Your first Self Assessment tax return can feel like a test you never studied for. HMRC expects you to know the deadlines, keep the right records and pay the right amount, often in a year when you have just started a business or let out a property.
Most first-time mistakes are small. A missed letter, a late registration, a return filed a few days after the deadline. The problem is that HMRC penalties start automatically and keep growing the longer a mistake goes unfixed.
This checklist covers what to do, when to do it, and what it costs if you get it wrong.
The first-time filer checklist
Work through these steps in order. Each one has a deadline or a penalty attached.
- Register with HMRC now. If you need to file for the 2025-26 tax year and have not filed before, the deadline to tell HMRC was 5 October 2026. Registering late can lead to a penalty, so do not wait any longer.
- Get your Unique Taxpayer Reference (UTR). HMRC sends this after you register. You cannot file without it, so do not leave registration until January.
- Keep your records together. Sales invoices, rent received, receipts and bank statements all count. You must keep them for at least 5 years after the 31 January deadline for that tax year.
- Claim your allowable expenses. Costs you have to pay to run your business or let your property reduce your tax bill. First-time filers often miss these because nobody asked about them.
- File by 31 January 2027. This is the online deadline for the 2025-26 tax year. Paper returns are due earlier, by 31 October 2026.
- Pay by 31 January 2027. Filing and paying are separate. You can file on time and still be charged for paying late.
What happens if you miss a deadline
HMRC runs two sets of penalties at the same time. One is for filing late. The other is for paying late.
The late filing penalty starts the day after the deadline:
- An initial £100 penalty, even if you owe no tax.
- After 3 months, £10 for each day the return is late, up to £900.
- After 6 months, a further 5% of the tax due or £300, whichever is greater.
- After 12 months, another 5% or £300, whichever is greater.
Late payment adds its own charges. HMRC adds a penalty of 5% of any tax still unpaid at 30 days, again at 6 months, and again at 12 months. Interest is charged on top of all of this.
A worked example: Tom's first year as a landlord
Tom lets out a flat for the first time in the 2025-26 tax year. After expenses, he owes £2,000 in tax.
Tom registers with HMRC, but life gets busy and he misses the 31 January 2027 deadline. He finally files and pays in late August 2027, about 7 months late.
Here is what that costs him:
| Penalty | Amount |
|---|---|
| Initial late filing penalty | £100 |
| Daily penalties after 3 months (maximum) | £900 |
| 6-month filing penalty (greater of 5% or £300) | £300 |
| Late payment at 30 days (5% of £2,000) | £100 |
| Late payment at 6 months (5% of £2,000) | £100 |
| Total | £1,500 |
That is £1,500 in penalties on a £2,000 tax bill, before interest is added. Tom's tax bill is now three quarters bigger than it needed to be, and none of the extra money reduces what he owes.
If Tom had filed and paid on time, his penalties would have been £0.
Making Tax Digital is coming next
Your first return also matters for what comes after it. From April 2027, Making Tax Digital for Income Tax applies to sole traders and landlords with qualifying income over £30,000, based on the 2025-26 tax year. From April 2028, the threshold drops to £20,000.
Qualifying income means your income from self-employment and property before expenses. If you are caught, you will need digital records, compatible software and quarterly updates to HMRC. Your first Self Assessment tax return is likely to be the one that decides when you start.