How to File Your Self Assessment Tax Return in 2025/26

The Accounted Business Team·26 February 2026·6 min read

Who Needs to File a Self Assessment Tax Return?

If you earned income that isn't taxed at source through PAYE, you almost certainly need to file a Self Assessment return. That includes:

Sole traders and freelancers with any self-employment income
Landlords receiving rental income
Company directors (even if you only pay yourself a salary)
Higher earners with income above £150,000
Anyone with untaxed income — savings interest above your allowance, foreign income, capital gains

If HMRC has sent you a notice to file, you must do so — even if you don't owe any tax. Failing to file when required triggers automatic penalties.

Key Deadlines for 2025/26

The 2025/26 tax year runs from 6 April 2025 to 5 April 2026. Here are the deadlines that matter:

5 October 2026: Deadline to register for Self Assessment if you're new to it
31 October 2026: Paper return deadline (though almost nobody should be filing on paper)
31 January 2027: Online return deadline AND deadline to pay the tax you owe
31 January 2027: First payment on account for 2026/27 (if applicable)
31 July 2027: Second payment on account for 2026/27

Miss the 31 January deadline and you'll face an immediate £100 penalty — even if you don't owe any tax. Further penalties accumulate at 3 months, 6 months, and 12 months late. The penalties guide has the full breakdown.

What You'll Need Before You Start

Gather everything before you sit down to file. Hunting for missing documents halfway through is how returns get abandoned and deadlines get missed.

Essential Documents

Unique Taxpayer Reference (UTR): Your 10-digit number from HMRC. It's on previous tax correspondence.
National Insurance number: On your payslip, P60, or HMRC letters.
Government Gateway login: You'll need your user ID and password for the HMRC online portal.
Income records: Invoices raised, bank statements, payment records from all income sources.
Expense records: Receipts, bank statements, mileage logs — everything you plan to claim.
P60 or P45: If you have employment income alongside your self-employment.
Bank interest certificates: Your bank can provide these, or check your annual summary.
Pension contributions: Details of any private pension payments made during the year.
Gift Aid donations: Records of charitable donations you want to claim relief on.

For Landlords

Rental income records (rent received, not rent invoiced)
Mortgage interest statements
Letting agent statements
Repair and maintenance receipts
Insurance premiums

For CIS Subcontractors

CIS payment and deduction statements from contractors
Records of materials costs (excluded from CIS deductions)

Step-by-Step: Filing Your Return Online

Step 1: Log Into HMRC

Go to the HMRC Self Assessment portal and sign in with your Government Gateway credentials. If you've lost your login details, allow a few days to recover them — don't leave this until 30 January.

Step 2: Select the Correct Sections

HMRC's online return is modular. You'll tick boxes to indicate which sections apply to you:

Self-employment (SA103): For sole traders and freelancers
UK property (SA105): For landlords
Partnership (SA104): If you're in a business partnership
Capital gains (SA108): If you've sold property, shares, or other assets
Foreign income (SA106): For overseas earnings

Only complete the sections relevant to your situation.

Step 3: Enter Your Income

For self-employment, you'll need your total turnover (all income received or invoiced during the tax year) and your allowable expenses. You can either list expenses by category or claim the trading allowance of £1,000 if your expenses are minimal.

Be accurate. HMRC cross-references your figures against bank data, CIS records, and information from third parties. Rounding to the nearest hundred might seem harmless, but discrepancies flag reviews.

Step 4: Claim Your Expenses

Enter your allowable business expenses by category. The main categories HMRC uses are:

Cost of goods sold
Car, van, and travel expenses
Wages, salaries, and other staff costs
Rent, rates, power, and insurance
Repairs and maintenance
Phone, fax, stationery, and other office costs
Advertising and business entertainment
Interest on bank and other loans
Bank, credit card, and other financial charges
Depreciation and loss/profit on sale of assets
Other allowable expenses

If you work from home, don't forget to claim your working from home allowance. It's one of the most commonly missed deductions.

Step 5: Review Your Tax Calculation

Once you've entered everything, HMRC's system calculates your tax liability. Check it makes sense. The calculation should show:

Your total income from all sources
Your Personal Allowance (£12,570 for 2025/26)
Income Tax at 20%, 40%, or 45% as applicable
Class 2 National Insurance (£3.45 per week for 2025/26 if profits exceed £12,570)
Class 4 National Insurance (6% on profits between £12,570 and £50,270, 2% above that)
Any payments on account already made

Step 6: Submit and Pay

Once you're satisfied the figures are correct, submit the return. You'll receive a confirmation with a reference number — save this.

Then pay what you owe. HMRC accepts direct bank transfer, debit card, and budget payment plans. Direct Debit is the safest way to ensure you don't miss the deadline.

Common Mistakes That Cost You Money

Forgetting to Claim Expenses

Every legitimate expense you miss is tax you overpay. The complete expenses list covers everything from mileage to working-from-home costs.

Mixing Up Turnover and Profit

HMRC asks for your turnover (total income) and your expenses separately. Some people accidentally enter their profit figure as turnover, which understates their expenses and overstates their tax.

Missing the Payment Deadline

Filing on time and paying on time are separate requirements with separate penalties. Even if your return is submitted in November, the tax isn't due until 31 January. But if you miss the payment deadline, interest starts accruing immediately.

Not Claiming Capital Allowances

If you bought equipment, a laptop, or tools for your business, you can claim capital allowances. The Annual Investment Allowance lets you deduct the full cost of most equipment (up to £1 million) in the year you bought it.

Ignoring Payments on Account

If your tax bill is above £1,000 and less than 80% of your tax was collected at source, HMRC will require payments on account — advance payments towards next year's bill. These are due on 31 January and 31 July. Forgetting about them leads to unexpected bills and penalties.

How Accounted Simplifies Self Assessment

Filing a tax return doesn't have to involve a frantic January weekend sorting through a year's worth of receipts. With Accounted, your financial records are maintained throughout the year:

Bank transactions are automatically categorised as they come in
Receipts are captured via WhatsApp — just snap a photo and send
Your Self Assessment data is compiled as you go, not reconstructed at year-end
When it's time to file, your figures are ready — you just review and submit

The difference between filing with good records and filing from memory is the difference between a 20-minute task and a stressful weekend.

Get Started with Accounted

Stop dreading Self Assessment. Accounted keeps your books in order year-round so filing is a formality, not a crisis. See our pricing for details — plans start from less than the cost of a single accountant visit. Start your free trial today — no credit card required.

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