How to File Your Self Assessment tax Return 2026

The 2025/26 Self Assessment tax return is not just another HMRC form to get out of the way. It can affect your tax bill, cash flow, payments on account, penalty risk and how confidently you move into the next tax year.

For many people, the real worry is not the return itself. It is the fear of missing something important: a rental income figure, a dividend voucher, a CIS deduction statement, a business expense, a student loan repayment, or the payment-on-account amount sitting quietly at the bottom of the calculation.

That is where a calm, organised approach makes all the difference. Whether you are self-employed, a landlord, a company director, a freelancer, a contractor, or someone with untaxed income, this guide explains how to file your 2025/26 Self Assessment tax return properly, what to prepare, what to watch out for, and when it may be sensible to ask an accountant for help.

Quick answer: How do you file your Self Assessment tax return for 2025/26?

You file your 2025/26 Self Assessment tax return after the tax year ends on 5 April 2026. You can submit it online from 6 April 2026, and the main online filing and payment deadline is 31 January 2027. To file, you need your UTR, Government Gateway access, records of income, allowable expenses, tax already paid, and details of any reliefs or allowances. Once submitted, HMRC calculates what you owe, including any balancing payment and payments on account.

What is a Self Assessment tax return?

Self Assessment is the system HMRC uses to collect Income Tax from people whose income is not fully taxed at source.

If you are employed, your employer normally deducts tax through PAYE before paying your wages. But if you earn money outside normal PAYE, HMRC may not know the full picture until you report it.

A Self Assessment tax return tells HMRC about your income, expenses, allowances, tax reliefs, and tax already paid. HMRC then uses that information to work out whether you owe more tax, have overpaid tax, or need to make payments on account for the next tax year.

For the 2025/26 tax year, the return covers income and gains from 6 April 2025 to 5 April 2026.

Who needs to file a Self Assessment tax return for 2025/26?

You may need to file a return if you had income or tax circumstances that HMRC cannot fully deal with through PAYE.

Common examples include:

• You were self-employed as a sole trader and earned more than £1,000 before deducting expenses.
• You were a partner in a business partnership.
• You received rental income from UK property.
• You received tips, commission or casual income not taxed through PAYE.
• You had dividend income, savings income or investment income to report.
• You had foreign income.
• You sold an asset and may need to pay Capital Gains Tax.
• You need to pay the High Income Child Benefit Charge.
• You are a company director with dividends or other untaxed income.
• You are a CIS subcontractor and want to claim a repayment or report income properly.
• HMRC has issued you with a notice to file a tax return.

Some people file voluntarily because they need to claim tax relief, prove income for a mortgage, claim a refund, or keep their National Insurance record correct.

If you are unsure, it is better to check early. Waiting until January often turns a small admin job into a stressful tax problem.

Key Self Assessment deadlines for the 2025/26 tax year

Here are the key dates for the 2025/26 return:

• Tax year covered: 6 April 2025 to 5 April 2026
• Earliest online filing date: 6 April 2026
• Register for Self Assessment if you are new: usually by 5 October 2026
• Paper tax return deadline: 31 October 2026
• Deadline to file online if you want HMRC to collect tax through your PAYE code: 30 December 2026
• Online filing deadline: 31 January 2027
• Tax payment deadline: 31 January 2027
• Second payment on account deadline, where applicable: 31 July 2027

The 31 January deadline is the big one. It is not only the filing deadline; it is also the payment deadline for any tax due for 2025/26, plus the first payment on account for 2026/27 if one applies.

What information do you need before filing?

Good records make Self Assessment much easier. Before you start, gather everything in one place.

You will usually need:

• Your 10-digit UTR number
• National Insurance number
• Government Gateway login details
• Bank interest statements
• Dividend vouchers or investment reports
• Rental income and property expense records
• Sole trader sales, invoices, receipts and bank statements
• CIS deduction statements, if relevant
• P60, P45 or P11D forms, where applicable
• Pension contribution details
• Gift Aid donations
• Student loan or postgraduate loan details
• Records of tax already paid
• Details of any capital gains
• Mileage logs, home-working costs or business-use calculations
• Accounting software reports, if you use cloud bookkeeping

For sole traders and landlords, the quality of your records matters. A shoebox of receipts is better than nothing, but it is not ideal. Clear digital records reduce the risk of missed expenses, duplicated income, and HMRC queries.

Step-by-step: how to file your Self Assessment tax return 2025/26

1. Check whether you need to file

Start by confirming whether you actually need to send a return. If HMRC has issued a notice to file, you normally need to submit one unless HMRC agrees to withdraw it.

If your income changed during 2025/26, do not assume last year’s position still applies. A new rental property, freelance project, dividend payment, side hustle or crypto disposal can change things quickly.

2. Register with HMRC if you are new

If this is your first Self Assessment return, you need to register. HMRC will issue a UTR number. This can take time, so do not leave it until January.

Sole traders, partnerships and individuals with untaxed income may need to register in slightly different ways. If in doubt, ask an accountant to check the correct route before submitting the wrong form.

3. Set up or access your Government Gateway account

Online filing usually requires a Government Gateway account. Make sure you can log in well before the deadline. Lost passwords, old phone numbers and missing activation codes are more common than people think.

4. Organise your income

Separate your income by type. For example:

• Self-employment income
• Property income
• Employment income
• Dividends
• Interest
• Pension income
• Foreign income
• Capital gains

This matters because different types of income may be taxed differently.

5. Review allowable expenses

Allowable expenses reduce taxable profit, not tax directly. For example, if your business earns £40,000 and has £8,000 of allowable expenses, your taxable profit is £32,000 before allowances and adjustments.

Typical allowable expenses may include:

• Accountancy fees
• Business insurance
• Office costs
• Software subscriptions
• Travel for business purposes
• Marketing and website costs
• Phone and internet business use
• Training linked to your trade
• Bank charges
• Equipment and tools
• Repairs and maintenance for rental property

The expense must be wholly and exclusively for business purposes. Where there is mixed personal and business use, only the business proportion should be claimed.

6. Complete the return carefully

HMRC’s online system will guide you through the return, but the questions can still be easy to misunderstand. You may need supplementary pages for self-employment, property, capital gains, foreign income or partnership income.

Take your time. A wrong box can change the tax calculation.

7. Check the calculation before submitting

Before you press submit, review the tax calculation. Look at:

• Total income
• Taxable profit
• Income Tax due
• Class 4 National Insurance
• Student loan repayments
• Payments on account
• Tax already deducted
• Final amount payable by 31 January 2027

If the figure looks far too high or suspiciously low, pause. It may be correct, but it is worth checking before submission.

8. Submit and save proof

Once submitted, save your submission receipt, tax calculation and copy of the return. You may need these for mortgage applications, loan checks, future tax planning or HMRC correspondence.

What income must be included?

Your Self Assessment return should give HMRC the full picture of your taxable income for the year.

This may include:

• Sales from self-employment
• Rental income from residential or commercial property
• Employment income and benefits
• Company dividends
• Bank and building society interest
• Foreign income
• Pension income
• Trust income
• Partnership profit share
• Capital gains
• CIS income and deductions
• Side-hustle income from online platforms

A common mistake is assuming small income does not count. Some allowances may apply, such as the £1,000 trading allowance or property allowance, but you should not ignore income simply because it was casual, irregular or paid into a personal account.

What expenses and allowances can you claim?

The right expenses can make a meaningful difference to your tax bill. The wrong expenses can create problems if HMRC asks questions later.

For sole traders, common claims include:

• Goods for resale
• Materials
• Subcontractor costs
• Business mileage
• Use of home as office
• Professional fees
• Advertising
• Website hosting
• Accounting software
• Protective clothing or uniforms
• Repairs to business equipment

For landlords, common claims may include:

• Letting agent fees
• Repairs and maintenance
• Landlord insurance
• Accountancy fees
• Service charges and ground rent
• Safety certificates
• Replacement domestic items relief, where applicable

Mortgage interest relief for residential landlords works differently from normal expenses. It is usually given as a basic-rate tax credit rather than a direct deduction from rental profit, so landlords should be careful with the calculation.

For company directors, dividends, salary, benefits and director’s loan account movements should be reviewed properly. A director’s personal tax return should line up with company payroll records, dividend paperwork and the company accounts.

How much tax might you pay? Practical examples

The figures below are simplified examples for illustration. Real tax bills can change because of other income, pension contributions, student loans, Scottish tax rates, benefits, losses, capital allowances and other reliefs.

Example 1: Sole trader with income and expenses

Amira is a self-employed designer in Manchester.

• Sales income: £48,000
• Allowable expenses: £9,500
• Taxable profit: £38,500

Using the 2025/26 Personal Allowance of £12,570, her taxable income is:

£38,500 – £12,570 = £25,930

If she is a basic-rate taxpayer, Income Tax at 20% is:

£25,930 x 20% = £5,186

Class 4 National Insurance is broadly due on profits above £12,570:

£38,500 – £12,570 = £25,930
£25,930 x 6% = £1,555.80

Estimated tax and Class 4 NIC: £6,741.80

Because the bill is over £1,000, Amira may also need to make payments on account towards the following tax year.

Example 2: Landlord with rental income

David owns one rental property in Birmingham.

• Rental income: £18,000
• Letting agent fees: £1,800
• Repairs: £2,200
• Insurance and compliance costs: £600
• Accountancy fees: £300

Allowable property expenses total:

£1,800 + £2,200 + £600 + £300 = £4,900

Rental profit before finance cost adjustments:

£18,000 – £4,900 = £13,100

If David also has employment income using his Personal Allowance already, this rental profit may be taxed at his marginal rate. If he is a higher-rate taxpayer, the property profit could create a sizeable liability. If he also has mortgage interest, the treatment needs care because residential finance cost relief is not deducted in the same way as ordinary repairs.

Example 3: Limited company director with salary and dividends

Sophie is a company director in Leeds.

• Salary: £12,570
• Dividends: £35,000
• Dividend allowance: £500

Her salary uses the Personal Allowance. Her taxable dividend income is:

£35,000 – £500 = £34,500

If the dividends fall within the basic-rate band for 2025/26, dividend tax is:

£34,500 x 8.75% = £3,018.75

This is a simplified example. In real life, the calculation depends on Sophie’s total income, tax band, any benefits, pension contributions, other dividends, savings income and whether the company has prepared proper dividend vouchers and board minutes.

Payments on account explained in plain English

Payments on account are advance payments towards your next Self Assessment tax bill.

HMRC usually asks for them when your Self Assessment bill is £1,000 or more and less than 80% of your tax has already been collected at source.

Each payment is normally half of the previous year’s tax bill. The first is due by 31 January, and the second is due by 31 July.

Here is a simple example.

If your 2025/26 Self Assessment bill is £4,000, HMRC may ask you to pay:

• £4,000 balancing payment for 2025/26 by 31 January 2027
• £2,000 first payment on account for 2026/27 by 31 January 2027
• £2,000 second payment on account for 2026/27 by 31 July 2027

That means the January payment could be £6,000, not £4,000.

This catches many first-time filers by surprise. Filing early gives you time to plan rather than finding out the number a few days before the deadline.

Common Self Assessment mistakes that cost taxpayers money

Most Self Assessment errors are not dramatic. They are ordinary mistakes made by busy people.

Common issues include:

• Forgetting the 31 January payment deadline
• Missing payments on account
• Claiming personal expenses as business expenses
• Not claiming legitimate expenses
• Using bank deposits as income without checking transfers
• Forgetting CIS deductions
• Ignoring dividend income
• Missing rental income from part of the year
• Treating capital purchases incorrectly
• Forgetting student loan repayments
• Using the wrong accounting basis
• Not keeping records for long enough
• Filing without checking the calculation

A good accountant is not there just to “fill in the boxes”. The real value is in reviewing the figures, asking the right questions, spotting gaps, and helping you avoid problems before HMRC does.

What happens if you miss the deadline?

If you miss the online filing deadline, HMRC can charge an automatic £100 late filing penalty. This can apply even if there is no tax to pay.

Further penalties can apply if the return remains outstanding, and late payment can trigger interest and additional penalties. HMRC can also charge penalties linked to unpaid tax at later points.

The practical point is simple: do not ignore the return because you cannot pay the full bill. Filing and paying are connected, but they are not the same thing. If you cannot pay everything by the deadline, submit the return anyway and speak to HMRC or your accountant about payment options.

Should you file your tax return yourself or use an accountant?

Some people can file a straightforward return themselves. If you have one income source, tidy records and no unusual tax points, HMRC’s online system may be enough.

But professional help is worth considering if:

• You are self-employed with growing income.
• You have rental property.
• You are a company director.
• You have dividends, benefits or a director’s loan account.
• You are a CIS subcontractor.
• You have capital gains.
• You have foreign income.
• You are close to a higher-rate or additional-rate tax band.
• You are affected by payments on account.
• Your records are incomplete.
• You want advice, not just filing.

An accountant can help you understand the figures, claim the correct expenses, avoid common errors, and plan ahead.

Self assessment tax return UK 2026: how Bloom Financials can help

Bloom Financials supports individuals, landlords, sole traders, contractors, freelancers, company directors and small business owners with accounting, tax planning, bookkeeping, statutory compliance and Self Assessment support.

For your 2025/26 return, Bloom Financials can help with:

• Checking whether you need to file
• Registering for Self Assessment
• Finding or applying for your UTR number
• Reviewing income sources
• Organising bookkeeping records
• Checking allowable expenses
• Preparing the Self Assessment tax return
• Calculating tax and National Insurance
• Reviewing payments on account
• Advising on HMRC deadlines
• Supporting landlords and property income reporting
• Supporting sole traders and CIS subcontractors
• Helping directors report salary, dividends and benefits
• Planning ahead for Making Tax Digital for Income Tax

The aim is not to make tax feel complicated. It is to make sure the return is accurate, compliant and submitted with confidence.

Making Tax Digital for Income Tax: why 2025/26 matters

Making Tax Digital for Income Tax is becoming increasingly important for sole traders and landlords.

From 6 April 2026, sole traders and landlords with qualifying income from self-employment and property over £50,000 will need to use Making Tax Digital for Income Tax. This means keeping digital records, using compatible software, sending quarterly updates to HMRC, and finalising the year-end position.

Even if you are not immediately within the first threshold, it is sensible to improve your record-keeping now. Clean digital records make Self Assessment easier, reduce missed expenses, and help you understand your tax position before the deadline arrives.

Bloom Financials can help you review your current setup and decide whether cloud accounting software, bookkeeping support or MTD-compatible processes would be useful.

Final checklist before submitting to HMRC

Before you submit your 2025/26 Self Assessment tax return, check:

• Your UTR and personal details are correct.
• All income sources have been included.
• Employment income matches your P60 or P45.
• CIS deductions have been entered correctly.
• Rental income and expenses have been reviewed.
• Dividends and savings interest have been included.
• Allowable expenses are supported by records.
• Pension contributions and Gift Aid have been considered.
• Student loan details are correct.
• Payments on account have been checked.
• The final tax calculation looks reasonable.
• You have saved a copy of the return and submission receipt.

A final review can save a lot of stress later.

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