11 September 2026 · 6 min read · Bruce Thomas
This is the question we are asked more than any other, usually by someone who has started earning a bit on the side and is not sure whether HMRC needs to hear about it. The answer depends on the type of income and how much of it there is, not on whether you consider yourself to be running a business.
The usual reasons a return is needed
- You are self-employed as a sole trader and your gross trading income is above the trading allowance
- You are a partner in a business partnership
- You receive rental income from property above the property allowance
- You have untaxed income such as dividends, tips, commission or interest that your tax code does not deal with
- You have to pay the High Income Child Benefit Charge
- You have capital gains to report, for example from selling a second property or shares
- HMRC has issued you with a notice to file — once issued, the return is due even if your tax bill is nil
The trading allowance and casual income
There is an allowance for small amounts of casual trading income. If your gross income from self-employment sits below it, you generally do not need to register or report it. Above it, you must register, although you can still choose to deduct the allowance instead of your actual expenses if that gives a better result.
The important word is gross. It is measured on what you receive before costs, so a business turning over more than the allowance but making very little profit still needs to file.
Being a director is not automatically a trigger
Being a company director does not, by itself, create a Self Assessment obligation. What usually does is the dividend income that goes with it, or a notice to file that HMRC has already issued. If you have had a notice, filing is not optional — it has to be submitted or formally withdrawn by HMRC.
Registration deadlines worth diarising
- Register by 5 October following the end of the tax year in which the income arose
- File online and pay by the following 31 January
- Paper returns are due earlier, by 31 October
- Payments on account, where they apply, fall on 31 January and 31 July
If you have already missed a year
Late registration is common and it is fixable. Register, file the outstanding returns and pay what is owed. Penalties and interest apply, but they are far smaller when you come forward than when HMRC opens an enquiry. If the underpayment was genuinely a mistake rather than deliberate, say so clearly in the disclosure.
If you are not sure whether a return is needed for your situation, it takes a short conversation to settle rather than a guess that has to be unwound later.
This article is general guidance for business owners in the UK and does not amount to advice for your particular circumstances. Tax rules change and the right answer depends on your figures — please take specific advice before acting.
