Almost everything that goes wrong with a tax return goes wrong because filing and paying are treated as one event. They are two deadlines, and missing either has its own penalty.
Which tax year we are talking about
GOV.UK states it plainly: “The last tax year started on 6 April 2025 and ended on 5 April 2026.” Everything below is about that year.
Deadline one: getting the return in
- On paper, HMRC must receive your paper tax return by 11:59pm on 31 October 2026, or you will get a late filing penalty.
- Online, you must submit your online tax return by 11:59pm on 31 January 2027, or you will get a late filing penalty.
Those are two different dates for the same return, three months apart, decided purely by how you send it.
Deadline two: paying what you owe
GOV.UK: “You need to pay your Self Assessment tax by 11:59pm on 31 January 2027 or you'll get a penalty.” And, for anyone who filed on paper in October and assumed that was that: “You must still pay the tax you owe by 11:59pm on 31 January 2027.”
This is the one that catches people. Filing early does not move the payment date, and filing early is still the right thing to do, because it tells you the number months before you have to find it.
Payments on account: the deadline in July
The January payment is not always the only one. GOV.UK notes a second payment deadline of 31 July for anyone who makes payments towards their bill in advance, these are known as payments on account.
If they apply to you, the year has two payment dates in it and neither is optional. It is the single most common reason somebody who thought they had settled up in January hears from HMRC in the summer.
Two dates people miss
30 December 2026
If you want to pay your Self Assessment bill through your tax code, GOV.UK says you must submit the return by 11:59pm on 30 December 2026. That is a month earlier than the ordinary online deadline, and it is the only way to spread the bill through PAYE.
5 October 2026
If you have not sent a return before, or you registered previously but did not need to send one for the 2024 to 2025 tax year, HMRC must be told by 5 October. GOV.UK also notes that if you register after 5 October 2026, HMRC will send a letter or email with a different deadline, three months from the date on it.
You can estimate the bill before you file
GOV.UK points out that you can estimate your Self Assessment tax bill before sending the return, and says why: to give you an idea of what your bill is likely to be, and to let you budget and pay on time.
That is the whole argument for preparing early. The return itself is a deadline; the number is a cash-flow problem, and you want the number as far in advance of the deadline as you can get it.
Two situations with different dates
If you cannot file online
Trustees of a registered pension scheme and non-resident companies cannot send a return online. GOV.UK gives them a deadline of 31 January 2027, on paper.
Partnerships with a company as a partner
If a partnership's accounting date falls between 1 February and 5 April and one of the partners is a limited company, GOV.UK sets the deadlines from the accounting date instead of the tax year: 12 months from it for online returns, nine months for paper.
What we would do about it
Get the return prepared in the autumn, so the figure is known, and then decide how to deal with it. That order round is the whole difference between a January that is administrative and a January that is stressful.
Deadlines move for some situations, and penalties have their own rules. Check the GOV.UK page below before you rely on any date here.