Mortgage Tax Calculator UK 2025/26

Reviewed by Mustafa Bilgic
UK Calculator Editor · Updated May 2026

How a UK mortgage interacts with three different taxes

A typical buy-to-let landlord encounters three tax events in the life of a mortgaged property: Stamp Duty Land Tax at purchase, income tax on rent received during ownership, and Capital Gains Tax (or Corporation Tax inside a company) at sale. The mortgage itself is not directly taxable, but the interest paid affects the income tax liability via the Section 24 restriction introduced from April 2017 and fully phased in by April 2020.

For owner-occupiers, the picture is simpler: SDLT at purchase, no income tax during ownership, and Private Residence Relief eliminating CGT at sale. Only when an owner-occupier converts to a let (or vice versa) does the picture become more complex — that is when this calculator is most useful.

Section 24 in detail

Before April 2017 a UK landlord deducted mortgage interest from gross rents to compute taxable rental profit, paying tax on the net figure at their marginal rate. Section 24 of the Finance (No. 2) Act 2015 reversed this. The interest is no longer a deductible expense; instead, HMRC awards a 20% basic-rate tax credit applied after income tax is computed.

The practical effect: a higher-rate landlord lost half the value of their interest relief. A landlord with rents nudging income above the £100,000 personal allowance taper threshold can lose even more, because the gross rental income (before interest) rather than the net is what counts for taper purposes.

Basic-rate landlord

Rent £18,000, expenses £3,000, interest £6,000, PAYE £35,000. Profit £15,000. Total income £50,000 (still basic rate). Tax on profit at 20% = £3,000. Credit at 20% × £6,000 = £1,200. Net rental tax: £1,800.

Higher-rate landlord

Rent £24,000, expenses £4,000, interest £9,000, PAYE £55,000. Profit £20,000. Total income £75,000 (40% band on rental). Tax on profit at 40% = £8,000. Credit at 20% × £9,000 = £1,800. Net rental tax: £6,200.

Personal allowance taper case

Rent £40,000, expenses £6,000, interest £18,000, PAYE £88,000. Profit £34,000 pushes total income to £122,000 — within the £100k–£125,140 PA taper. Marginal effective rate on the rent slice approaches 60%. Calculator output shows the actual cliff-edge.

Common mistakes to avoid

• Treating mortgage interest as expense and then claiming the credit too — double-counting flagged by HMRC software.
• Forgetting that the 20% credit caps at adjusted income above PA; a low-earner landlord may waste relief.
• Including the capital element of the mortgage payment as interest.
• Applying the credit before income tax (it's a tax reducer applied after).
• Missing carry-forward of unused interest (Box 45 SA105 next year).

When to use this calculator

Use this calculator before deciding whether to remortgage to a higher rate, before incorporating into a limited company, and at year-end to project your January tax bill.

How this differs in Scotland, Wales and Northern Ireland

Section 24 is a UK-wide rule but Scottish landlords pay tax at Scotland's bands (19/20/21/42/45/48%), worsening the squeeze for higher-rate Scots. Welsh and NI landlords use the same rates as England.

When this calculator is and isn't the right tool

The Mortgage Tax Calculator UK 2025/26 above is built for the most common UK 2025/26 scenarios in this tax area. It will be the right tool when your situation maps cleanly onto the inputs — single property or simple aggregation, standard HMRC rates and bands, and individual taxpayer (rather than complex trust or partnership structures). It is informational and does not replace tailored advice from a chartered tax adviser, especially for transactions above £500,000, cross-border situations, or where reliefs interact with each other. For year-end filings, always reconcile with HMRC's own free calculators on gov.uk before pressing submit on Self Assessment.

Closely related calculators

• Mortgage Tax Return Calculator — useful when you need a different angle on the same UK property tax topic, especially if your situation involves multiple types of property income or disposal.
• Section 24 Mortgage Interest Calculator — useful when you need a different angle on the same UK property tax topic, especially if your situation involves multiple types of property income or disposal.
• UK Landlord Tax Calculator — useful when you need a different angle on the same UK property tax topic, especially if your situation involves multiple types of property income or disposal.
• Rental Income Tax Calculator — useful when you need a different angle on the same UK property tax topic, especially if your situation involves multiple types of property income or disposal.
• CGT on Property Calculator — useful when you need a different angle on the same UK property tax topic, especially if your situation involves multiple types of property income or disposal.

Glossary of UK property tax terms

Section 24

The 2017 rule replacing full mortgage interest deduction with a 20% tax credit for individual residential landlords.

SA105

The Self Assessment supplementary form for UK property income.

Box 44

The SA105 box for residential property finance costs (mortgage interest).

Property allowance

£1,000 tax-free band for individual property income, claimable instead of expenses.

Rent-a-Room scheme

Separate £7,500 allowance for letting a furnished room in your main home.

Tax planning checklist

1. Confirm the figures input above match your actual position — purchase contract, mortgage offer or completion statement.
2. Cross-check the year (2025/26) — figures change every April. The tax year 2026/27 starts 6 April 2026.
3. Use HMRC's official calculator at gov.uk for the final filing figure; this calculator is informational.
4. Keep records for at least 6 years — HMRC's normal enquiry window. 21 years for fraud investigations.
5. Discuss any unusual transaction (joint purchase, gift, divorce settlement, trust) with a qualified tax adviser.
6. Submit your return online via Government Gateway — paper deadlines are earlier and penalties harsher.