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What does Section 24 actually cost you?

Since 2017, mortgage interest on a UK buy-to-let isn't fully deductible — HMRC restricts it to a 20% basic-rate credit. Most landlord software shows your accounting net and stops there. Enter a few rough numbers and we'll show you the gap between that and what HMRC actually sees.

Annual interest only — not capital repayments

Insurance, repairs, agent fees — exclude mortgage interest

Salary, self-employment, pension etc.

Nothing is stored. Numbers are POSTed once and discarded. Output is an estimate; your accountant reconciles to your return. The arithmetic is the same one HMRC uses.

How much does Section 24 cost a landlord?

"Section 24" is the restriction on finance-cost relief for individual residential landlords. Mortgage interest is no longer deducted from rental profit; instead you get a tax reduction worth 20% of those costs. A basic-rate taxpayer is roughly unaffected, but anyone whose rental profit pushes them into the higher or additional rate pays more — sometimes on profit they did not make. This calculator shows the difference on your own figures.

What this tool checks

  • Tax on your rental profit under the current rules, against the old full-deduction rules
  • How much the restriction costs you each year
  • Whether adding the rental profit moves you into a higher band

Where it applies

Individual landlords across the UK, since it is income tax. It does not apply to limited companies. The calculator uses the income tax bands for England, Wales and Northern Ireland; Scottish taxpayers pay different rates, so their figures will differ.

How it works

Enter the rent you received, the mortgage interest you paid, your other deductible expenses and your other taxable income for the year. The calculator works out your property profit, the 20% basic-rate credit and the tax the restriction adds.

What it can’t tell you

  • It is an estimate for planning, not tax advice. It does not model pension contributions, Gift Aid, the personal allowance taper in detail, or jointly owned property splits.
  • Property income tax rates are due to change from April 2027; the calculator reflects the rates for the current tax year.

Sources

Tekniti keeps the ledger through the year and produces the year-end SA105 figures and exports on every plan.

Tekniti for landlords →

General information, not legal or tax advice. Checked against the sources above in September 2026.