When is Self Assessment due, and why is the first bill so large?

For the 2025 to 2026 tax year, an online Self Assessment return and any tax owed are both due by 11:59pm on 31 January 2027. A paper return was due by 31 October 2026. If this is your first return, the bill may be larger than you expect, because payments on account can add half as much again, a point the example further down works through.

Sole traders must register for Self Assessment if they earn more than £1,000 in a tax year, and GOV.UK's checker covers the other reasons you might need to file. What follows reads as a calendar. Start at the date nearest today.

By	What
6 April 2026	Tax year ended on the 5th. Returns can be sent from now.
5 October 2026	Last date to tell HMRC you need to file, if you have never filed or did not file for 2024 to 2025.
31 October 2026	Paper return due.
30 December 2026	Online return due if you want tax collected through your tax code.
31 January 2027	Online return due. Balance of tax due. First payment on account for 2026 to 2027 due, if one applies.
31 July 2027	Second payment on account due, if one applies.

October: registration

The registration date for this return was 5 October 2026. Telling HMRC later could bring a penalty, but it is better done now than in January. Per GOV.UK's deadlines page, if you register after that date, HMRC gives you a return deadline three months from the date on its letter or email. The payment date stays at 31 January 2027. Allow time for HMRC's reply, because GOV.UK has a separate tool for checking when you can expect one.

November to January: filling it in

Self-employment income goes on supplementary pages. The short form, SA103S, is aimed at people with relatively simple affairs and turnover below the VAT threshold for the year. You enter totals for income and expenses and you do not attach the receipts, though you must keep them, for the period described in our guide to records.

This is the point where messy books cost you time. If you need to rebuild a year from statements, sort the bank lines first into business and personal, then into the categories in what sole traders can claim, and check whether your choice of cash basis or traditional accounting changes which year each item falls in.

31 January: the two-part bill

Here is the surprise most new filers meet. Payments on account are advance payments towards your next bill, including Class 4 National Insurance. Each is half of last year's bill, due on 31 January and 31 July. They do not apply if last year's bill was under £1,000, or if more than 80% of your tax was already paid outside Self Assessment, for instance through your tax code.

Take a first-time filer with no other income, living outside Scotland, whose whole profit is £30,000 in the 2026 to 2027 tax year. Using the current Income Tax rates and Class 4 rates:

Working	Dr. due (£)
Income Tax: (£30,000 − £12,570 Personal Allowance) × 20%	3,486.00
Class 4 National Insurance: (£30,000 − £12,570) × 6%	1,045.80
Bill for the year	4,531.80
First payment on account for 2027 to 2028 (half)	2,265.90
Payable by 31 January 2028	6,797.70
Payable by 31 July 2028 (second half)	2,265.90

So a £4,531.80 tax bill means £6,797.70 leaving the account in one go, with more due six months later. The second-year balance is then adjusted against what you actually owe. People in Scotland pay Scottish Income Tax, and the bands are different, so the Income Tax line above would not match.

If your profits are falling, you can ask HMRC to reduce your payments on account, online or on form SA303. You have to state what you expect to owe, and you will be charged interest if the real bill turns out higher.

After 31 January: what lateness costs

The GOV.UK penalty table treats late returns and late payment as separate problems, and you can be charged for both.

How late	Late return	Late payment
Straight away	£100	Interest on the amount owed
30 days		5% of tax unpaid
3 months	£10 a day, up to £900
6 months	5% of tax due or £300, whichever is greater	A further 5%
12 months	A further 5% or £300, whichever is greater	A further 5%

If you cannot pay in full, it still pays to send the return on time, because the £100 and the daily charges attach to the filing. Contact HMRC early about the payment. Debt worries are worth discussing with an adviser before the 30-day mark.

Three small checks before you press submit

First, confirm you actually need to file. HMRC may have written asking for a return, in which case you must send one by the date in its letter, even if you think nothing is owed. If you believe you no longer need to file, contact HMRC and explain why, because silence does not cancel the request. The checker linked above does not send your answers to HMRC, so there is no harm in trying it.

Second, check what HMRC thinks you have already paid. In your online account, choose your latest return and then "View statements" to see payments made and those still due. Any penalty charged has to be paid within 30 days of the date on the notice, so look there too if you were late last year.

Third, compare your totals with your books one last time. The sum on the return should trace back to your cash book or software, and any figure you had to estimate should be declared as an estimate or as provisional when you file.

Looking ahead: this calendar is changing

Some sole traders now also send quarterly updates under Making Tax Digital for Income Tax. It does not move the payment dates. For a first return with several income sources, or a sum that you cannot make sense of, an accountant is likely to be money well spent, and HMRC's guidance remains the last word on the dates.

General education, not personal advice. Links in this article go to the GOV.UK pages we read while writing it, opened again on 8 November 2026. Rates and rules change, so check the current figure on GOV.UK before relying on it.