Mortgage interest under MTD: the box you put it in decides your tax bill

Do you report mortgage interest quarterly, or not?

You do. HMRC's Making Tax Digital Update Notice sets out the quarterly reporting categories for a UK property business, and two of them are Residential property finance costs and Residential finance costs brought forward.

This matters because a large amount of published advice says the opposite. You will read that mortgage interest is “not reported quarterly”, that it is “a year-end adjustment only”, that it is “handled at the final declaration rather than in quarterly updates”. Vendor guides say it. Accountancy blogs say it. It is repeated confidently enough that you would be forgiven for believing it.

Nobody is lying. They are collapsing three separate things into one: reporting the cost, HMRC excluding it from your profit, and the 20% reducer being computed. All three happen. Only the last is a year-end event. The first is quarterly, mandatory, and the one that goes wrong.

Why the category you choose is the whole game

A UK property expense breakdown has a field called financialCosts and a separate field called residentialFinancialCost. They look like near duplicates. They are not. HMRC's published calculation logic sums your allowable expenses from a specific list, and residential finance costs are deliberately not on it. They are pulled out and handled under allowances and reliefs instead.

So the same £10,000 of mortgage interest produces two completely different tax outcomes depending on which box it lands in.

This is a quiet failure. Miscode the interest and your submission is still perfectly valid. HMRC accepts it. Your profit comes out too low, your relief comes out at zero, and the numbers look reasonable enough that nobody notices. We know of no validation rule that catches it, though we cannot prove none exists, and we would not want to bet a tax return on the difference.

How much relief do you actually get?

Not 20% of your mortgage interest. That is the shorthand everyone uses and it is only true when nothing else bites. HMRC applies the basic rate to the lowest of three figures (PIM2058):

Anything you cannot relieve this year is carried forward with no time limit, but only within the same property business. That caveat matters more than it looks. If the business ceases, if you sell up, any unrelieved balance is not banked. It is lost.

So Section 24 usually defers the relief rather than destroying it, which is a cash-flow problem rather than a tax problem, and a great deal less comforting than it sounds when you are the one funding the gap. But it is only a deferral for as long as you keep letting.

If you have more than one property business, HMRC apportions the income cap between them using a formula in PIM2058. The total relief comes out the same.

Why your tax bill rises even though your profit did not

Because your mortgage interest is no longer deducted, HMRC's idea of your property profit is larger than your actual profit, and that larger number feeds into your total income. GOV.UK has a worked example showing exactly this. It is also, we discovered, a decade out of date, and almost every article that quotes it repeats figures that no longer hold.

The example: a landlord with £35,000 of self-employment income, £18,000 of rent, £2,000 of other expenses and £8,000 of mortgage interest. GOV.UK computes his tax rising from £6,400 to £8,000, a £1,600 cost. Those numbers are correct. They are also calculated on 2016-17 thresholds: a personal allowance of £11,000 and a higher-rate threshold of £43,000.

Run the same landlord through 2026-27 thresholds, with a £12,570 allowance and a higher-rate threshold of £50,270, and the picture changes completely.

He still crosses into the higher-rate band, so the mechanism GOV.UK is illustrating is real and unchanged. But the higher-rate threshold has moved from £43,000 to £50,270, so only £730 of his income now sits above it rather than £8,000. The headline cost falls from £1,600 to roughly £146.

Do not read that as good news. It is a statement about this particular landlord, who is barely leveraged. Section 24 scales with borrowing: the more of your rent goes to the lender, the more you are taxed on income you never keep. A landlord with £18,000 of rent and £14,000 of interest is in a very different position from one with £8,000, and no amount of threshold drift rescues him.

GOV.UK's page also warns the example could drag him into the High Income Child Benefit Charge, because his total income exceeds £50,000. That warning is now dead too: the charge starts at £60,000 from 2024-25, so his £51,000 triggers none of it. The mechanism is unchanged; the threshold moved. This is what happens when guidance is quoted rather than checked.

Can you trust the in-year tax estimate your software shows you?

Treat it as a rough indication, and here is the honest reasoning rather than a confident answer.

HMRC's published calculation logic excludes residential finance costs from expenses and runs the reducer on the data you submit quarterly, so an in-year estimate should handle Section 24 correctly by design. We have not found any HMRC statement confirming that in terms, and we are not going to assert it. What we can say is that the estimate is unreliable for a more fundamental reason.

Two of the three caps are whole-year tests. Your property profit and your adjusted total income are both incomplete in July. Your employment income, dividends and savings may not have reached HMRC at all yet. Applying a whole-year cap to a part-year data set produces a number that is arithmetically valid and economically meaningless. HMRC evidently agrees, because its own minimum standards require your software to show you a disclaimer saying the calculation is based only on what it has received so far.

What we could not verify. Whether HMRC's in-year calculation applies the reducer in practice, as opposed to in design. We intend to test it in the HMRC sandbox and publish what comes back, because as far as we can tell nobody has. If you have done it, we would like to hear from you.

A naming mess worth knowing about

The same figure has four names, which is its own small explanation of why this goes wrong so often. Your unrelieved interest from earlier years is called:

• Residential finance costs brought forward in HMRC's Update Notice
• residentialFinancialCostsCarriedForward in the Property Business API, which says carried where the notice says brought
• residentialFinanceCostCarriedForward in HMRC's calculation logic, which drops the -ial from Financial, so the API and the engine spell the same word differently
• Box 45 on the SA105 (box 44 is the current year)

They are the same number. It is the unrelieved amount coming into this year from earlier ones, added to your current interest before the caps are applied.

Two changes that catch people out

Common questions

Sources

Primary sources. Where the market disagrees with HMRC, we have cited HMRC.

• HMRC Property Income Manual PIM2054 (who is caught and which loans are in scope, updated 21 May 2026)
• HMRC Property Income Manual PIM2058 (the calculation, the caps and the carry-forward)
• GOV.UK: Making Tax Digital for Income Tax Update Notice (27 March 2026, the quarterly category list)
• GOV.UK: Tax relief for residential landlords, how it is worked out (the worked examples)
• HMRC Tax Logic service guide: allowances and reliefs (residential finance costs excluded from allowable expenses)
• GOV.UK: abolition of the furnished holiday lettings regime (from 6 April 2025)

Related

Why your second MTD update looks twice as big as your first explains the cumulative model that puts this figure in front of you four times a year.