Every year, over 12 million people in the UK are required to file a self assessment tax return with HMRC, and a significant number miss the deadline or get it wrong. This guide explains who must file, what deadlines apply, what happens if you are late, and how to decide whether to file yourself or hand it to an accountant.

Why self assessment affects more people than they realise

Self assessment is not just for the self-employed. According to HMRC, you must file a tax return if you are a sole trader earning more than £1,000 in a tax year, a partner in a business partnership, or if you receive untaxed income such as rental income, dividends, or foreign earnings. Many people discover they are required to file only after receiving a letter from HMRC, which is always an uncomfortable moment.

In the 2024-25 tax year, over 12 million customers were expected to file, and around 1 million missed the deadline. That figure matters because missing the deadline triggers automatic penalties regardless of whether you owe any tax. If you are unsure whether you need to file, the starting point is to check whether any of HMRC’s criteria apply to your income for that tax year.

Where most people go wrong

The self assessment system asks the right questions, but it is easy to answer them incorrectly if you do not know what counts as income, which expenses are allowable, and what reliefs you are entitled to claim. Most errors are not the result of dishonesty. They come from confusion about what to include.

Missing the registration deadline

HMRC requires you to notify them by 5 October following the end of the tax year in which you first needed to file. If you became self-employed in the 2025-26 tax year, you must register by 5 October 2026. Registering late does not remove the obligation to file, and it can lead to penalties stacking up before you have even submitted anything.

Underclaiming allowable expenses

Sole traders and landlords are entitled to deduct legitimate business expenses before calculating their taxable profit. Many people underclaim because they are not confident about what is allowable, particularly for home working costs, mileage, and professional subscriptions. Leaving out valid expenses means paying more tax than you legally owe.

What to do (step by step)

Filing a self assessment tax return follows a clear sequence. Each step builds on the last, so rushing or skipping steps is usually what creates the problems people bring to an accountant after the fact.

1. Register for self assessment with HMRC if you have not already done so. You can do this online via your Government Gateway account. Do this as early as possible, because HMRC can take up to 10 working days to send your Unique Taxpayer Reference (UTR), which you cannot file without.
2. Gather your records for the tax year in question. This includes income from all sources (self-employment, rental, dividends, interest), receipts for allowable expenses, P60 or P45 if you also had employment income, and records of any capital gains if applicable. The more organised your records, the faster and more accurate the filing process.
3. Complete and submit your return via HMRC’s online portal or compatible software before the deadline. The online filing deadline is 31 January following the end of the tax year. Any tax owed must also be paid by 31 January. If you expect to owe more than £1,000, HMRC may also require payments on account for the following year.

If you are filing for the 2025-26 tax year, the return can be submitted from 6 April 2026, and the deadline to pay and file is 31 January 2027. Filing early means you know your tax bill sooner and have more time to plan payment if needed.

Costs and what to expect

The cost of filing a self assessment tax return depends on whether you do it yourself or work with an accountant. DIY filing is free in terms of HMRC fees, but the time involved, the risk of errors, and the possibility of missing allowances can make it more expensive in practice. An accountant’s fee is often recovered many times over through correctly claimed expenses and reliefs that a first-time filer would miss entirely.

How to get started today

If your self assessment return is already overdue, the first step is to file as quickly as possible. Late filing penalties start at £100 and increase after 3 months with daily charges of £10 per day up to £900, then a further 5 percent of tax owed or £300 after 6 months, whichever is greater. Filing late does not make the penalties disappear, but it does stop them growing. If you are not yet registered, notify HMRC first and then gather your records.

• Check whether you need to register for self assessment by reviewing HMRC’s criteria at gov.uk/self-assessment-tax-returns and register immediately if you meet any of the conditions.
• Collect your income and expense records for the relevant tax year, then decide whether to file yourself or book a free call with David to go through your situation and get a fixed-price quote.