Self Assessment 2025/26: Deadlines, Penalties and What to File

Quick answer

The 2025/26 tax year runs from 6 April 2025 to 5 April 2026. If you receive income that is not fully taxed at source, you will need to report it to HMRC via a Self Assessment tax return. The key milestone to keep in mind: register by 5 October 2026, submit your online return by 31 January 2027, and pay any tax owed by 31 January 2027. Miss that online deadline by a single day and a £100 fixed penalty lands automatically — regardless of whether you owe any tax at all.

For most self-employed people and higher earners, the January bill is also larger than expected because it includes a payment on account — a 50% advance payment towards your 2026/27 tax bill. A tax bill of £4,000 for 2025/26 therefore requires a January payment of £6,000 (£4,000 balancing payment plus £2,000 first payment on account).

All the key dates at a glance

These dates apply to the vast majority of taxpayers. There are limited exceptions — for example, if HMRC issues you a notice to file after 31 October 2026, you get three months from the date of that notice for the online submission rather than the standard 31 January 2027 date.

Who must file Self Assessment for 2025/26

HMRC does not automatically send Self Assessment forms to everyone. You need to register and file if any of the following applied to you during the 2025/26 tax year:

Self-employment and trading income You must file if your self-employment income exceeded £1,000 gross in the year. The first £1,000 is covered by the Trading Allowance and does not need to be reported. If you earned £1,001 or more from freelancing, contracting, a side business, or gig economy work, you must file.

Rental income Rental income above £2,500 net (after allowable expenses) in the year must be reported via Self Assessment. Below £2,500, HMRC may instead adjust your PAYE tax code to collect the tax — but you may still choose to file a return. Below £1,000, the Property Allowance means you have no tax to pay and no obligation to report.

High earners Any income above £100,000 in 2025/26 requires a Self Assessment return. At £100,000, your Personal Allowance starts to taper away at £1 for every £2 earned, creating the well-known 60% effective marginal rate trap between £100,000 and £125,140.

High Income Child Benefit Charge If either you or your partner earned above £60,000 and one of you received Child Benefit payments, the higher earner must file a Self Assessment return to pay back some or all of the Child Benefit received. The charge is 1% of the Child Benefit for every £200 of income above £60,000, reaching 100% at £80,000.

Capital gains If your total capital gains in 2025/26 exceeded the £3,000 annual exempt amount — from selling shares, a second property, cryptocurrency, or other assets — you must report them. Note the exempt amount fell sharply from £6,000 in 2024/25 to £3,000 in 2025/26.

Other situations requiring a return

The penalty structure in full

HMRC's penalty regime for Self Assessment is designed to escalate sharply the longer you delay. Critically, the initial £100 penalty applies even if you owe zero tax — there is no "nothing owed, nothing to pay" exception.

Late filing penalties

A return filed 13 months late with £3,000 of tax owed would therefore attract: £100 (day 1) + £900 (days 3-6 months) + £300 (6 months, greater than 5% of £3,000 = £150) + £300 (12 months) = £1,600 in filing penalties alone, before any interest.

Late payment penalties and interest

Separate from filing penalties, HMRC charges interest and surcharges on unpaid tax:

If you owe £5,000 and do not pay for 12 months from January 2027, you face approximately £338 of interest (6.75% x £5,000) plus surcharges of £250 + £250 + £250 = £750 in surcharges, totalling over £1,000 in additional charges on a £5,000 bill.

Reasonable excuse HMRC can waive penalties if you have a reasonable excuse — serious illness, bereavement of a partner, or a technical failure on HMRC's own system. Simply forgetting or not knowing the deadline does not qualify. If you think you have grounds, appeal within 30 days of the penalty notice.

Payments on account: the January surprise

First-time filers are often caught out by the payment on account system. When HMRC calculates that you owe more than £1,000 for 2025/26 (and that less than 80% of your total tax liability was collected at source through PAYE), they require advance payments towards 2026/27.

How it works:

• First payment on account = 50% of your 2025/26 tax bill, due 31 January 2027
• Second payment on account = 50% of your 2025/26 tax bill, due 31 July 2027
• Balancing payment for 2026/27 = actual 2026/27 tax minus the two payments on account, due 31 January 2028

Example:

If your income is lower in 2026/27 than in 2025/26, you can apply to reduce your payments on account. Log into your Government Gateway account and submit a claim to reduce them, giving your estimated 2026/27 income. If you reduce them incorrectly, interest is charged on any shortfall.

What you will need to file

Gathering the right documents before you start will make the process significantly faster. HMRC's online system does not automatically populate your return with figures from third parties — you are responsible for entering everything correctly.

Income documents

Deductions and reliefs

Collect evidence for anything you plan to claim: pension contributions (to calculate relief at source or carry-forward), Gift Aid donations (to extend your basic rate band), professional subscriptions, and allowable business expenses if self-employed.

Your UTR and Government Gateway login

You need your 10-digit Unique Taxpayer Reference (UTR). If you have lost it, it appears on previous HMRC correspondence. For first-time filers, register at gov.uk/register-for-self-assessment — your UTR will arrive by post within 10 working days. Do not leave this until October; postal delays can push you past the registration deadline.

After you file: correcting mistakes

If you spot an error after submitting your return, you can amend it online via your Government Gateway account. The amendment window for a 2025/26 return is 31 January 2028 — 12 months after the original online filing deadline. After that date, you must write to HMRC to request an amendment, and HMRC has four years from the end of the relevant tax year to raise an assessment.

If HMRC has already processed your return and sent a statement, notify them promptly. Underpaid tax following an error may incur interest from the original payment deadline, even if the mistake was innocent.

Time to Pay: if you cannot pay

If you cannot pay your January 2027 bill in full, contact HMRC before the deadline — not after. HMRC's online Time to Pay service is available if:

• You owe £30,000 or less
• You have no other HMRC debts
• You do not have any existing payment plans
• The deadline was within the last 60 days (or is approaching)

You can set up a plan for up to 12 months online at gov.uk. Interest continues to accrue during the arrangement at Bank of England base rate + 2.5%, but you avoid the 5% late payment surcharges as long as you stick to the agreed schedule.

If you owe more than £30,000 or need more than 12 months, call HMRC's Self Assessment helpline on 0300 200 3310 to discuss options. Lines are busy in January — call early in the morning or in November or December if you know a problem is coming.

HMRC's current interest rate on late tax

The interest rate HMRC charges on overdue tax is set at the Bank of England base rate plus 2.5 percentage points. With the Bank of England base rate at approximately 4.25% as of spring 2026, the HMRC late payment rate stands at approximately 6.75% per annum. This is not a small number — a £10,000 unpaid bill left for 12 months accrues approximately £675 of interest, on top of surcharges.

HMRC also pays interest on tax you have overpaid, but at a lower rate: Bank of England base rate minus 1%, currently around 3.25%. If you have made payments on account that exceed your actual liability, you will receive interest on the overpayment once your return is processed.