SA105 Calculator 2025–26: Fill In Your UK Property Pages Box by Box
Enter your rent and costs; get every UK property box of your 2025–26 tax return, the Section 24 credit and the tax to pay by 31 January 2027. Free, private, no sign-up.
Your UK property pages
Estimate from HMRC’s own working sheet. It does not file anything. Not advice.
Your SA105 boxes
| Box | What it is | Enter |
|---|---|---|
| Enter your rent above. | ||
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This SA105 calculator fills in the UK property pages of your Self Assessment return for you. Enter the year’s rent and costs and it returns the figure for every box HMRC asks about — Box 20 income, Boxes 24–29 expenses, Box 38 profit or Box 41 loss, Box 40 taxable profit, the losses to carry in Box 43, and the Box 44 mortgage interest that earns the 20% Section 24 credit. It also works out the extra tax your rent adds, whether you face payments on account, and what to pay by 31 January. Landlord software and filing services charge for this; here it is free and nothing is stored — the maths runs in your browser.
How the calculator fills each box
It follows HMRC’s own working sheet from the 2025–26 UK property notes. Box 38 (or Box 41 for a loss) = Boxes 20 + 22 + 23 + 30 + 31, minus Boxes 24–29, 32–35, 36, 37 and 20.1. Brought-forward losses go in Box 39, capped at Box 38, and Box 40 is Box 38 minus Box 39. Anything unused rolls into Box 43 for next year. Residential mortgage interest never goes in the expense boxes: it goes in Box 44, and HMRC turns it into a tax reduction instead. Premiums (Box 22), reverse premiums (Box 23), balancing charges (Box 31), capital allowances (Boxes 32–35) and Rent a Room (Box 37) are rare for ordinary buy-to-lets and are left out — if you have them, add them on the form yourself or ask an accountant.
The Section 24 credit, worked out the HMRC way
The credit is 20% of the lowest of three numbers: your finance costs (Box 44 plus any Box 45 carried forward), your property profit after losses (Box 40), and your adjusted total income above the personal allowance. It cannot create a refund. If the credit was capped by profit or income, the difference carries forward and goes in next year’s Box 45 — the calculator tells you that figure.
Actual expenses or the £1,000 property allowance?
You can deduct up to £1,000 in Box 20.1 instead of every expense — but HMRC’s notes are explicit that you then “cannot deduct any allowable expenses or claim any other allowances”, and you lose the Section 24 credit too. With “Pick the cheaper for me” selected, the calculator runs both routes and uses the one with less tax. Gross property income of £1,000 or less is covered by the allowance and does not need reporting at all.
A worked example
An employee on £45,000 in England lets one flat for £14,400 a year. Costs: £900 insurance and ground rent (Box 24), £1,200 repairs (Box 25), £1,440 agent fees (Box 27), £300 certificates (Box 29), and £6,000 of mortgage interest (Box 44).
- Box 38 adjusted profit: £14,400 − £3,840 = £10,560 (Box 40 is the same, no losses).
- Income tax on salary alone: £6,486. With the rent added: £9,656.
- Section 24 credit: 20% × the lowest of £6,000, £10,560 and income over the allowance = £1,200.
- Extra tax because of the rent: £1,970. PAYE covered 77% of the year’s tax, so payments on account apply: £985 each, the first due with the bill on 31 January and the second on 31 July.
Deadlines and how to pay
For the 2025–26 tax year, paper returns are due by 31 October 2026 and online returns by 31 January 2027, when the tax is also due. If you already pay tax through PAYE, owe less than £3,000 and file online by 30 December 2026, HMRC will normally collect the bill through your tax code instead of a lump sum. Payments on account — two advance halves of next year’s bill — apply unless the bill is under £1,000 or more than 80% of your tax was already taken at source. Landlords mandated into Making Tax Digital from April 2026 (qualifying income over £50,000) report 2026–27 through MTD software instead of the SA105 — check whether that is you.
Sources, read 30 September 2026: HMRC SA105 2026 form and UK property notes (2025–26, gov.uk, last updated 6 April 2026); gov.uk “Changes to tax relief for residential landlords: how it’s worked out”; gov.uk Income Tax in Scotland 2025 to 2026; gov.uk Self Assessment payments on account and paying through your tax code. 2026–27 rates from our verified dataset, checked 2026-08-29. General information, not advice — the return is yours to check and submit.
Asked constantly
Yes. It runs in your browser, stores nothing and needs no sign-up. It does not submit anything to HMRC — you copy the box figures into your online return or the paper SA105.
Box 44, residential property finance costs — never Boxes 24–29 or Box 26. HMRC gives a 20% tax reduction from Box 44 instead of deducting the interest from profit.
Box 20.1 on the 2026 SA105 form. Box 21 is for tax already taken off your rent, which is usually blank. If you use the allowance you cannot claim any expenses or the Section 24 credit.
Each owner files their own SA105 with their share. Married couples and civil partners are taxed 50/50 by default unless they hold unequal beneficial shares and send Form 17. Enter whole-property figures and your share percentage and the calculator scales every box.
The shortfall is a loss: it goes in Box 41 and carries forward in Box 43 against future property profits. It cannot normally be set against your salary. The calculator shows the Box 43 figure to carry to next year.
HMRC asks for two advance payments towards next year's bill (31 January and 31 July), each half of this year's Self Assessment tax, unless that tax is under £1,000 or more than 80% of your total tax was already paid at source. In a first year of letting this can make January's bill about 1.5 times the tax on the rent.
For the 2025–26 return (due 31 January 2027), yes. From 2026–27, landlords and sole traders with qualifying income over £50,000 report quarterly through MTD software instead; everyone else keeps filing the SA105.
Doing this every quarter, not just once?
Keep the same figures all year instead of re-typing them: rent and expenses per property, the Section 24 finance costs and the MTD quarterly totals, in one workbook.
Landlord Tax Lab stays free. We also sell the Landlord Tax Pack: one Excel workbook for up to 10 properties that logs rent and expenses per property, maps every line to its SA105 box, keeps Section 24 finance costs out of the expense boxes, and rolls up the MTD quarterly totals. £19 once — no subscription.
Disclosure: we make and sell the Tax Pack — this is our own product, not a paid recommendation. It does not file anything with HMRC and it is not tax advice; filing goes through you, your accountant or software on HMRC’s compatible-software list.