Every year, millions of UK residents need to complete a self assessment tax return. Whether you are self-employed, a landlord, a company director, or someone with untaxed income, your self assessment tax return must be filed accurately and on time. Miss a deadline or make a mistake and HMRC will fine you, even if you do not owe any tax at all.
At Sepera Accounting, we help sole traders, landlords, and limited company directors across London and Stockport file their self assessment tax return every year. This guide covers everything you need to know for the 2025/26 tax year, including key deadlines, allowable expenses, and what happens if you file late.
What Is a Self Assessment Tax Return?
A self assessment tax return is the form you submit to HMRC to declare your income and calculate how much tax you owe. Unlike employees on PAYE, where tax is deducted automatically from wages, you are responsible for reporting your own income and calculating your own tax liability.
Your self assessment tax return covers income from multiple sources, including self-employment profits, rental income, dividends, savings interest, capital gains, and foreign earnings. It is called self assessment because you assess your own liability, rather than having an employer or HMRC do it for you.
Who Needs to File a Self Assessment Tax Return?
You need to file a self assessment tax return for the 2025/26 tax year if any of the following apply:
- You were self-employed as a sole trader and earned more than £1,000
- You are a partner in a business partnership
- You received rental income from a UK or overseas property
- You are a company director in most circumstances
- Your total income exceeded £100,000
- You received Child Benefit and your income was above £60,000
- You had untaxed income above £2,500 from savings or investments
- You earned dividends above the £500 dividend allowance
If you are not sure whether you need to file, HMRC has a free eligibility checker at GOV.UK. Our accounting for self-employed service includes an eligibility check as standard.
Self Assessment Tax Return Deadlines for 2025/26
Getting your self assessment tax return in on time is not optional. HMRC charges automatic penalties for late filing, regardless of whether you owe any tax.
The key dates for the 2025/26 tax year are:
- 5 October 2026: register for self assessment if you are filing for the first time
- 31 October 2026: paper return deadline
- 31 January 2027: online return deadline and payment deadline
- 31 July 2027: second payment on account deadline
Most people file their self assessment tax return online. This gives you three extra months compared to the paper deadline, and it is far faster.
You can file your self assessment tax return from 6 April 2026, as soon as the 2025/26 tax year closes. Filing early means you know exactly what you owe well in advance of the January deadline.
What Do You Need to Complete Your Self Assessment Tax Return?
Before you start, gather everything you need. Having your records in order before you begin your self assessment tax return saves time and reduces the risk of errors.
For self-employment income:
- Total turnover and income received
- All business expense records
- Capital allowances details
- Your Unique Taxpayer Reference (UTR)
For rental income:
- Total rent received
- Mortgage interest paid (restricted relief applies)
- Letting agent fees, repairs, and other allowable costs
For other income:
- P60 or P45 from any employment
- Dividend vouchers from any company shareholdings
- Bank and savings interest statements
- Capital gains records including sale price and acquisition cost
Our limited company accounting service covers full record gathering and submission for directors.
Allowable Expenses on Your Self Assessment Tax Return
Claiming the right expenses on your self assessment tax return is one of the most effective ways to reduce your tax bill. Many sole traders underclaim, paying more tax than they need to.
Common allowable expenses include:
- Office costs such as stationery, postage, and software subscriptions
- Business travel and mileage at HMRC-approved rates
- Staff and subcontractor costs
- Marketing, advertising, and website costs
- Professional fees including accountancy and legal advice
- A proportion of home costs if you work from home
- Business-related training and development
Expenses must be incurred wholly and exclusively for business purposes. Mixed personal and business use requires careful apportionment. HMRC publishes the full list of allowable expenses at GOV.UK.
How to File Your Self Assessment Tax Return
Filing your self assessment tax return online is the most straightforward option for the vast majority of people. Here are the steps:
- Register for self assessment at GOV.UK if you have not filed before. You will need your National Insurance number and a Government Gateway account.
- Receive your UTR. HMRC posts your Unique Taxpayer Reference within 10 working days of registration.
- Gather your records as listed above.
- Log in to HMRC online services and complete your return.
- Submit by 31 January 2027 for the 2025/26 tax year.
- Pay any tax owed by the same deadline to avoid interest and surcharges.
If your tax affairs are complex, or you simply do not want the hassle, an accountant can manage the entire process. Our self assessment service covers preparation, review, and online submission.
Self Assessment Tax Return Penalties
HMRC applies automatic penalties for a late or incorrect self assessment tax return. Here is what to expect if you miss the deadline.
Late filing penalties:
- 1 day late: automatic £100 penalty
- 3 months late: £10 per day for up to 90 days (up to £900)
- 6 months late: 5% of tax owed or £300, whichever is greater
- 12 months late: a further 5% or £300
Late payment penalties:
- 30 days late: 5% of tax owed
- 6 months late: a further 5%
- 12 months late: a further 5%
Interest also accrues on any unpaid tax from 1 February. Penalties on a single self assessment tax return can accumulate very quickly, and HMRC charges them even where no tax is owed.
If you have a genuine reasonable excuse for late filing, you can appeal. The success rate for appeals is low, however. The only reliable protection is to file your self assessment tax return on time.
How Sepera Accounting Helps With Your Self Assessment Tax Return
Filing a self assessment tax return takes time and expertise. Errors can trigger HMRC enquiries, and missed expenses mean paying more tax than necessary.
Sepera Accounting is an AAT-licensed, ACCA-affiliated practice with over 30 years of combined experience. We manage the entire process for sole traders, landlords, and company directors across London and Stockport. Every self assessment tax return we file is reviewed by a qualified accountant before submission.
We check for missed expenses, planning opportunities, and HMRC compliance risks as standard. We also liaise directly with HMRC on your behalf if any questions arise.
Get in touch via our contact page or call us on +44 20 7071 8676. For official HMRC guidance, visit the self assessment pages on GOV.UK.
Frequently Asked Questions: Self Assessment Tax Return
What is a self assessment tax return? A self assessment tax return is the annual form you submit to HMRC to declare your income and calculate your tax liability. It is required for self-employed people, landlords, company directors, and anyone with untaxed income above HMRC thresholds.
Who needs to file a self assessment tax return? You need to file if you are self-employed and earn more than £1,000, a landlord, a company director, earn over £100,000, or have untaxed income above £2,500. HMRC has a free tool at GOV.UK to help you check.
What is the self assessment tax return deadline for 2025/26? The online deadline is 31 January 2027. The paper deadline is 31 October 2026. New filers must register by 5 October 2026.
What happens if I miss the self assessment tax return deadline? HMRC charges an automatic £100 penalty for a late self assessment tax return, even if you owe nothing. Penalties then increase at three months, six months, and twelve months, plus interest on unpaid tax from 1 February.
Can I file my self assessment tax return early? Yes. You can file from 6 April 2026 for the 2025/26 tax year. Filing early is recommended as it gives you time to budget for any tax owed.
What expenses can I claim on a self assessment tax return? Allowable expenses include office costs, business travel, staff costs, marketing, professional fees, and a proportion of home costs. All expenses must be wholly and exclusively for business use.
Do I need an accountant for my self assessment tax return? You are not required to use one, but a qualified accountant ensures accuracy, identifies missed expenses, and reduces the risk of HMRC penalties or enquiries. Many people find it saves both money and time.
How do I register for a self assessment tax return? Register online at GOV.UK using your National Insurance number and a Government Gateway account. Your UTR number arrives by post within 10 working days. For 2025/26, the registration deadline is 5 October 2026.
What is a payment on account? If your tax bill exceeds £1,000, HMRC requires two advance payments towards the following year. Each is 50% of your current year’s bill. They are due 31 January and 31 July.
Can Sepera Accounting file my self assessment tax return for me? Yes. We manage preparation, review, and online submission for clients across London and Stockport. Contact us to arrange a quote.
This article provides general guidance on the self assessment tax return process for 2025/26. Tax rules and deadlines change regularly. Please contact us for advice tailored to your specific circumstances.
