There are two Self Assessment deadlines, and most people have only heard of one of them.
The well-known one is 31 January — the date your tax return must be filed and your tax paid. The one that catches people out comes earlier: the deadline to tell HMRC you need to file at all.
The notification deadline
If you had untaxed income during a tax year and you are required to file a return, you generally need to notify HMRC by 5 October following the end of that tax year.
So for the 2025/26 tax year, which ran from 6 April 2025 to 5 April 2026, the notification deadline was 5 October 2026. The filing deadline for that same year is 31 January 2027.
Who this applies to
- Sole traders with gross trading income over £1,000
- Landlords with taxable rental income
- People with other untaxed income that needs reporting
- People receiving dividend income who meet the relevant Self Assessment criteria
- Partners in a business partnership
The important point is that being employed does not mean you are exempt. You can have a PAYE salary and still have a Self Assessment obligation because of other income.
If you have missed 5 October
Registering late does not automatically mean a fixed penalty.
If you register after 5 October but pay all the tax you owe by the 31 January payment deadline, HMRC says you may not receive a failure-to-notify penalty. If tax remains unpaid, however, a failure-to-notify penalty may apply.
So if you have missed it, the sensible response is to register now and make sure the tax is paid on time — not to wait, and certainly not to assume the problem has gone away.
Why not just leave it all until January?
Because registering takes time. You need to register with HMRC and obtain your Unique Taxpayer Reference (UTR), and you cannot file without it. Leaving everything to the last minute can make it much harder to get the return completed and submitted on time.
If you miss the online filing deadline of 31 January, the initial late-filing penalty is £100, with further penalties and interest if the delay continues.
There is also a practical reason. Preparing a return in January, from records that have not been kept up during the year, is how people end up guessing, missing allowable expenses and paying more tax than they needed to.
The payment side is separate
Registering and filing are not the same as paying. If this is your first return, be aware that your January bill may include more than you expect — payments on account can add half as much again on top of the tax for the year just completed.
A note on Making Tax Digital
For many sole traders and landlords, the picture has changed. Making Tax Digital for Income Tax applies from 6 April 2026 to those with qualifying income over £50,000, bringing digital record keeping and quarterly updates alongside the year-end return.
If that applies to you, keeping records up to date during the year stops being good practice and becomes a requirement.
In short
If you started a business, began receiving rental income, or had another source of untaxed income, check whether you need to register. It takes very little time to deal with now, and it avoids a far more stressful January.
If you are not sure whether any of this applies to you, book a consultation.
This article sets out general information and is not advice for your circumstances.
