Landlord Self-Assessment: A Step-by-Step Walkthrough for the 2025-26 Tax Return

The 2025-26 tax year ended on 5 April 2026. The online self-assessment deadline is 31 January 2027. For most landlords the return itself is not complicated, but two areas catch people out every year: recording rental income and expenses correctly, and handling mortgage interest under Section 24.

Get those right and the rest of the return follows straightforwardly. This guide walks through the full process, step by step, with the key deadlines, a worked Section 24 example, and an explanation of why your January bill may be larger than you expect.

Do You Need to File?

You must register for self-assessment and submit a return if you received rental income during 2025-26, with one exception. If your total gross rental income was under £1,000, the property allowance covers it in full, no return is needed and the income is tax-free.

If gross rental income was between £1,001 and £2,500, HMRC asks you to contact them directly rather than register online. Above £2,500, you register through GOV.UK, receive a Unique Taxpayer Reference, and submit a full return.

Already registered from a previous year? Your UTR remains valid. No re-registration needed.

Source: GOV.UK guidance on income tax when you let property [verified May 2026]

The Two Forms: SA100 and SA105

Self-assessment uses a main return, the SA100, with supplementary pages attached for different income sources. Landlords need to include the SA105 UK property pages alongside their SA100.

Your SA100 covers all income, reliefs, and tax already paid. The SA105 is where rental income, allowable expenses, and mortgage interest are recorded. All UK properties owned personally are pooled on a single SA105. You do not need one per property.

If you are filing on paper, both forms are downloadable from GOV.UK. Filing online through HMRC's portal or any commercial software, the SA105 fields are built into the property income section automatically.

Calculating Your Rental Income

Rental income is the gross rent you received, or were entitled to receive, during the 2025-26 tax year from 6 April 2025 to 5 April 2026. HMRC uses the arising basis: rent falls in the year it was due, not necessarily when you were actually paid.

If a tenant pays late and rent for March 2026 arrives in May 2026, it still belongs in the 2025-26 return because that is when it was due.

A few points worth knowing:

Letting agent arrangements: if an agent collects rent on your behalf, your gross rental income is the rent before the agent's fees. The fees are then deducted as an allowable expense.
Deposits: not income when received. Only taxable if you retain all or part at the end of the tenancy.
Multiple properties: add all rental income together across every property you own personally. Expenses from one property can be offset against income from another.

Source: GOV.UK guidance, income tax when you rent out a property [verified May 2026]

Frequently Asked Questions

When is the deadline for the 2025-26 self-assessment tax return?

Online returns for the 2025-26 tax year must be filed by 31 January 2027. Paper returns have an earlier deadline of 31 October 2026. Any tax owed for 2025-26 is also due on 31 January 2027.

How do landlords claim mortgage interest on self-assessment?

Mortgage interest on a residential buy-to-let cannot be deducted as an allowable expense. Instead, you enter the total finance costs in the residential property finance costs section of the SA105 supplementary pages (box 44). HMRC then calculates a 20% basic rate tax credit against your income tax liability. This is the Section 24 restriction, fully in force since April 2020.

What expenses can landlords deduct from rental income?

Allowable expenses include letting agent fees, property maintenance and repairs (not improvements), landlord insurance, ground rent and service charges, accountancy fees for the property accounts, and advertising costs. Mortgage interest is not an allowable expense under Section 24, though a 20% tax credit applies. Capital expenditure on improvements is not deductible against income.

What is payment on account and does it affect landlords?

Payments on account are advance payments towards your next year's tax bill. If your 2024-25 self-assessment tax bill exceeded £1,000 and more than 20% was not collected at source via PAYE, HMRC requires two advance payments for 2025-26: each is 50% of the previous year's bill. The first falls on 31 January 2027, the second on 31 July 2027. This means the January payment is often much larger than the tax on a single year's income alone.

Does Section 24 affect basic-rate taxpayers?

Section 24 has less direct impact on basic-rate taxpayers because the 20% credit roughly matches the basic rate of tax. However, it can still affect them indirectly. Because mortgage interest is no longer deducted before taxable profit is calculated, gross rental income is included in total taxable income. Landlords with a salary close to £50,270 may find that rental income pushes them into the higher rate band, making the Section 24 restriction more costly than it first appears.