What is a payment on account?

It is an advance payment towards next year's tax, made in two instalments on 31 January and 31 July. It applies when your bill is over 1,000 pounds and most of it was not taxed at source, which is why a first January bill can be roughly 150% of what you expected.

What happens if I miss the Self Assessment deadline?

There is an automatic 100 pound penalty even if you owe nothing, then 10 pounds a day after three months up to 900 pounds, with further charges at six and twelve months. Paying late is separate and adds interest plus surcharges.

Self Assessment UK: the 31 January Deadline & Who Files

Q: When is the Self Assessment deadline?

For the 2025/26 tax year, the online return and payment are due by 31 January 2027. Paper returns are due by 31 October 2026, and if it is your first time you should register by 5 October 2026.

Q: Do I need to file a Self Assessment?

Usually yes if you were self-employed with turnover over 1,000 pounds, had rental income over 1,000 pounds, had dividends or savings interest above your allowances that is not taxed at source, made capital gains above 3,000 pounds, or you or your partner earned over 60,000 pounds while claiming Child Benefit. A large PAYE-only salary no longer requires a return on its own.

Q: Do I have to declare dividends and savings interest?

If they are above your allowances (500 pounds for dividends, and your Personal Savings Allowance for interest) and are not already collected through your tax code, then yes, they need to go on your return.

Q: Does WealthR file my tax return?

No. WealthR is a planner, not a filing tool. You file on gov.uk or through an accountant. It keeps your income, pensions and adjusted net income in one place so the return is quick to complete, and its UK tax calculators are free to use.

Every January, a few million people sit down at 11pm on the 30th, discover they can't remember their Government Gateway password, and file a tax return in a cold sweat. I built the calculators a lot of them end up Googling that night — the adjusted-net-income one, the child benefit charge one, the dividend one. So here's the plain-English version: what the deadline actually asks of you, who genuinely has to file, and where people quietly overpay. I'm a founder, not an accountant, so treat this as a map, not advice — the final word is always gov.uk or a good accountant.

The dates that actually matter

Self Assessment runs a year behind. The return you file by 31 January 2027 covers the 2025/26 tax year — 6 April 2025 to 5 April 2026. The dates that catch people out:

5 October — register for Self Assessment if it's your first time. Leave it and you can still file, but you're already on the back foot.
31 October — deadline for a paper return. Almost nobody files on paper now, but if you do, this is your date, not January.
30 December — file online by now and, if you owe under £3,000 and have a PAYE income, you can ask HMRC to collect the tax through next year's tax code instead of in one lump. Genuinely handy, widely missed.
31 January — the big one. File online and pay what you owe for 2025/26, plus your first payment on account for 2026/27 (more on that below).
31 July — second payment on account, if you make them.

Do you even have to file?

Plenty of people file who don't need to, and plenty who should, don't. You generally need to send a return if any of these were true in 2025/26:

You were self-employed and turned over more than £1,000 (the trading allowance), or were a partner in a partnership.
You had rental income over £1,000.
You, or your partner, earned over £60,000 and one of you claimed Child Benefit — the High Income Child Benefit Charge. Work out the charge →
You had dividends above the £500 allowance, or savings interest above your Personal Savings Allowance, that isn't already taxed through your code. Dividend tax → · Savings interest →
You made capital gains above the annual exempt amount (£3,000).
You had untaxed income — a side hustle, foreign income, tips, a bit of freelancing on top of a job.

One myth worth killing: earning a big salary on PAYE alone no longer forces you into Self Assessment — HMRC dropped that trigger. It's the untaxed stuff that pulls you in.

The bits that quietly cost money

The return itself is mostly typing. The money is in the boxes people skip:

Higher-rate pension relief. If you pay into a personal pension and you're a higher-rate taxpayer, the extra 20–25% relief usually isn't automatic — you claim it back on the return. Miss it for a few years and it's real money left on the table.
Adjusted net income. The £100,000 personal-allowance taper, the £60k child benefit charge and pension relief all hinge on one number — your adjusted net income — and pension contributions and Gift Aid pull it back down. Get it wrong and you either overpay or walk into a charge. Check your adjusted net income →
Marriage Allowance. If one of you earns under the personal allowance and the other is a basic-rate taxpayer, you can move a slice of allowance across. Small, but free. Marriage Allowance →

Payments on account — the January shock

This is the one that blindsides first-time filers. If your bill is over £1,000 and most of it wasn't already taxed at source, HMRC asks you to pay next year's tax in advance, in two instalments. So your first January can land as roughly 150% of the bill you expected: the year you owe, plus half of next year on top. It isn't a penalty and it evens out — but nobody warns you, so budget for it.

What it costs to miss the deadline

HMRC's late-filing penalties don't care why:

£100 the moment you're a day late — even if you owe nothing.
After 3 months, £10 a day, up to £900.
At 6 and 12 months, further charges (the greater of 5% of the tax or £300 each time).
Late payment is separate — interest, plus 5% surcharges at 30 days, 6 months and 12 months.

Filing on time and paying late is far cheaper than the other way round, so if the money's tight, file anyway and talk to HMRC about a payment plan.

Free calculators for the tricky bits

Each figure below is one people get wrong on the return, and each has its own free calculator — no account needed. They work out the exact number HMRC wants, so you can copy it straight across:

Adjusted Net Income Calculator — the one number that drives the £100,000 personal-allowance taper, the £60,000 child-benefit charge and your higher-rate pension relief. Get this right and most of the rest falls into place.

Child Benefit charge (HICBC) Calculator — if you or your partner earn over £60,000 and someone claims Child Benefit, this is the charge you'll owe on the return.

Dividend Tax Calculator — the tax on dividends above the £500 allowance, worked out at your band.

Savings Interest Tax Calculator — whether your interest tips over your Personal Savings Allowance, and what it costs if it does.

Marriage Allowance Calculator — the slice of unused personal allowance you can move to a basic-rate partner. Small, but free.

How to make next January boring

The reason January is stressful is that people reconstruct a whole year in one night. The fix isn't a clever trick — it's knowing your numbers before you sit down: what you earned, what's already been taxed, what your pension took, where your adjusted net income landed. That's most of what WealthR is for. It doesn't file your return — you do that on gov.uk or with an accountant — but it keeps the figures in one place all year so the form is a copy-out, not an archaeology dig. Every calculator linked above is free, no account needed.

The free UK money tracker

Make next January a copy-out job

WealthR keeps your income, pensions, dividends and adjusted net income in one place, updated month by month — so when the return is due, the numbers are already sitting there instead of scattered across a year of statements. It will not file for you, but it turns the worst night of the tax year into ten minutes of copying. Free to start, no bank linking, built in the UK.

Start tracking free →

General information for the 2025/26 UK tax year, not tax advice — thresholds change, so check gov.uk or speak to an accountant for your own situation.

Frequently asked

When is the Self Assessment deadline?
For 2025/26, the online return and payment are due by 31 January 2027. Paper returns by 31 October 2026; register by 5 October 2026 if it's your first time.

Do I need to file a Self Assessment?
Usually yes if you were self-employed over £1,000, had rental income, had dividends or savings interest above your allowances that isn't taxed at source, made capital gains over £3,000, or you or your partner earned over £60,000 while claiming Child Benefit. A big PAYE-only salary no longer requires one on its own.

What's a payment on account?
An advance payment toward next year's tax, in two instalments (31 January and 31 July), due if your bill tops £1,000 and most wasn't taxed at source. It's why a first January bill can be about 150% of what you expected.

What happens if I miss the deadline?
An automatic £100 penalty even if you owe nothing, then £10/day after three months up to £900, with further charges at six and twelve months. Late payment adds interest and surcharges on top.

Do I have to declare dividends and savings interest?
If they're above your allowances (£500 for dividends, your Personal Savings Allowance for interest) and not already collected through your tax code — yes.

Does WealthR file my tax return?
No. WealthR is a planner, not a filing tool — you file on gov.uk or via an accountant. It keeps your income, pensions and adjusted net income in one place so the return is quick to fill in, and its tax calculators are free.