How to Prepare Your First Self Assessment (Checklist Included)

First, confirm you actually need to file

Before anything else: sole traders, landlords with rental income, company directors, and anyone with untaxed income (dividends, freelance earnings on top of a salary, foreign income) generally need to file. If you're not certain, HMRC's own checker gives a quick answer. Getting this wrong in either direction causes problems: filing when you don't need to wastes time, not filing when you do need to risks a penalty you didn't see coming.

Step 1: Register with HMRC, as early as possible

If this is genuinely your first year of self-employment or rental income, you need to register for Self Assessment with HMRC by 5 October following the end of the tax year you started in. But don't wait until that deadline: registering triggers your Unique Taxpayer Reference (UTR) being issued, and you need it before you can file anything. Since HMRC's registration service update in September 2026, it can appear in your online account within 72 hours; the traditional postal letter is still sent as a fallback but takes up to 10 working days, longer if you're abroad. Either way, this is the single biggest bottleneck for a first-time filer, and the easiest one to avoid just by not leaving it late.

Step 2: Gather your records while the year is still recent

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

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

If your records are already a mess by this point, our bookkeeping service can get a year sorted retrospectively, it's more common than you'd think for a first return.

Step 3: Decide how you're actually going to file

You've got three realistic options: HMRC's own online portal, third-party software, or an accountant. A straightforward first return, one income source, simple expenses, is genuinely manageable yourself. It's worth paying for help once there's any real complexity: multiple income sources, a payment on account you didn't expect, or expense treatment you're not confident about. There's also a simpler reason people choose an accountant for the first one specifically: it sets the pattern, correcting a wrong assumption in year one is easier than unwinding it three years in.

Step 4: Know your numbers before you sit down to file

Work out, roughly, your total income and total allowable expenses before you open the actual return. Filing is much faster when you're transcribing numbers you've already checked, rather than calculating them for the first time inside HMRC's form. This is also when to check whether you'll owe a payment on account, an advance instalment towards next year's tax, which catches a lot of first-time filers by surprise because nobody mentioned it until the bill arrived.

The first-time filer checklist

Everything above, in one place:

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

Common first-time mistakes

Almost all of them come down to timing rather than the tax itself: registering too close to the deadline and being stuck waiting on a UTR, not realising a payment on account applies until the bill lands, and trying to reconstruct a year of records from memory in January instead of tracking them as you go. None of this is complicated once it's a routine. The first year is just the one where there is no routine yet.

How REYS Accountants can help

Our Self Assessment service is built for exactly this: we can handle the registration, tell you what records we actually need, calculate your bill with enough notice to plan for it, and explain every number before anything goes to HMRC. If you're not sure whether you need to file at all, or you're behind on registering, get in touch and we'll tell you plainly where you stand.

Frequently Asked Questions