Tax Guide · 2025/26
UK Self-Assessment Guide
Roughly 12 million UK taxpayers file Self-Assessment each year — sole traders, landlords, company directors, high earners and anyone with untaxed income. This guide explains who must file, the key deadlines, payments on account, what penalties look like, and how to register with HMRC for the first time.
Who Must File
You must complete a Self-Assessment return for 2025/26 if any apply:
- Self-employed with gross income above £1,000 (the Trading Allowance)
- Partner in a partnership
- Company director with untaxed income (excluding non-trading dormant directors)
- Untaxed property income above £1,000 (the Property Allowance)
- Total income above £150,000 from any source
- Subject to the High Income Child Benefit Charge (one partner earns over £50,000 and you claim Child Benefit)
- Dividend income over £10,000, or above £500 if you want to declare/owe more
- Capital gains above the £3,000 allowance, or proceeds over £50,000
- Untaxed foreign income
- HMRC has issued a notice to file
Key Deadlines for 2025/26
| Date | Action |
|---|---|
| 5 April 2026 | End of 2025/26 tax year |
| 5 October 2026 | Register for Self-Assessment if new to it |
| 31 October 2026 | Paper SA100 deadline |
| 30 December 2026 | Online deadline to collect tax through PAYE (under £3,000) |
| 31 January 2027 | Online filing deadline + tax payment + first payment on account |
| 31 July 2027 | Second payment on account due |
Payments on Account
If your tax bill (excluding tax already taken via PAYE and excluding CGT) for the year is more than £1,000, HMRC requires two advance payments toward the following year. Each is 50% of your current year's bill.
Example. Your 2025/26 tax bill is £4,000. On 31 January 2027 you pay: the £4,000 + £2,000 first payment on account toward 2026/27 = £6,000. On 31 July 2027 you pay the second £2,000.
The first time payments on account kick in is a cash-flow shock for many sole traders — you effectively pay 1.5 years' tax in one go. If you know next year's profits will be lower, file form SA303 to reduce the payments — but HMRC charges interest if you under-estimate.
Penalties for Late Filing
| How late | Penalty |
|---|---|
| 1 day | £100 fixed (even if no tax due) |
| 3 months | +£10/day, max £900 |
| 6 months | + greater of £300 or 5% of tax due |
| 12 months | + greater of £300 or 5% of tax due (up to 100% for deliberate) |
On top of late-filing penalties, late-payment penalties apply: 5% of unpaid tax at 30 days, again at 6 months, and again at 12 months. Interestruns on unpaid tax from 1 February at HMRC's late-payment rate (Bank of England base + 4%, currently around 7.75%).
Supplementary Pages (Beyond SA100)
The main SA100 covers basic income and allowances. Add pages as needed:
- SA102 — Employment income (one per employer)
- SA103S / SA103F — Self-employment (short or full)
- SA104 — Partnerships
- SA105 — UK property (including BTL and FHL)
- SA106 — Foreign income
- SA108 — Capital Gains, including crypto and property
- SA109 — Residence and domicile
- SA110 — Tax calculation summary if you do it manually
Making Tax Digital (MTD)
MTD for Income Tax Self-Assessment (MTD ITSA) starts April 2026 for sole traders and landlords with combined gross income above £50,000. April 2027 brings in the £30,000+ tier, and April 2028 the £20,000+ tier.
From those dates, in-scope taxpayers must use MTD-compatible software to keep digital records and file quarterly updates plus a final declaration. The old once-a-year SA100 will gradually be phased out for affected groups — check your status on gov.uk.