How to Prepare Your First Self Assessment (Checklist Included)

The short answer

• ✓ Register by 5 October after the end of the tax year you started in. For income in 2025/26, that's 5 October 2026.
• ✓ Wait for your UTR. You can't file without your Unique Taxpayer Reference, so register early.
• ✓ File online and pay by 31 January. For 2025/26, that's 31 January 2027. Paper returns are due by 31 October 2026.
• ✓ Budget for a payment on account. If your bill is £1,000 or more, you'll usually pay half of next year's tax in advance on the same day.

All dates and figures in this article were checked against GOV.UK on 1 October 2026.

Do you actually need to file a Self Assessment?

You usually need to file if you're a sole trader earning over £1,000, a landlord, or you have other income that isn't taxed at source, such as foreign income or larger amounts of dividends.

If you're not sure, HMRC's checker gives a quick answer. Getting it wrong either way causes problems: an unneeded return wastes time, and a missed one risks a penalty.

Step 1: When should you register with HMRC?

If this is your first year of self-employment or rental income, you must register for Self Assessment by 5 October after the end of the tax year you started in. Register after that and you could get a penalty.

Don't wait for the deadline. Registering is what gets you a Unique Taxpayer Reference (UTR), and you need it before you can file anything.

Waiting times change, so use HMRC's tool to check when you can expect your UTR. Waiting on a UTR is the most common hold-up we see with first-time filers.

Step 2: What records do you need to gather?

The further you get from the tax year, the harder it is to piece together. Pull together:

• ✓ Bank statements for the full tax year, business and personal if they're mixed
• ✓ Sales invoices or a record of all income received
• ✓ Receipts or records for allowable business expenses (see our guide to what sole traders can claim if you're not sure what counts)
• ✓ Details of any other income: employment (P60 or P45), dividends, savings interest and rent
• ✓ Your UTR and Government Gateway login, once you've registered

Keep these records for at least 5 years after the 31 January deadline for that tax year, according to HMRC's record-keeping rules.

If your records are already a mess, our bookkeeping service can sort out a past year. That's common for a first return. To keep next year's in order as you go, see what records to keep.

Step 3: Should you file yourself or use an accountant?

You have three realistic options: HMRC's online service, third-party software, or an accountant. A simple first return, with one income source and simple expenses, is manageable yourself.

Help is worth paying for once there's real complexity: several income sources, a payment on account you didn't expect, or expenses you're unsure about.

There's also a reason to get help with the first one in particular. It sets the pattern, and fixing a wrong assumption in year one is easier than unpicking it three years later.

Step 4: How do you work out what you'll owe?

Roughly work out your total income and allowable expenses before you open the return. Filing is much faster when you're copying in numbers you've already checked.

This is also when to check for a payment on account. If your bill is £1,000 or more, and less than 80% of your tax was taken at source, HMRC usually asks for two advance payments towards next year, each half of this year's bill.

The first is due on 31 January with your bill, and the second on 31 July. HMRC sets out the rules in its guide to payments on account.

Illustrative example: your first bill, for 2025/26, is £3,000. On 31 January 2027 you pay that £3,000 plus a £1,500 payment on account for 2026/27, so £4,500 in total. A further £1,500 is due on 31 July 2027.

The first Self Assessment checklist

Everything above, in one place:

• ✓ Confirmed you actually need to file (HMRC's checker, or ask us)
• ✓ Registered for Self Assessment by 5 October and received your UTR
• ✓ Gathered bank statements, invoices and expense records for the full tax year
• ✓ Noted any other income: employment, dividends, savings and rent
• ✓ Decided whether to file yourself or get help
• ✓ Worked out roughly what you owe, including any payment on account
• ✓ Filed before 31 January, not the week of it (see our full deadline and penalty breakdown)

What are the most common first-time mistakes?

Almost all of them come down to timing rather than the tax itself:

• ✓ Registering close to the deadline, then waiting on a UTR
• ✓ Not knowing a payment on account applies until the bill lands
• ✓ Rebuilding a year of records from memory in January instead of keeping them as you go

The bottom line

Your first Self Assessment is mostly about timing. Register well before 5 October, keep records as you go, and budget for the 31 January bill plus any payment on account. Do that once and the following years become routine.

How REYS Accountants can help

We're a small practice in Catford, SE6, and we help first-time filers every year.

Our Self Assessment service can handle the registration, tell you which records we need, work out your bill with time to plan for it, and explain every number before anything goes to HMRC.

Freelancers should see our page for freelancers in Hither Green and Lee, and first-time landlords our page on rental income tax in Brockley and Ladywell. Not sure whether you need to file? Get in touch.

Frequently Asked Questions