Self Assessment deadline 31 January 2027: what to file and pay

Updated 21 October 2026 · For the 2025/26 tax year (return due 31 January 2027)

If you earned money from a side hustle between 6 April 2025 and 5 April 2026, the date that matters is 11:59pm on Saturday 31 January 2027. That's the last moment to submit your 2025/26 online Self Assessment return and pay what you owe. Here's exactly what's due, what it costs to miss it, and a simple plan to get it done without the last-week panic.

The key dates for the 2025/26 tax year

Date	What happens
5 April 2026	2025/26 tax year ended.
5 October 2026	Deadline to tell HMRC you need to file (register for Self Assessment) if you're new or didn't file for 2024/25.
31 October 2026	Paper return deadline.
30 December 2026	File online by now if you want a bill under £3,000 collected through your PAYE tax code.
31 January 2027	Online return deadline, balancing payment for 2025/26, and first payment on account for 2026/27 (if applicable).
31 July 2027	Second payment on account for 2026/27 (if applicable).

You can file any time from 6 April 2026, so there's no advantage in waiting. Filing early doesn't mean paying early: the payment is still due on 31 January 2027, but you'll know the number months in advance.

What you actually pay on 31 January

For many first-time side hustlers the 31 January bill is bigger than expected, because it can include two things:

1. The balancing payment – your total 2025/26 Income Tax and Class 4 National Insurance, minus anything already paid on account.
2. Your first payment on account for 2026/27 – an advance equal to half of your 2025/26 bill.

Payments on account apply unless your last Self Assessment bill was under £1,000, or more than 80% of your tax was already collected at source (for example through PAYE). So a first-year bill of £1,400 could mean paying £2,100 on 31 January (the £1,400 plus a £700 payment on account), then another £700 on 31 July. If you expect your profit to fall, you can ask HMRC to reduce your payments on account.

What it costs to miss the deadline

• £100 automatic penalty the day after, even if you owe no tax or pay in full.
• After 3 months: £10 a day for up to 90 days (a maximum of £900).
• After 6 months: a further 5% of the tax due or £300, whichever is greater – and again at 12 months.
• Paying late: 5% of the unpaid tax at 30 days, 6 months and 12 months, plus interest.

If you have a genuine reasonable excuse you can appeal, but HMRC's bar is high. It's far cheaper to file on time, even with your best estimate, and amend later (you have 12 months from the deadline to amend).

A six-week countdown plan

Mid-December: get access sorted

Check you can sign in to your HMRC online account and that you have your 10-digit Unique Taxpayer Reference (UTR). If you've never registered, do it now – activation codes come by post and take time, and January is HMRC's busiest month.

Late December: pull your numbers together

Download your platform sales reports (Etsy, eBay, Vinted, Depop, PayPal, Stripe) for 6 April 2025 to 5 April 2026 and total your gross sales before fees. List your costs: materials, postage, packaging, platform and payment fees, mileage. Find your P60 if you also have a job.

Early January: allowance or expenses?

Decide whether the £1,000 trading allowance beats your actual expenses. If your costs are under £1,000, the allowance usually wins; if they're over £1,000, claiming expenses usually does. See our trading allowance vs expenses guide.

Mid-January: file

Fill in the self-employment pages (the online version of the SA103S – our box-by-box guide walks through it). Check the calculation HMRC shows before you submit, and save the submission receipt.

By 31 January: pay

Bank transfers using your UTR plus the letter K as the reference can take up to 3 working days if not sent by Faster Payments, so don't leave it to the final evening.

If you're behind

If you missed the 5 October 2026 registration deadline, register straight away. HMRC will send a letter with a filing deadline 3 months from the date of that letter, but your tax must still be paid by 31 January 2027 to avoid late payment penalties and interest – so estimate it and pay something on time. If you can't pay in full, you may be able to set up a Time to Pay arrangement online.

Making Tax Digital is coming – but not for this return

From 6 April 2026, Making Tax Digital for Income Tax applies to sole traders and landlords with qualifying income over £50,000, dropping to over £30,000 from April 2027 and over £20,000 from April 2028. It doesn't change how you file your 2025/26 return by 31 January 2027.

Frequently asked questions

When is the Self Assessment deadline for 2025/26?

The online return for the 2025/26 tax year (6 April 2025 to 5 April 2026) must reach HMRC by 11:59pm on 31 January 2027. Any tax you owe must also be paid by then. Paper returns were due by 31 October 2026.

What happens if I miss 31 January 2027?

You get an automatic £100 late filing penalty, even if you owe no tax. After 3 months £10 a day is added for up to 90 days (up to £900), with further penalties at 6 and 12 months. Late payment adds 5% of unpaid tax at 30 days, 6 months and 12 months, plus interest.

I haven't registered yet – what should I do?

The registration deadline for 2025/26 was 5 October 2026. Register now anyway: HMRC will give you a filing deadline 3 months from its letter, but the tax must still be paid by 31 January 2027.

What is due on 31 July 2027?

If payments on account apply to you, your second payment on account for 2026/27 is due by 31 July 2027.

Sources (GOV.UK, checked 21 October 2026)

• Self Assessment deadlines
• Self Assessment penalties
• Understanding your Self Assessment tax bill: payments on account
• Register for Self Assessment
• Making Tax Digital for Income Tax

General information, not tax advice. Rules can change – always check GOV.UK or speak to an accountant. January Sorted is not affiliated with HMRC.

More side hustle tax guides

• Do I need to tell HMRC about my side hustle?
• Etsy seller tax UK: what to declare for 2025/26
• SA103S explained box by box (2025/26)
• Trading allowance vs expenses: which saves you more tax?