Self Assessment 2026: Deadlines, Penalties & How to File

Self Assessment is the system HMRC uses to collect income tax from individuals whose tax affairs are not fully settled through PAYE. If you are self-employed, a company director receiving dividends, a higher earner with investment income, or fall into any of the other categories requiring a tax return, you must file a Self Assessment return each year. Getting it wrong — or filing late — can be expensive. This guide covers everything you need to know about Self Assessment for the 2025/26 tax year, with returns due in 2026.

Who Needs to File a Self Assessment Return?

You must file a Self Assessment tax return if any of the following apply to you during the 2025/26 tax year:

• You were self-employed as a sole trader with income over £1,000 (the trading allowance)
• You were a partner in a business partnership
• You had total taxable income above £150,000 (even if all tax was collected through PAYE)
• You had untaxed income — for example, from renting out property, tips and commissions, or savings/investments
• You had income from abroad that you need to pay tax on
• You had capital gains above the annual exempt amount (£3,000 for 2025/26)
• You were a company director (unless it was a non-profit organisation and you received no pay or benefits)
• You or your partner received Child Benefit and your adjusted net income was above £60,000 (the High Income Child Benefit Charge threshold)
• You had income from a trust or settlement
• Your State Pension was more than your Personal Allowance
• You received a P800 from HMRC saying you did not pay enough tax

If you are not sure whether you need to file, HMRC provides an online tool to check. However, when in doubt, it is always safer to register and file — there is no penalty for filing a return that shows no tax is due, but there are significant penalties for failing to file when required.

Key Dates and Deadlines for 2025/26

The 2025/26 tax year runs from 6 April 2025 to 5 April 2026. Here are the critical dates:

The most important date is 31 January 2027. This is both the deadline for filing your online return AND for paying any tax you owe. Missing either deadline triggers automatic penalties.

Penalties for Late Filing

HMRC's penalty regime for late Self Assessment filing is deliberately harsh to encourage compliance. The penalties stack up quickly:

This means that a return filed more than 12 months late could attract penalties of at least £1,600 (£100 + £900 + £300 + £300) even if there is no tax to pay. If tax is owed, the penalties are even higher because they are calculated as a percentage of the outstanding tax.

Penalties for Late Payment

Separate from the filing penalties, HMRC also charges penalties for paying your tax late:

• 30 days late: 5% of the tax unpaid at that date
• 6 months late: a further 5% of the tax still unpaid
• 12 months late: a further 5% of the tax still unpaid

HMRC also charges interest on any tax paid late. The current late-payment interest rate is set at the Bank of England base rate plus 2.5%. With base rates elevated, this can add up to a significant additional cost.

How to File Your Self Assessment Online

Filing online through HMRC's Self Assessment portal is the most common method. Here is a step-by-step guide:

Step 1: Register for Self Assessment

If you have never filed before, you need to register with HMRC. For sole traders, this means registering for Self Assessment and Class 2 National Insurance. HMRC will send you a Unique Taxpayer Reference (UTR) — a 10-digit number — by post within 10 working days. You also need to set up a Government Gateway account if you do not already have one.

Step 2: Gather Your Records

Before you start filling in your return, make sure you have:

• P60 from your employer (if employed)
• P11D for any benefits in kind
• Records of all self-employment income and expenses
• Bank statements showing interest earned
• Dividend vouchers or statements
• Rental income and expense records
• Records of any capital gains or losses
• Pension contribution certificates
• Gift Aid donation records
• Your UTR and National Insurance number

Step 3: Complete the SA100 Main Return

The SA100 is the main tax return form. It captures your personal details, employment income, and total income summary. Depending on your circumstances, you will also need to complete supplementary pages:

• SA102 — Employment income (one per employer)
• SA103 — Self-employment (short or full version)
• SA104 — Partnership income
• SA105 — UK property income
• SA106 — Foreign income
• SA107 — Trusts and settlements
• SA108 — Capital gains
• SA109 — Residence, remittance basis
• SA110 — Tax calculation summary

Step 4: Submit and Pay

Once you have completed all relevant sections, review the tax calculation HMRC generates. If everything looks correct, submit the return. HMRC accepts payment by direct debit, debit card, bank transfer, and other methods. Setting up a Budget Payment Plan allows you to spread your tax bill across the year.

Payments on Account

If your Self Assessment tax bill is more than £1,000 and less than 80% of your total tax was collected at source (e.g., through PAYE), HMRC will require you to make “payments on account”. These are advance payments towards the following year's tax bill:

• First payment on account: due 31 January (same deadline as the current year's tax)
• Second payment on account: due 31 July
• Each payment is 50% of the previous year's Self Assessment liability
• A balancing payment (or refund) is made the following 31 January

Payments on account can catch people off guard. If you had a large tax bill last year, you could face paying 150% of that amount in the following January — the balancing payment for the current year plus the first payment on account for next year.

Common Mistakes to Avoid

After processing millions of returns, HMRC has identified the most frequent errors that delay processing or trigger enquiries:

• Forgetting to include all sources of income — HMRC receives data from banks, employers, and investment platforms and will flag discrepancies
• Claiming expenses that are not wholly and exclusively for business purposes
• Not keeping adequate records — HMRC requires you to keep records for at least 5 years after the filing deadline
• Missing the deadline for registration — new self-employed individuals must register by 5 October following the end of the tax year
• Entering figures in the wrong boxes — gross vs net, inclusive vs exclusive of VAT
• Forgetting to claim tax relief on pension contributions, especially if you are a higher rate taxpayer
• Not declaring cryptocurrency gains — HMRC has data-sharing agreements with major exchanges
• Overlooking the Marriage Allowance — worth up to £252 per year for eligible couples
• Not claiming overlap relief if you are a sole trader and have changed your accounting date

Tips for a Smooth Filing Experience

• File early: you do not have to pay any sooner by filing early, but you get certainty on what you owe
• Use accounting software to keep records organised throughout the year — this makes completing your return much easier
• Set aside money for tax as you earn it — a separate savings account earmarked for tax prevents January surprises
• Check your tax code throughout the year to ensure PAYE is collecting the right amount
• Consider using TaxWhizz.ai to automate your Self Assessment calculations and identify planning opportunities
• If you cannot pay on time, contact HMRC to arrange a Time to Pay agreement before the deadline passes