SA105 Self Assessment: The Landlord’s Box-by-Box Guide (2026/27)
Last updated 23 June 2026 · 12 min read · By the LandlordTaxAi Editorial Team
The short answer
The SA105 is the UK property page of your tax return. Put your total rent in Box 20, your running costs in Boxes 24–29, and — crucially — your residential mortgage interest in Box 44, not the expenses boxes, because it only gets a 20% credit. If your rent is under £1,000 you usually do not need it at all.
Every January, thousands of landlords stare at the SA105 wondering which figure goes where — and one wrong box can quietly inflate or deflate their tax. This is the plain-English, box-by-box walkthrough, with the one mistake almost everyone makes flagged clearly. For the wider return, see our landlord tax return guide; for the costs you can claim, our allowable expenses list.
The boxes that matter
| Box | What goes in it |
|---|---|
| 20 | Total rents and other income from property |
| 24 | Rent, rates, insurance and ground rents |
| 25 | Property repairs and maintenance |
| 26 | Non-residential loan interest and other financial costs |
| 27 | Legal, management and other professional fees |
| 28 | Costs of services provided, including wages |
| 29 | Other allowable property expenses |
| 44 | Residential property finance costs (your buy-to-let mortgage interest) |
| 45 | Unused residential finance costs brought forward |
The Box 44 trap — read this twice
Do not put residential mortgage interest in Boxes 24–29. Since Section 24, residential finance costs are not a normal deductible expense — they belong in Box 44, where HMRC applies a flat 20% tax-reducer instead. Put it in the expenses boxes and you deduct it in full, which is wrong and a classic trigger for an HMRC correction. Box 26 is only for non-residential finance costs.
Free calculator · no sign-up
Rental profit & tax calculator
Estimate the tax on your rental income for 2026/27
Result
- Taxable profit (rent − expenses)
- £11,200
- Income Tax at 40%
- £4,480
- Less mortgage interest credit (20%)
- − £1,000
- Tax due on this property
- £3,480
- Income after tax
- £7,720
Estimate based on verified 2026/27 UK rates. Informational only — not personal tax advice.
Let the boxes fill themselves
LandlordTaxAi reads your bank statements, sorts every transaction into the right SA105 box — including Box 44 — and flags anything that looks off.
See how it worksHow to fill it in, step by step
- 1
Enter your total rental income in Box 20
Add up all rent and other income received from your UK property in the tax year and put the total in Box 20. Use the rent due in the year if you use the accruals basis, or rent actually received if you use the cash basis.
- 2
Claim your running costs in Boxes 24 to 29
Split your allowable expenses across the boxes: Box 24 for rent, rates, insurance and ground rents; Box 25 for repairs and maintenance; Box 27 for legal, letting-agent and professional fees; Box 28 for services; Box 29 for other allowable costs. Do not put residential mortgage interest here.
- 3
Put residential mortgage interest in Box 44
Residential property finance costs — your buy-to-let mortgage interest — go in Box 44, not the expenses boxes. HMRC gives a 20% tax-reducer on this figure under Section 24, rather than a full deduction.
- 4
Decide between actual expenses and the £1,000 allowance
If your costs are low, you can claim the £1,000 property allowance instead of your actual expenses. Use whichever gives the bigger deduction, but you cannot claim both on the same income.
- 5
Work out and check your taxable profit
Income minus allowable expenses gives your profit. Carry any losses forward, apply the figures to your SA100 return, and keep all your records in case HMRC asks.
A worked example
Tom, a higher-rate taxpayer, lets one flat. He received £14,400 rent, spent £2,600 on allowable costs, and paid £5,000 of mortgage interest.
| Box 20 — total rent | £14,400 |
| Boxes 24–29 — allowable expenses | − £2,600 |
| Taxable profit | £11,800 |
| Income Tax at 40% | £4,720 |
| Box 44 — mortgage interest £5,000 × 20% credit | − £1,000 |
| Tax due on the property | £3,720 |
Notice the mortgage interest never reduced the £11,800 profit — it only gave a £1,000 tax credit at the end. Had Tom wrongly put it in the expenses boxes, his profit would have shown as £6,800 and his return would have been incorrect.
Frequently asked questions
What is the SA105 form?
The SA105 is the 'UK property' supplementary page of the Self Assessment tax return. Any landlord with UK rental income over £1,000 attaches it to their main SA100 return to declare rents received, claim allowable expenses, and report mortgage interest for the Section 24 tax credit. It is where your buy-to-let profit is actually calculated for tax.
Where does mortgage interest go on the SA105?
Residential mortgage interest goes in Box 44 — 'Residential property finance costs' — not in the expenses boxes (24 to 29). This is the single most common SA105 mistake. Putting it in the expenses boxes wrongly deducts it in full, when the rules only allow a 20% tax-reducer on it. Box 44 makes sure the Section 24 restriction is applied correctly.
Do I need to fill in an SA105 if my rent is under £1,000?
Usually not. The £1,000 property allowance means that if your gross property income for the year is £1,000 or less, it is covered and you do not need to report it. Above £1,000 you must complete the SA105, but you can still choose to deduct the £1,000 allowance instead of your actual expenses if that gives a better result.
What expenses can I claim on the SA105?
Allowable running costs: letting-agent and management fees, landlord insurance, repairs and maintenance, ground rent and service charges, council tax and utilities you pay, accountancy fees, and other costs incurred wholly for the letting. Capital improvements and residential mortgage interest are handled separately — improvements reduce Capital Gains Tax when you sell, and mortgage interest goes in Box 44.
Does Making Tax Digital replace the SA105?
For landlords brought into Making Tax Digital for Income Tax (qualifying income over £50,000 from April 2026), the quarterly updates and a final declaration through compatible software replace the traditional SA105 paper or online property pages. Until you are mandated, the SA105 remains how you report your rental income each year.
Written and reviewed by the LandlordTaxAi Editorial Team. Our guides are reviewed against current HMRC guidance and updated when the rules change. Operated by LandlordTaxAi, United Kingdom. Follow us on LinkedIn.
Last reviewed: 23 June 2026 · Based on HMRC’s SA105 UK property pages and the accompanying UK property notes, including the Section 24 finance cost restriction (Box 44) and the £1,000 property allowance. Box references are for the 2026 SA105; always check the current form, as box numbers can change year to year. This article is informational only and does not constitute tax advice. Check the latest details on GOV.UK or with a qualified accountant.