Self Assessment Tax Return 2025/26: Deadlines, Expenses and How to File

The 2025 to 2026 tax year ran from 6 April 2025 to 5 April 2026. The return for it can be filed from 6 April 2026 and has to be with HMRC by midnight on 31 January 2027 if you file online. This guide covers who has to file, what the deadlines actually are, what you can claim and how the payments are worked out.

Who has to file for 2025 to 2026

HMRC requires a return from you if, during the tax year, any of the following applied. You were self employed as a sole trader and your income before expenses was more than £1,000. You were a partner in a business partnership. You had Capital Gains Tax to pay on something you sold or gave away. You were liable to the High Income Child Benefit Charge and do not pay it through your tax code.

Beyond those, untaxed income is the usual reason. Rent from property or land, dividends, savings interest, tips and commission, foreign income and taxable UK income received while you were living abroad all have to be declared, and a return is normally where that happens. Some people file by choice, to prove self employment, to pay voluntary National Insurance or to claim a relief such as one for a community investment or a venture capital scheme.

If HMRC has sent you a notice to file, you have to file, even if you believe nothing is due. The way out of that is to ask HMRC to withdraw the notice, not to ignore it.

The 2025/26 deadlines

Each of the filing and payment deadlines is midnight on the day shown. If you register late, HMRC writes to you with a filing deadline three months from the date of its letter, but the date the tax is due does not move. That is worth understanding, because it means you can be inside your filing deadline and still be charged interest on tax paid after 31 January.

What you can claim

For a trade, the test is whether the cost was incurred wholly and exclusively for the business. HMRC's own list covers office costs and phone, travel including fuel, parking and train or bus fares, uniforms, staff and subcontractor costs, stock and raw materials, insurance and bank charges, premises costs such as heat, light and business rates, advertising and website costs, trade subscriptions and work related training such as a refresher course.

Where a cost is part private it is apportioned rather than refused outright. For vehicles and for working from home, HMRC publishes flat rates that are often simpler and no worse than working out the real figure.

For the 2025 to 2026 return the mileage rate for cars and goods vehicles was 45p for the first 10,000 miles, with 25p after that. The 55p rate applies from 6 April 2026, so it is the 2026 to 2027 return where you will use it. The working from home flat rate does not cover telephone or internet, which are claimed separately on the business proportion.

Things that sit outside the trade matter just as much to the final figure. Personal pension contributions, Gift Aid donations, the marriage allowance, capital allowances on equipment and vehicles, losses brought forward and, for landlords, the basic rate credit that replaced mortgage interest relief can each move the tax a long way. Our guide to allowable expenses for sole traders goes through the categories one by one.

Payments on account

Payments on account are advance instalments towards next year's bill. They are due on 31 January and 31 July, and each one is normally half of the tax you owed for the previous year. Class 4 National Insurance is included in them. Capital Gains Tax is not, so a gain is paid in full with the balancing payment rather than spread.

You do not have to make them if the tax you owed last year was under £1,000, or if more than 80% of your tax for last year was collected at source, through your tax code or by a bank deducting on savings interest.

The practical consequence catches people out in a first profitable year. On 31 January you pay the balancing payment for the year just filed and the first instalment for the year already running, so the January demand can be half again as large as the tax on the year you have just reported. If your income has fallen you can apply to reduce the instalments, but reducing them below what turns out to be due attracts interest, so it is worth doing with a calculation rather than a guess.

Six mistakes that cost money

• Reporting net rather than gross. If tax was taken off before you were paid, under the Construction Industry Scheme for example, your turnover is the gross figure and the deduction is claimed separately as a credit. Reporting the net amount means you lose the credit.
• Forgetting capital allowances. A van, tools, a computer or equipment is normally claimed through capital allowances rather than as a running cost, and the annual investment allowance usually means the whole cost comes off in the year you bought it.
• Missing pension and Gift Aid. Both extend your basic rate band. A higher rate taxpayer who leaves them off the return pays more tax than they need to, and HMRC will not add them for you.
• Getting a joint property split wrong. Spouses holding property jointly are treated as owning it equally for tax unless the beneficial ownership is different and a declaration has been made. Splitting income to suit the tax position without that is a correction waiting to happen.
• Filing without the paperwork. Interest certificates, dividend vouchers, P11Ds and completion statements routinely change the figures. Estimating and intending to correct it later rarely ends with the correction being made.
• Leaving it to January. Not a tax error, but the one that causes the most of them. A return prepared in May is prepared from records you still remember.

How to file

You file through your HMRC online account, through commercial software, or through an agent who files on your behalf. To file yourself you need your Unique Taxpayer Reference and a Government Gateway account, and if you have never filed before you need to allow time for HMRC to issue the reference and the activation code by post.

Whoever files it, work in this order. Register if you need to, in time for 5 October. Gather the income records and the cost records for the whole year. Reconcile them to your bank, so that what you are reporting matches what actually happened. Apply the expenses, the capital allowances and the reliefs. Then read the computation before it goes, because the figure you owe should make sense to you.

Keep the records for five years after the 31 January filing deadline if you are self employed or a landlord. They are what supports the return if HMRC asks about it.