Self Assessment Income Tax Returns

The 2025/26 self assessment tax return covers income and gains from 6 April 2025 to 5 April 2026. The online filing deadline is 31 January 2027. Payment of any tax due falls on the same date. If you are in self assessment and have not yet started gathering your records, now is the time to act. Our tax advisors at Tax Accountant prepare and file self assessment returns for individuals, sole traders, landlords, directors, expats and anyone with a more complex tax position. Filing on time, with everything claimed correctly, is how you avoid penalties and keep more of what you earn.

Who Needs to File a Self Assessment Tax Return for 2025/26?

Most employees in the UK pay income tax through PAYE and never need to file a return. A significant number of people do, however, need to complete self assessment — and HMRC expects you to know whether you are one of them, even if it has not written to you.

Common Reasons You Must File

Self-employed sole traders earning more than £1,000 in the year must file. Partners in a business partnership must file. Rental income from property triggers a filing obligation, as does untaxed income from savings, investments or dividends. Total taxable income exceeding £100,000 brings you into self assessment automatically. Child benefit claimants whose income — or their partner’s income — exceeded £60,000 face the high income child benefit charge and must file to account for it. Directors, those with foreign income, and anyone with capital gains from property, shares or other assets all need to file.

Receiving a notice to file from HMRC creates an unconditional obligation. Submit the return even if you believe you owe no tax. Ignoring a notice triggers exactly the same penalty structure as a genuinely late return. HMRC’s guidance on who must send a tax return sets out the full list of circumstances.

Key Deadlines for the 2025/26 Tax Year

The paper return deadline for 2025/26 is 31 October 2026. Any paper return submitted after this date automatically incurs a £100 late filing penalty, even where no tax is owed.

Online returns must reach HMRC by 31 January 2027. That date is also the deadline for paying any tax due for 2025/26, including the balancing payment and the first payment on account for 2026/27. The second payment on account for 2026/27 falls due on 31 July 2027.

First-time online filers need to register with HMRC before they can submit. An activation code arrives by post and can take up to ten working days. Leaving registration until January is a risk. Where you authorise our team to act as your agent, we manage the registration and filing process on your behalf through our personal tax services.

The Penalty Structure for Late Filing

HMRC applies a structured penalty regime for late self assessment returns. Critically, these penalties apply regardless of whether any tax is owed. A nil return filed one day late still attracts the initial £100 penalty.

The penalties for the 2025/26 return build as follows. Missing the 31 January 2027 deadline immediately triggers a £100 fixed penalty. From 1 May 2027 — after three months — daily penalties of £10 per day accumulate for up to 90 days, reaching a maximum of £900. From 31 July 2027 — at six months — a further charge of 5% of the tax due or £300 applies, whichever is higher. From 31 January 2028 — at twelve months — another 5% or £300 charge falls due. Deliberate withholding of information can push that final penalty to 100% of the tax owed.

These penalties compound quickly. A return filed two years late with a significant liability can attract charges exceeding the original tax itself. Not one of these penalties counts as a tax-deductible expense — each is a pure, unavoidable cost. Filing early with our team is the simplest way to avoid them entirely.

Late Payment Penalties and Interest

Filing on time does not protect you if the tax goes unpaid by 31 January 2027. From the day after the due date, HMRC charges interest on all overdue amounts. On top of interest, late payment surcharges apply. Thirty days late triggers a 5% surcharge on the unpaid tax. Six months late adds another 5%. Twelve months late adds a final 5%.

Cannot pay in full by the deadline? The worst response is to do nothing. HMRC operates a Time to Pay arrangement — a structured payment plan spread over an agreed period. Interest continues to accrue under the arrangement, but the late payment surcharges are suspended while it remains active. Contacting HMRC or speaking to our team as soon as a payment problem arises gives you the best chance of agreeing a workable plan.

Payments on Account

Where your 2025/26 tax bill exceeds £1,000 and less than 80% of it was collected through PAYE, HMRC requires payments on account toward the following year. Each payment equals half of your 2025/26 tax liability.

The first payment on account for 2026/27 falls on 31 January 2027 — the same day as your balancing payment for 2025/26. The second falls on 31 July 2027. Many people are caught out by the size of their January bill because it combines two obligations at once.

Expecting lower income in 2026/27 than in 2025/26? An application to reduce payments on account is straightforward. Bear in mind, though, that if your actual 2026/27 liability turns out higher than expected, HMRC charges interest on the shortfall. Our team reviews payments on account as a standard part of preparing every return.

What Income and Gains Must Be Declared?

Every source of taxable income and every chargeable gain for the year belongs in the 2025/26 return. That includes employment income not fully covered by PAYE, self-employment profits, partnership income, rental income from UK and overseas property, dividends, savings interest, pension income, foreign income and capital gains from the disposal of property, shares, crypto assets and other chargeable assets.

Capital Gains: The 60-Day Rule

The annual CGT exempt amount for 2025/26 is £3,000. Net gains above this are taxable at 18% for basic rate taxpayers and 24% for higher and additional rate taxpayers on most assets. Selling a UK residential property that produces a gain creates a separate obligation. A 60-day CGT return must reach HMRC within 60 days of completion. This sits alongside the self assessment return — it does not replace it. Our personal capital gains tax service handles both.

Allowances and Reliefs Worth Claiming

Self assessment is also the route through which you claim allowances and reliefs that reduce your bill. HMRC will not claim them for you. Omit them from your return and you simply lose them.

Tax Rates and the Personal Allowance

The personal allowance for 2025/26 is £12,570. Earnings above this become taxable. The basic rate of 20% applies up to £50,270. The higher rate of 40% applies between £50,270 and £125,140. Above £125,140, the additional rate of 45% applies. Note that the personal allowance reduces by £1 for every £2 earned above £100,000, disappearing entirely at £125,140.

Other Reliefs to Include

Pension contributions attract tax relief at your marginal rate and can substantially reduce your bill. Gift Aid donations, marriage allowance, qualifying loan interest and business expenses for the self-employed all reduce taxable income. Landlords can claim allowable property expenses, though mortgage interest no longer qualifies as a full deduction. Since April 2020, finance costs attract only a 20% tax credit instead. Our property income tax service covers this in full.

Amending a Return After Filing

Made an error after filing? Amendments to the 2025/26 return are accepted until 31 January 2028 — one year after the filing deadline. Log into your HMRC online account and update the relevant figures. An amendment increasing your liability means HMRC charges interest on the additional tax from the original due date. An amendment reducing it triggers a refund.

Errors spotted after the amendment window closes require a separate overpayment relief claim. The conditions are stricter and the process is more involved. Our team reviews every return for errors before filing, which is the most effective point at which to catch them.

What Records Do You Need to Keep?

HMRC can open an enquiry into a self assessment return up to twelve months after the filing date in straightforward cases — longer where careless underreporting or fraud is suspected. Every figure in the return needs supporting records. Keep them for at least five years after the 31 January filing deadline of the relevant year. For the 2025/26 return, records must be retained until at least 31 January 2032.

Self-employed individuals need records of all business income and expenses. Landlords need rental income records, expense receipts, mortgage statements and details of capital expenditure. Investors need acquisition costs, disposal proceeds and transaction dates for every disposal. HMRC’s guidance on keeping records for self assessment explains the full requirements. The ICAEW has also published practical guidance on record keeping and tax compliance worth reviewing.

How Our Tax Advisors Can Help

Self assessment is routine for some people and genuinely complex for others. Multiple income sources, property, overseas interests, capital gains or a history of HMRC correspondence all make professional preparation worthwhile.

Our team gathers your information, identifies every allowance and relief available, prepares the return accurately and files it on time. Payments on account, HMRC queries after filing and any amendments all fall within our standard service. Our personal tax services cover every income category including employment tax, foreign income, property income and capital gains.

31 January 2027 is the last date — not the target date. The earlier your return reaches us, the more time we have to review your position and identify planning opportunities before the year end closes them off. Get in touch with our team today.

Updated April 2026 for the 2025/26 tax year. Deadlines, rates and allowances reflect the 2025/26 position.