and still owe real tax on the mortgage currency component in the same tax year, on the same transaction, from the same buyer's perspective feeling like one single event.

When This Actually Gets Triggered

Section 988 gain or loss on a mortgage is generally recognised at the point the loan is repaid or otherwise settled — not while you're simply making ongoing monthly payments on an outstanding balance. The events that typically trigger it:

• Selling the property and repaying the mortgage in full as part of completion.
• Refinancing into a new mortgage, which counts as repaying the old loan even though you're borrowing again immediately afterward.
• Making a substantial early repayment or overpayment large enough to be treated as a partial settlement of the debt, depending on the specifics.

This is exactly why the mortgage rate environment matters here in a way most people wouldn't expect — refinancing to chase a better rate isn't just a UK interest-rate decision, it's also a US taxable event determined by currency movement since your original loan, independent of anything happening with rates themselves.

Could This Ever Work in Your Favour?

Yes — the mechanism cuts both ways. If the dollar weakens against the pound between taking out the mortgage and repaying it, the same calculation produces a Section 988 loss rather than a gain, which can potentially offset other ordinary income on your US return, subject to the normal rules and limitations around foreign currency losses. Nobody can control which direction exchange rates move over a multi-year mortgage term, but it's worth knowing the mechanism isn't inherently punitive — it's neutral, and currency movement has simply gone against most American mortgage-holders over sustained periods in recent years, which is why the gain scenario is the one specialist accountants discuss most.

What This Means for How You Should Plan

• Track your mortgage's original dollar value at inception, using the exchange rate on the exact date you drew down the loan — this is your baseline for any future Section 988 calculation, and it's far easier to establish this contemporaneously than to reconstruct it years later.
• Factor this into refinancing decisions, not just UK rate comparisons — a refinance that looks purely beneficial on the UK side could trigger a US tax bill the rate saving doesn't fully offset, depending on currency movement since your original mortgage.
• Don't assume Section 121 covers everything when budgeting for the US tax consequences of selling — model the mortgage currency component as a genuinely separate calculation from the property gain itself.
• Loop in a cross-border accountant before you refinance or sell, not after — this is exactly the kind of calculation where after-the-fact planning options are far more limited than before-the-fact ones.

How This Fits With the Rest of Your US Reporting

Section 988 is a separate mechanism from FBAR and FATCA (Form 8938) reporting — the property itself isn't a reportable financial account under either regime, but rental income, capital gains on sale, and this specific mortgage currency gain all interact with your broader US filing picture. See our UK inheritance tax guide and ownership structure guide for how this sits alongside the other cross-border tax questions a UK property purchase raises — these genuinely need to be modelled together with a specialist, not one at a time.

Frequently Asked Questions