Self Assessment — Deadlines, Penalties, and How to File for 2026/27

Self Assessment (SA) is HMRC's way of taxing income that doesn't go through PAYE. If you're a limited company director, a sole trader, a landlord, or anyone with significant untaxed income, you need to file one each year. Miss it and the penalties stack fast: £100 on day one, £10/day after three months, and 5% of unpaid tax after six months. The good news: the rules are stable, the deadlines are predictable, and the process is mostly typing numbers into a website.

This guide is what we wish every new AIS client read in October.

Do you need to file?

You must file Self Assessment for the tax year (6 April to 5 April) if any of these apply:

• Self-employed (sole trader, partner) with trading income over £1,000
• Limited company director drawing dividends, salary above PAYE threshold, or with other untaxed income
• Landlord with rental income over £1,000
• High earner (income over £150k from 2024/25 onwards — was £100k before)
• Foreign income to declare
• Capital gains above the annual exempt amount (£3,000 in 2026/27)
• Child Benefit High Income Charge (income over £60k)
• Anyone HMRC has written to asking for a return

If you're unsure, the official HMRC "Do I need to file?" tool answers in 5 minutes.

The four dates you need to know

Memorise these:

| Date | What's due | |------|-----------| | 5 October | Register for SA if this is your first year | | 31 October | Paper return deadline (if filing on paper) | | 31 January | Online return + balancing payment + first payment on account | | 31 July | Second payment on account |

For the 2025/26 tax year (6 April 2025 – 5 April 2026):

• Register by 5 October 2026
• Online return + payment by 31 January 2027
• Second payment on account by 31 July 2027

Most people miss the registration deadline because nothing happens visibly. Don't. You can't file SA online without a UTR, and getting a UTR can take 2–3 weeks. Register the moment you know you'll owe.

Payments on account — the trap that catches everyone

If your final tax bill for a year is over £1,000 and less than 80% of your income was taxed at source (PAYE), HMRC asks for two advance payments on next year's bill:

• 50% of the prior year's bill, due 31 January
• 50% of the prior year's bill, due 31 July

Then a balancing payment the following January.

Worked example — your first year filing

Say your 2025/26 tax bill comes out to £8,000.

31 January 2027 — first online return:

• Pay 2025/26 balance: £8,000
• Pay first payment on account for 2026/27: £4,000 (50% of last year)
• Total cash out: £12,000

31 July 2027:

• Pay second payment on account for 2026/27: £4,000

31 January 2028:

• If 2026/27 tax is exactly £8,000 (rare), you've paid in full via the payments on account, owe £0 balance
• If 2026/27 tax is £10,000, you owe £2,000 balance + first POA for 2027/28 of £5,000

The cash-flow shock: first-time filers see £12,000 leaving their account in one go. This is why every accountant tells new clients to put 25–30% of every payment in a separate savings account.

Reducing your payments on account

If you know next year's income will be lower (you stopped contracting, took a salaried role), file a claim to reduce payments on account via SA303. HMRC charges interest if you reduce and then end up owing more, so be conservative.

The penalty ladder

The 2026/27 penalty schedule for late filing of an online return:

| Lateness | Penalty | |----------|---------| | 1 day late | £100 flat (even if no tax due) | | 3 months late | £100 + £10/day (max 90 days) | | 6 months late | Above + greater of 5% of tax owed or £300 | | 12 months late | Same again, sometimes harder if deliberate |

Late payment is separate from late filing. Payment penalties:

| Lateness | Penalty | |----------|---------| | 30 days late | 5% of unpaid tax | | 6 months late | Another 5% | | 12 months late | Another 5% |

Plus interest at HMRC's late-payment rate (currently base rate + 2.5%).

Worst case scenario: £1,000 tax owed, filed 12 months late, paid 12 months late = £900 in penalties + ~£75 interest = £1,975 total. Almost double the original bill.

How to file online

1. Get your UTR — 10-digit number from HMRC, sent in the post after registration.
2. Create a Government Gateway account at gov.uk/log-in-file-self-assessment-tax-return.
3. Enrol for Self Assessment with your UTR + NI number.
4. Wait for the activation code (10 working days, sent by post).
5. Log in and start the return.

The return has sections matching your income types:

• Employment (each P60/P11D)
• Self-employment (trading profit/loss, expenses)
• UK property (rental income/expenses)
• Dividends (UK + foreign)
• Interest (banks, savings)
• Capital gains
• Foreign income
• Pensions (contributions in excess of relief)
• Charitable giving (Gift Aid)

Each section has its own SA10X supplementary page. The online system asks the right questions automatically — you don't need to know the form numbers.

What you can claim as a sole trader

Common allowable expenses:

• Office costs (rent, utilities, broadband, business phone)
• Travel (mileage at 45p / 25p, or actual costs)
• Subsistence on overnight business travel
• Professional fees (accountant, lawyer, professional bodies)
• Software, equipment (capital allowances)
• Marketing and advertising
• Insurance (professional indemnity, public liability)
• Training that maintains existing skills

Not allowable:

• Travel from home to a regular workplace (commuting)
• Lunch on a normal working day
• Suits and "workwear" (unless it's actual uniform/PPE)
• Training that gives you new skills (capital, not revenue)
• Client entertainment (this catches a lot of people)

What directors need to declare

If you're a limited-company director, your SA covers your personal tax — separate from the company's CT600. The most common items:

• Salary from your own Ltd (P60 figure)
• Dividends from your own Ltd (gross dividend received)
• Benefits in kind if any (P11D figures)
• Other employment if you have a day job alongside
• Side income (rental, second business, etc.)
• Pension contributions above auto-enrolment (for higher-rate relief)

Even if all your income is from the company and there's nothing else, directors must still file. HMRC asks for it specifically.

Common mistakes

1. Forgetting to register — costs you 2–3 weeks and risks missing the 31 January deadline.
2. Putting dividends in the wrong box — UK company dividends go in the Dividends section, not Other Income.
3. Mixing personal and business expenses — HMRC can disallow the lot if your records are sloppy.
4. Not saving the receipt — keep paper trail for 5 years (sole trader) or 6 years (Ltd director).
5. Missing the 31 July payment — assuming the bill in January was "the lot".
6. Skipping the High Income Child Benefit Charge — if you earn over £60k and your partner claims Child Benefit, you have to declare it on SA.
7. Forgetting Gift Aid relief — every £100 you Gift Aid as a higher-rate taxpayer saves you £25 in personal tax.

How AIS handles Self Assessment

/tax/self-assessment aggregates your salary, dividends, expenses, and other income across the tax year — then exports a CSV you can paste into the HMRC online return, or a PDF you can hand to an accountant.

/tax/sa-payments tracks your scheduled balancing payment + payments on account across the cycle, so the 31 January and 31 July dates never sneak up on you.

Next steps

• Read Optimal Director Salary for 2026/27
• Read UK Corporation Tax Rates for 2026/27
• See your SA estimate on the Self Assessment page
• Or start a free trial