How to File a Self Assessment Tax Return 2025/26 (The Complete Step-by-Step UK Guide)
Quick Summary: 12.1 million people filed a Self Assessment tax return for 2024/25. If you’re self-employed, earn over £100,000, have rental income, savings interest above £10,000, or fall into several other categories — you need to file too. Miss the 31 January 2027 deadline and the penalties start immediately. This guide covers every step, every form, and every deadline.
Table of Contents
- Do You Actually Need to File Self Assessment?
- Key Dates and Deadlines for 2025/26
- How to Register for Self Assessment
- Setting Up Your Government Gateway Account
- What Information and Documents You Need
- How to Complete the SA100 Return: Section by Section
- Allowable Expenses: What You Can Deduct
- Payments on Account: The Trap Most First-Timers Miss
- How to Pay Your Self Assessment Bill
- If You Can’t Pay: Time to Pay Arrangements
- Penalties, Interest, and How to Appeal
- How to Amend a Submitted Return
- How to Stop Filing Self Assessment
- Making Tax Digital: What’s Coming
- Frequently Asked Questions
1. Do You Actually Need to File Self Assessment? {#who-needs}
HMRC’s PAYE system collects the right tax automatically for most employees. Self Assessment exists for everyone whose tax situation is too complex for PAYE to handle alone. You must file a Self Assessment return if any of the following applied in the 2025/26 tax year (6 April 2025 – 5 April 2026):
┌──────────────────────────────────────────────────────────────────┐│ WHO MUST FILE SELF ASSESSMENT FOR 2025/26 ││ ││ Trigger Threshold ││ ────────────────────────────────────────────────────────────── ││ Self-employed / sole trader income Over £1,000 ││ Company director (most) Any income ││ Total income Over £100,000 ││ Untaxed income (e.g. freelance) Over £2,500 ││ Savings/investment interest Over £10,000 ││ Rental / property income Over £2,500 net ││ Foreign income Any taxable amount ││ Capital gains Over £3,000 ││ Child Benefit + partner earns >£60,000 HICBC applies ││ Tax owed not collectable via PAYE Any amount ││ HMRC issued a notice to file Mandatory │└──────────────────────────────────────────────────────────────────┘
Common situations people get wrong:
- Savings interest between £1,000 and £10,000: HMRC usually collects this via a tax code change or Simple Assessment — you don’t necessarily need Self Assessment. But if your total interest exceeds £10,000, Self Assessment is mandatory.
- Employed AND self-employed: If you have a PAYE job and earn over £1,000 from freelancing or a side hustle, you must register.
- Pension income: Most people with only pension income don’t need Self Assessment — HMRC codes pensions through PAYE. But if you have multiple pension sources plus rental income or large savings, you may tip over the threshold.
- Company directors: Most directors must file Self Assessment even if they take no salary, because HMRC requires a return to reconcile all income including dividends.
Not sure? Use HMRC’s free online tool: gov.uk/check-if-you-need-tax-return
💡 Related reading: HMRC Savings Tax Bills: The Complete Guide · HMRC Penalties: How They Work and How to Appeal
2. Key Dates and Deadlines for 2025/26 {#deadlines}
Missing any of these dates triggers automatic penalties or interest. Put them in your calendar now.
┌──────────────────────────────────────────────────────────────────┐│ SELF ASSESSMENT CALENDAR: 2025/26 TAX YEAR ││ ││ 6 April 2025 Tax year 2025/26 begins ││ ││ 5 April 2026 Tax year 2025/26 ends ││ ││ 5 October 2026 ⚠️ REGISTRATION DEADLINE ││ Must register if filing for first time ││ for 2025/26. Even if not yet registered. ││ ││ 31 October 2026 Paper return deadline (2025/26) ││ Very few people use this — online is better ││ ││ 31 January 2027 🔴 MAIN DEADLINE (online return + payment) ││ File online return AND pay all tax owed ││ for 2025/26 by midnight ││ ││ 31 January 2027 First Payment on Account due ││ (50% of estimated 2026/27 tax bill) ││ ││ 31 July 2027 Second Payment on Account due ││ (remaining 50% of estimated 2026/27 tax) ││ ││ 31 January 2028 Balancing payment for 2026/27 │└─── ───────────────────────────────────────────────────────────────┘
Why the registration deadline matters so much:
HMRC needs time after you register to post your Unique Taxpayer Reference (UTR) — a 10-digit number required to file your return. UTRs take up to 10 working days by post (21 days if you’re abroad). If you register in October, your UTR may arrive with barely enough time to file before January. Register as soon as the tax year ends — ideally in April or May.
💡 Related reading: HMRC Penalties: How They Work and How to Appeal
3. How to Register for Self Assessment {#register}
Registration is separate from filing. You register once and then file every year thereafter until you tell HMRC to stop.
Step 1: Go to gov.uk/register-for-self-assessment
Select the correct registration route:
- Self-employed / sole trader: Use the CWF1 online form. This registers you for Self Assessment AND Class 2 National Insurance simultaneously.
- Not self-employed (landlord, director, high earner etc.): Use the SA1 form.
- Partnership: Use form SA400.
- Already registered but previously stopped: Call HMRC on 0300 200 3310 to reactivate.
Step 2: Provide your details
You’ll need:
- Full name and date of birth
- National Insurance number
- Home address and phone number
- Nature of income (self-employment, rental, etc.)
- Date you started the activity
- Business name and address (if self-employed)
Step 3: Wait for your UTR
HMRC will post your Unique Taxpayer Reference within 10 working days. This 10-digit number is yours permanently — keep it somewhere safe. You’ll use it every time you deal with HMRC about Self Assessment.
Step 4: Activate your Government Gateway account
HMRC sends a separate letter with an activation code. You have 28 days to use it to set up your online account. If you miss this window, request a new code through gov.uk.
┌──────────────────────────────────────────────────────────────────┐│ SELF ASSESSMENT REGISTRATION TIMELINE ││ ││ Day 1 Submit online registration ││ Days 1–10 HMRC processes; posts UTR letter ││ Days 5–14 Government Gateway activation code arrives ││ Day 14–28 Log in, activate account, set up online access ││ Day 28+ Begin completing your return (from 6 April) ││ ││ Important: UTRs can take 21 days for overseas addresses ││ Register immediately — do not wait until October │└──────────────── ──────────────────────────────────────────────────┘
4. Setting Up Your Government Gateway Account {#gateway}
The Government Gateway is HMRC’s secure digital portal. You need it to file your return, check your tax position, and manage your account.
If you don’t have one:
- Go to gov.uk/log-in-register-hmrc-online-services
- Click “Create sign-in details”
- Provide your email address and create a password
- Verify your identity using one of: UK passport, driving licence, payslips, P60, or credit record check
- Enter your UTR when prompted to add Self Assessment to your account
- Enter the activation code from your UTR letter
Strengthening your account:
HMRC now requires two-factor authentication (2FA). Set up an authenticator app (Google Authenticator or similar) rather than SMS verification — SMS can be intercepted and HMRC scams frequently spoof text messages.
The HMRC app:
Download the HMRC app (iOS and Android) to check your tax position, view your UTR, see payments owed, and even claim tax refunds. It’s significantly more user-friendly than the desktop Government Gateway.
5. What Information and Documents You Need {#documents}
Before you start your return, gather everything in this checklist. Hunting for documents mid-filing is the single biggest cause of errors and missed deadlines.
┌──────────────────────────────────────────────────────────────────┐│ SELF ASSESSMENT DOCUMENTS CHECKLIST ││ ││ Employment income: ││ ☐ P60 from each employer (shows total pay + tax for year) ││ ☐ P45 if you left a job during the year ││ ☐ P11D if you received benefits in kind (company car etc.) ││ ││ Self-employment income: ││ ☐ Total turnover (gross income from all clients) ││ ☐ All business expenses receipts/records ││ ☐ Mileage log if claiming vehicle costs ││ ☐ Home office calculations if working from home ││ ││ Property / rental income: ││ ☐ Rent received (all properties) ││ ☐ Allowable expenses: repairs, agent fees, insurance ││ ☐ Mortgage interest (restricted to 20% credit) ││ ││ Savings and investments: ││ ☐ Bank/building society interest statements ││ ☐ Dividend vouchers or online statements ││ ☐ Investment income documents ││ ││ Capital gains: ││ ☐ Records of assets sold (date, cost, proceeds) ││ ☐ Improvement costs for property ││ ││ Other: ││ ☐ Pension contribution statements (for higher-rate relief) ││ ☐ Gift Aid donation records ││ ☐ Foreign income documents ││ ☐ Student loan repayment status ││ ☐ Child Benefit received (if income > £60,000) │└──────────────────────────────────────────────────────────────────┘
Record keeping rules:
HMRC requires you to keep your tax records for at least 5 years after the 31 January submission deadline. For self-employed people, this means keeping records until 31 January 2032 for the 2025/26 return. Penalties for failing to keep records: up to £3,000.
6. How to Complete the SA100 Return: Section by Section {#sa100}
The SA100 is the main Self Assessment tax return form. You complete it online via Government Gateway or on paper (paper must arrive by 31 October). Most people also need supplementary pages.
┌──────────────────────────────────────────────────────────────────┐│ SELF ASSESSMENT FORM STRUCTURE ││ ││ Main Form Purpose ││ ────────────────────────────────────────────────────────────── ││ SA100 Core return — personal details, income ││ summary, reliefs, tax calculation ││ ││ Supplementary Pages: ││ SA101 Additional income (pensions, state pension) ││ SA102 Employment income (one per employer) ││ SA103S/F Self-employment (short or full) ││ SA104S/F Partnership income ││ SA105 UK property income ││ SA106 Foreign income ││ SA107 Trusts and settlements ││ SA108 Capital gains ││ SA109 Residence and domicile │└──────────────────────────────────────────────────────────────────┘
Section-by-section walkthrough of the SA100:
Pages 1–2: Personal details Your name, address, UTR, NI number, and contact details. Double-check your UTR — entering it wrong can cause your return to be misallocated.
Pages 3–4: Income sources Check boxes for each type of income you’re declaring. The system then presents the relevant supplementary pages. Don’t skip this — if you don’t tick “self-employment” you won’t see the SA103.
Pages 5–6: Reliefs and deductions Enter pension contributions, Gift Aid donations, and any other reliefs. If you’re a higher-rate taxpayer who contributes to a pension via relief at source, you must enter your total pension contribution here to claim the extra 20% relief HMRC owes you.
Pages 7–8: Tax calculation If filing online, HMRC calculates your tax automatically. Review this before submitting — common errors include:
- Pension contributions not entered
- Gift Aid not entered
- Previous year’s payments on account not shown correctly
- Savings interest figure differing from your actual bank statements
The tax calculation page:
┌──────────────────────────────────────────────────────────────────┐│ EXAMPLE TAX CALCULATION: 2025/26 ││ ││ Total income received: ││ Employment income: £38,000 ││ Self-employment profit: £12,000 ││ Savings interest: £1,800 ││ Total gross income: £51,800 ││ ││ Less deductions: ││ Personal allowance: -£12,570 ││ Pension contributions (GPA): -£4,000 ││ Adjusted net income: £35,230 ││ ││ Tax calculation: ││ Basic rate 20% on £34,230: £6,846 ││ PSA: £1,000 savings at 0%: £0 ││ Remaining £800 savings at 20%: £160 ││ Total income tax: £7,006 ││ ││ Less already paid: ││ PAYE deducted: -£5,086 ││ Payments on account: -£0 (first year) ││ Balancing payment owed: £1,920 │└──────────────────────────────────────────────────────────────────┘
7. Allowable Expenses: What You Can Deduct {#expenses}
For self-employed people, allowable expenses reduce your taxable profit — and therefore your tax bill. These must be wholly and exclusively for business use.
┌──────────────────────────────────────────────────────────────────┐│ ALLOWABLE SELF-EMPLOYMENT EXPENSES ││ ││ Category Examples Deductible ││ ────────────────────────────────────────────────────────────── ││ Office costs Stationery, software 100% ││ Travel Business mileage (45p/mi) 100% ││ Clothing Uniforms, protective gear 100% ││ Staff Wages, NI, pension 100% ││ Premises Rent, utilities (bus only) 100% ││ Finance Bank charges, interest 100% ││ Marketing Advertising, website 100% ││ Training Relevant skills courses 100% ││ Home working £6/week simplified rate 100% ││ Vehicle costs Business proportion only Partial ││ Mixed use costs Business proportion only Partial ││ ││ NOT deductible: client entertainment, fines, personal costs │└──────────────────────────────────────────────────────────────────┘
The £1,000 trading allowance: If your self-employment income is £1,000 or less, you can claim the trading allowance and declare zero profit — no need to itemise expenses. If income exceeds £1,000, you choose between claiming actual expenses or the flat £1,000 allowance (whichever gives the better result).
Home working simplified expenses: You can claim £6/week (£312/year) for working from home without any receipts. For higher claims, you must keep records of your actual additional household costs attributable to working from home.
Pension contributions: As a self-employed person, you claim pension tax relief through your return — you don’t get it automatically. Enter your total gross pension contribution in the “pension payments” section. Higher-rate taxpayers receive an additional 20–25% relief this way.
💡 Related reading: HMRC and Side Hustles: The £1,000 Trading Allowance Explained ·
8. Payments on Account: The Trap Most First-Timers Miss {#payments-on-account}
Payments on account are advance payments toward your next year’s tax bill, collected alongside your current year’s balancing payment. They catch almost every first-time Self Assessment filer by surprise.
How they work:
If your Self Assessment tax bill exceeds £1,000 AND more than 20% of it was not collected at source (via PAYE), HMRC requires you to make two advance payments toward next year’s tax.
┌──────────────────────────────────────────────────────────────────┐│ PAYMENTS ON ACCOUNT: HOW THEY WORK ││ ││ Your 2025/26 tax bill: £3,000 ││ ││ Due 31 January 2027: ││ Balancing payment (2025/26): £3,000 ││ + 1st POA (50% of £3,000): +£1,500 ││ TOTAL DUE 31 JANUARY 2027: £4,500 ││ ││ Due 31 July 2027: ││ 2nd POA (remaining 50%): £1,500 ││ ││ Due 31 January 2028: ││ Balancing payment (2026/27): Actual bill minus POAs ││ ││ The shock: Your FIRST January bill is 1.5x your actual tax ││ Set money aside from day one of your self-employment │└──────────────────────────────────────────────────────────────────┘
Can you reduce payments on account?
Yes. If you expect your next year’s bill to be lower (e.g., lower income, major expense year), you can apply online to reduce your payments on account. Log into your Government Gateway, go to “Self Assessment”, and select “Reduce payments on account”. You must have a reasonable basis for the reduction — HMRC can charge interest if you reduce too aggressively and the actual bill turns out higher.
9. How to Pay Your Self Assessment Bill {#how-to-pay}
HMRC accepts payment in several ways. Online bank transfer is fastest and recommended.
┌──────────────────────────────────────────────────────────────────┐│ SELF ASSESSMENT PAYMENT METHODS ││ ││ Method Speed Reference Required ││ ────────────────────────────────────────────────────────────── ││ Online bank transfer Same day 10-digit UTR + 'K' ││ HMRC app Same day Log in and pay ││ Debit card (online) 1–3 days UTR reference ││ BACS / CHAPS Same–1 day UTR + 'K' ││ Budget payment plan Ongoing Set up via Gateway ││ Cheque (post) 5–7 days Payable to HMRC only ││ PAYE code (< £3k) Monthly spread Via Government Gateway ││ ││ HMRC bank account: ││ Sort code: 08-32-10 ││ Account: 12001039 ││ Account name: HMRC Cumbernauld ││ Reference: Your 10-digit UTR followed by K (e.g., 1234567890K) │└──────────────────────────────────────────────────────────────────┘
Collect through PAYE (underpayments under £3,000): If you owe less than £3,000 and want to spread the payment, HMRC can collect it through your PAYE tax code the following year — spreading the cost across 12 monthly payslips. You must file your return online by 30 December (a month before the main January deadline) to use this option.
Budget payment plan: You can make regular voluntary advance payments through Government Gateway any time during the year, reducing your January shock. Even £100/month from April can meaningfully reduce the lump sum due in January.
10. If You Can’t Pay: Time to Pay Arrangements {#time-to-pay}
If you genuinely cannot pay your Self Assessment bill by the deadline, do not simply ignore it. HMRC offers a Time to Pay arrangement that lets you spread payments over up to 12 months (sometimes longer for significant hardship).
How to apply:
- Self-service (under £30,000 owed): Apply online via Government Gateway under “Self Assessment” → “Set up a payment plan”. Available 24/7.
- Over £30,000 or complex situations: Call the HMRC Payment Support Service on 0300 200 3835.
Important: You must apply before penalties kick in — ideally before the 31 January deadline, or as close to it as possible. Interest (currently 7.25%) continues to accrue even under a Time to Pay arrangement, but the daily and percentage penalties are suspended while you’re compliant with the plan.
11. Penalties, Interest, and How to Appeal {#penalties}
┌──────────────────────────────────────────────────────────────────┐│ SELF ASSESSMENT LATE FILING PENALTIES ││ ││ 1 day late (1 Feb): £100 automatic penalty ││ 3 months late (1 May): £10/day for up to 90 days (max £900) ││ 6 months late (1 Aug): 5% of tax due OR £300 (whichever more) ││ 12 months late (1 Feb 2028): Further 5% OR £300 ││ ││ Late payment interest: 7.25% per annum (accrues daily) ││ Late payment penalty: 5% of unpaid tax at 30 days ││ 5% at 6 months ││ 5% at 12 months ││ ││ Maximum total penalties: Can exceed the original tax bill ││ for significant delays │└──────────────────────────────────────────────────────────────────┘
Reasonable excuse — when you can appeal:
HMRC accepts appeals against penalties if you have a “reasonable excuse” — a genuine reason beyond your control that prevented timely filing. Accepted excuses include: serious illness or hospitalisation, bereavement of a close relative, natural disaster affecting your records, technical failure of HMRC’s own systems. Not accepted: forgetting, being too busy, relying on an accountant who failed to file.
How to appeal:
- Log into your Government Gateway account
- Navigate to “Self Assessment”
- Select “Appeal a penalty”
- Describe your reasonable excuse clearly and concisely
- Attach any supporting evidence (hospital letters, death certificates etc.)
HMRC responds within 45 days. If rejected, you can escalate to the First-tier Tax Tribunal within 30 days of HMRC’s decision.
12. How to Amend a Submitted Return {#amend}
Made a mistake after filing? You can amend your Self Assessment return online within 12 months of the original 31 January deadline.
For the 2025/26 return (filed by 31 January 2027), the amendment deadline is 31 January 2028.
To amend online:
- Log into Government Gateway
- Go to “Self Assessment”
- Select “View / change your return”
- Make the corrections and resubmit
After 12 months, you can request a “Overpayment Relief Claim” in writing to HMRC, up to four years after the end of the tax year in question. For 2025/26, that’s 5 April 2030.
13. How to Stop Filing Self Assessment {#stop}
Once registered, HMRC automatically expects a return every year. If your circumstances change and you no longer need to file, you must tell HMRC — they will not remove you automatically.
How to deregister:
- Online: Log into Government Gateway → “Self Assessment” → “Stop filing returns”
- By phone: Call HMRC on 0300 200 3310
You can stop if: you are no longer self-employed, your income has fallen below all relevant thresholds, you have stopped receiving rental income, or HMRC agrees your tax can be collected through PAYE.
Important: Even after you request to stop, HMRC may ask you to file one final return for the year in which you ceased the activity.
14. Making Tax Digital: What’s Coming {#mtd}
Making Tax Digital for Income Tax Self Assessment (MTD ITSA) is the most significant change to Self Assessment in decades. It replaces the annual return with quarterly digital submissions.
┌──────────────────────────────────────────────────────────────────┐│ MTD ITSA ROLLOUT SCHEDULE ││ ││ April 2026: Mandatory for self-employed and landlords ││ with total gross income over £50,000 ││ ││ April 2027: Extended to income over £30,000 ││ ││ April 2028: Expected extension to income over £20,000 ││ (subject to confirmation) ││ ││ What changes: ││ - Quarterly updates submitted via MTD-compatible software ││ - Final declaration replaces the annual SA100 ││ - Must use HMRC-recognised accounting software ││ - Paper returns abolished for MTD-mandated taxpayers ││ ││ HMRC-approved software includes: QuickBooks, Xero, FreeAgent, ││ Sage, and many others. Check gov.uk/guidance/find-software- ││ thats-compatible-with-making-tax-digital-for-income-tax │└──────────────────────────────────────────────────────────────────┘
If your income is over £50,000, you are already affected from April 2026. Start using MTD-compatible software now if you haven’t already.
15. Frequently Asked Questions {#faq}
What is a Unique Taxpayer Reference (UTR)?
A UTR is a 10-digit number HMRC assigns when you register for Self Assessment. It identifies you on all Self Assessment correspondence. It’s different from your National Insurance number. Keep it secure — it never changes.
What happens if I miss the 5 October registration deadline?
Register as soon as possible. HMRC can still process late registrations, but you may have a compressed timeline to receive your UTR, set up your account, and file your return before 31 January. Late registration that causes a late return will still trigger the £100 penalty.
Can I file Self Assessment if I’m employed?
Yes. Many employed people also need to file — particularly those earning over £100,000, those with significant savings interest, side hustle income, or rental income. Your PAYE income still goes on the return alongside your other income.
Do I need an accountant?
No — millions of people file Self Assessment successfully without professional help, especially for straightforward returns. HMRC’s online system calculates your tax automatically and includes guidance at every step. For complex situations (multiple business streams, rental portfolio, capital gains, foreign income), a professional can save more than they cost.
What if HMRC’s figures are different from mine?
HMRC pre-populates some fields (particularly employment income and savings interest) from third-party data. Always check these against your own records. If there’s a discrepancy, use your own figures and keep the evidence. If HMRC challenges it, you can provide your documentation.
How do I know if I’ve been selected for an enquiry?
HMRC will write to inform you that your return has been selected for enquiry. This can happen randomly or because something on your return triggered an automated check. Do not panic — most enquiries are routine and resolved without additional tax being owed. See our guide to HMRC Penalties and Investigations.
Self Assessment at a Glance
┌──────────────────────────────────────────────────────────────────┐│ SELF ASSESSMENT QUICK REFERENCE ││ ││ Register: gov.uk/register-for-self-assessment ││ Registration: 5 October 2026 (for 2025/26) ││ Paper deadline: 31 October 2026 ││ Online deadline: 31 January 2027 ││ Payment: 31 January 2027 (+ 1st POA same day) ││ Late penalty: £100 from day 1; escalates fast ││ Records: Keep for 5 years after filing deadline ││ ││ Do I need to file? gov.uk/check-if-you-need-tax-return ││ Pay online: gov.uk/pay-self-assessment-tax-bill ││ UTR + 'K' → Sort 08-32-10, Acc 12001039 │└──────────────────────────────────────────────────────────────────┘
Information correct as of May 2026. Tax rules change — always verify current rules at gov.uk. This article does not constitute personal tax advice.
Internal Links:
- HMRC Savings Tax Bills 2025/26
- HMRC Penalties: How They Work and How to Appeal
- HMRC and Side Hustles: The £1,000 Trading Allowance
- UK Tax Codes Explained: What Every Code Means
- Am I Owed an HMRC Tax Refund?
- Marriage Allowance: How to Claim Up to £1,260
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