Self-Assessment Tax Return: Deadlines, Penalties and How to Avoid Them

The self-assessment tax return is the one deadline every business owner knows — and dreads. But the rules are simple, the penalties are predictable, and with a little organisation you never need to face them.

The 2025/26 deadlines

The tax year runs 6 April 2025 to 5 April 2026. Your 2025/26 return is due:

• 31 October 2026 — paper returns (HMRC now expects most people to file online)
• 31 January 2027 — online returns, and the date any balancing payment is due
• 31 July 2027 — second payment on account (if applicable)

Online filing is the default for nearly everyone. If you're registered for Self Assessment, HMRC sends a notice to file each April — you don't have to wait for it.

What are the penalties for filing late?

HMRC's late-filing penalties are automatic and harsh:

• £100 immediately — even if you have no tax to pay
• £10 per day (up to £900) after 3 months late
• 5% of the tax due or £300 (whichever is greater) after 6 months
• A further 5% or £300 after 12 months, rising to 100% of the tax due in serious cases

So a return filed six months late can cost well over £1,000 before you've paid a penny of the underlying tax.

What about paying late?

Filing on time but paying late still hurts:

• Interest is charged on all late payments — the rate moves with the Bank of England base rate
• A 5% late-payment penalty applies if tax is still outstanding 30 days after the due date
• Further 5% penalties accrue at 6 and 12 months

From April 2027, late-payment penalties for income tax increase, so the cost of delay is going up, not down.

Payments on account — the trap within the trap

If your tax bill is over £1,000, HMRC normally requires payments on account: half of your previous year's tax is due on 31 January, and half on 31 July, before you've filed the current year's return. First-time filers are frequently caught out — the January bill can be roughly 150% of what they expected (current year's balancing payment plus the first payment on account).

Knowing this in advance is the single best way to avoid a cash-flow crisis in January.

How to avoid penalties entirely

1. File early. Even if you don't know your exact figures, your accountant can file provisional figures (with adjustments later) or simply get everything done by November.
2. Keep records as you go. Receipts, invoices and bank statements — cloud accounting software makes this automatic.
3. Put money aside monthly. Set aside roughly 20–30% of profit for tax, and you'll never scramble in January.
4. Use an accountant. Deadline management is part of the job. Most firms (including us) file returns early and handle HMRC correspondence on your behalf.

What if you've missed the deadline?

Don't panic and don't ignore it. The penalty regime rewards action:

• File as soon as possible — penalties stop accruing once you file
• If you have a reasonable excuse (serious illness, bereavement, HMRC system failure), you can appeal
• If the return was filed late but the tax was on time, some penalties may be reduced

An accountant can assess your position and often reduce or appeal penalties where there's a genuine excuse.

Does MTD change this?

If you're in scope for Making Tax Digital for income tax (sole traders and landlords earning over £50,000 from April 2026), your quarterly updates replace the year-end scramble. HMRC also confirmed late-submission penalties for quarterly updates are waived for 2026/27 as a soft landing — but the final declaration and payment deadlines still matter.

Let us handle it

At NTM Associates Ltd, we file self-assessment returns early, manage payments on account, and keep you penalty-free year after year. Contact us for a fixed-fee self-assessment quote.

Deadlines and penalties as of August 2026. Penalty and interest rates can change — always confirm current figures with HMRC or your accountant.