Self Assessment feels intimidating mostly because of unfamiliarity, not because it's genuinely complicated for most people. Here's the process broken down.
Step 1: Check if you need to file
You generally need to register for Self Assessment if you're self-employed, a company director, earn rental income, have income over £150,000, or receive income from savings, investments, or dividends above certain thresholds. If you're unsure, it's worth checking — HMRC penalises unregistered self-employment even if no tax turns out to be owed.
Step 2: Register and get your UTR
You'll need a Unique Taxpayer Reference (UTR) — a 10-digit number that identifies you to HMRC. Registration can take a couple of weeks to process, so don't leave it until January.
Step 3: Gather your records
For a typical return you'll need:
- Income records — invoices, bank statements, P60/P45 if employed
- Expense records — receipts, mileage logs, home office costs
- Bank interest and dividend statements
- Details of any pension contributions or Gift Aid donations
- Rental income and expenses, if applicable
Step 4: Complete the relevant sections
The core form is SA100, with supplementary pages depending on your situation — SA103 for self-employment, SA105 for property income, SA108 for capital gains, and so on. You only complete the pages relevant to you.
Step 5: Know your deadlines
| Deadline | What it's for |
|---|---|
| 5 October | Register for Self Assessment (if new) |
| 31 October | Paper return deadline |
| 31 January | Online return deadline + balancing payment due |
| 31 July | Second payment on account due |
Step 6: Pay what you owe
If your bill is over £1,000, HMRC usually asks for payments on account — advance payments toward next year's bill, paid in two instalments. This catches a lot of people out in their second year of self-employment, when they're paying last year's balance and next year's advance at the same time.
Common mistakes to avoid
- Missing allowable expenses because records weren't kept through the year
- Forgetting to declare all income sources, including small side income
- Not budgeting for payments on account
- Filing late — even a one-day miss triggers an automatic £100 penalty
If this is your first year filing, or you're switching from doing it yourself, starting three months before the deadline rather than three days gives you room to actually get it right.
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