Hamster blog

Self Assessment deadline 31 January 2027: what's due and what to do

By the Hamster team · Published 25 October 2026

Last checked: 25 October 2026 against GOV.UK · Figures are for the 2026–27 tax year unless stated

What is due on 31 January 2027

31 January is the main Self Assessment deadline for anyone with side hustle income from the 2025–26 tax year, which ran from 6 April 2025 to 5 April 2026. By 11:59pm on 31 January 2027 you need to:

• file your online tax return for 2025–26
• pay the tax you owe for 2025–26, known as the balancing payment
• pay your first payment on account for 2026–27, if you have to make them

Filing and paying are separate obligations with separate penalties. Filing on time but paying late still costs you, and so does paying on time but filing late.

The dates around it

• 5 October 2026: the deadline to register for Self Assessment if 2025–26 was your first year over the £1,000 trading allowance.
• 31 October 2026: the deadline for paper returns. Most people file online instead.
• 30 December 2026: file online by this date and, if you already pay tax through PAYE and owe less than £3,000, HMRC can collect the bill through your tax code over the following year.
• 31 January 2027: online return and payment deadline.
• 31 July 2027: the second payment on account for 2026–27, if you make them.

If you missed 5 October, you are not too late to file on time. Since September 2026, registering online puts your Unique Taxpayer Reference in your online account within 72 hours. Our guide to missing the registration deadline covers what to do.

Payments on account

If your Self Assessment bill is £1,000 or more, HMRC usually asks for payments on account: two advance payments towards next year, each half of this year's bill. You do not have to make them if your bill was less than £1,000, or if more than 80% of all the tax you owed was collected at source, for example through PAYE on your wages.

In your first year that means the January payment can be about one and a half times your tax bill. Payments on account explained goes through it in detail.

Penalties for missing the deadline

For a late return:

• £100 straight away, even if you owe no tax or pay on time
• £10 a day after 3 months, up to a maximum of £900
• the greater of 5% of the tax due or £300 after 6 months
• the greater of 5% of the tax due or £300 again after 12 months

For late payment, there are penalties of 5% of the unpaid tax at 30 days, 6 months and 12 months, plus interest on the amount owed.

A worked example

Amira works in a shop on £28,000 a year and grooms dogs at weekends. In 2025–26 she took £9,300 and spent £2,800 on clippers, shampoo, insurance and a grooming table.

• Profit: £9,300 − £2,800 = £6,500. Her costs beat the £1,000 allowance, so she claims them.
• Income Tax: the 2025–26 bands were the same as this year's, and her salary used her Personal Allowance, so the profit is taxed at 20%: £1,300.
• National Insurance: her profit is under £12,570, so there is no Class 4.
• Payments on account: her PAYE tax on £28,000 was £3,086, about 70% of her total tax of £4,386. That is under 80% and her bill is over £1,000, so she makes payments on account of £650 each.

So on 31 January 2027 Amira pays £1,950: £1,300 for 2025–26 plus £650 towards 2026–27. Another £650 is due on 31 July 2027.

If she filed on 10 February instead, she would get the £100 penalty. If the £1,300 was still unpaid 30 days after the deadline, she would owe a further 5%, or £65, plus interest.

Your checklist

1. Find your UTR and sign-in details. Your 10-digit Unique Taxpayer Reference is on letters from HMRC about Self Assessment.
2. Total your side hustle income for 6 April 2025 to 5 April 2026, across every platform, including tips and the value of anything you were paid in kind.
3. Total your costs, or decide to claim the £1,000 trading allowance instead. You cannot do both. Our guide to allowable expenses covers what counts.
4. Use the right mileage rate. Business miles driven before 6 April 2026 are 45p a mile for the first 10,000, not the newer 55p.
5. Have your employment details to hand, such as your P60, if you also have a job.
6. Decide on voluntary Class 2. If your 2025–26 profit was under £6,845, you can choose to pay Class 2 National Insurance to protect your State Pension record. You cannot pay it voluntarily through Self Assessment after 31 January 2027.
7. File, then pay, leaving yourself time before the deadline. Our box-by-box guide to the SA103S self-employment pages walks through the form.

If you cannot pay in full

File on time anyway, so you avoid the late filing penalties. Then contact HMRC: you may be able to set up a payment plan to pay in instalments. HMRC checks whether the plan is affordable for you.

If you do not think you need to file

If HMRC has asked you to file a return for 2025–26 but you do not think you need to, the SA103S notes say you must tell HMRC by 31 January 2027 to avoid penalties.

Keeping track

The return is quickest when the year's figures already exist. Hamster is a free Android app for logging side hustle earnings and costs as they come in, with your estimated tax and National Insurance worked out as you go, so next January starts with the numbers ready.

This is general information, not personal tax advice. Rules and rates change — check the linked GOV.UK pages or speak to an accountant about your own situation.