Register for Self Assessment by 5 October — or Pay for It Later

If you started working for yourself in the 2025/26 tax year, HMRC needs to know by 5 October 2026. Miss it and the penalty is based on the tax you owe, not a flat fee.

Two deadlines get confused every year, and only one of them is in January. Before you can file a tax return you have to exist as a Self Assessment taxpayer, and telling HMRC that is a separate job with its own date: 5 October following the end of the tax year you started in.

Who this applies to

If you started working for yourself at any point between 6 April 2025 and 5 April 2026, your deadline is 5 October 2026. The same applies if you started letting a property, or picked up untaxed income that takes you past the £1,000 trading allowance.

It catches more people than expected. Driving for Uber or Bolt is self-employment. So is selling online beyond the point of clearing out a loft, and so is a weekend trade alongside a PAYE job. The employer deducting tax from your salary does not deal with any of it.

Registering is not filing

Registration gets you a Unique Taxpayer Reference, the ten-digit number every return needs. It arrives by post, usually in about ten working days and sometimes considerably longer. You cannot file without it.

This is the practical reason to register now rather than in January. Every year people sit down in the third week of January, discover they have no UTR, and miss the filing deadline waiting for the postman. The £100 late filing penalty applies even where no tax is owed.

The penalty is not a flat fee

Late registration is charged as a failure to notify penalty, and it is a percentage of the tax you owe rather than a fixed sum. The percentage turns on two things: whether the failure was careless or deliberate, and whether you told HMRC or HMRC found you.

That second point is worth reading twice. An unprompted disclosure — you coming forward before any HMRC contact — attracts a much lower penalty, and where there is a reasonable excuse it can be reduced to nothing. Waiting to see whether you are noticed is the expensive option.

Register even if you owe nothing

A first year that made a loss, or profits below the personal allowance, still needs registering. The obligation is triggered by the source of income, not by whether tax falls due. Registering also starts your National Insurance record, which matters for the state pension even in a year you pay nothing.

What to do this month

Register online through GOV.UK, keep the UTR somewhere you will find it in January, and start keeping records now rather than reconstructing them later. If you are not certain whether you need to register, ask before October rather than after — the answer takes five minutes and the difference in penalty can run to hundreds of pounds.

Getting help with this

AJH Accountants handles self assessment for clients across the UK. We deal with HMRC, tell you what you can claim, and file on time, so nothing rests on you remembering a deadline. See how the service works, ask for a fixed-fee quote, or call the office on 01204 840303.

Originally published at ajhaccountant.co.uk/blog/register-for-self-assessment-october-deadline.