Filing a Self Assessment tax return for the first time can feel intimidating, mostly because nobody explains the process until you’re already up against a deadline. The good news is that once you understand the sequence — register, get your reference number, then file — it’s a manageable, one-time learning curve.
If you’d rather have someone check your registration and return before you submit, book a free 15-minute consultation with Felix Accountants — it’s a quick way to make sure your first return is right.
Who Needs to Register for Self Assessment?
You generally need to register if, in the tax year in question, you had income from self-employment over £1,000, rental income, foreign income, capital gains, dividends or savings above certain thresholds, or if you need to pay the High Income Child Benefit Charge. Self Assessment isn’t limited to the self-employed — many first-time filers are landlords, company directors, or people with a side income alongside employment.
The Registration Deadline
If you need to file for the first time, you must register with HMRC by 5 October following the end of the tax year in which the income arose. For example, if you started earning untaxed income at any point between 6 April 2025 and 5 April 2026 (the 2025/26 tax year), you need to register by 5 October 2026. Miss this and you can still register late, but you risk penalties if it causes you to miss the return deadline too.
How to Register: Step by Step
- Decide your category. HMRC’s registration route differs slightly depending on whether you’re self-employed, a landlord, a partner in a business, or none of the above but still need to file.
- Register online with HMRC. Use the appropriate GOV.UK registration service for your circumstances and set up a Government Gateway account or sign in with GOV.UK One Login.
- Receive your Unique Taxpayer Reference (UTR). HMRC posts this, usually within around 10 working days in the UK (longer if you’re abroad). You cannot file a return without it.
- Activate your online account. A separate activation code arrives by post, which you’ll need to complete sign-in for the online filing service.
- Set up your personal tax account. This is worth doing early — see our guide on how to set up your personal tax account for the details.
Filing Deadlines You Need to Know
| Deadline | What it’s for |
|---|---|
| 5 October | Register for Self Assessment for the tax year just ended |
| 31 October | Paper return deadline |
| 31 January (following year) | Online return deadline and balancing payment due |
Our guide to the UK tax year and key dates covers how these deadlines fit into the wider tax calendar, including payments on account.
What Happens If You Miss the Deadline?
Missing the registration deadline alone doesn’t always trigger an automatic penalty, particularly if you don’t end up owing tax. Missing the filing deadline is different: HMRC applies an automatic £100 penalty even if you owe no tax, rising to daily penalties after three months. Full details are in our article on HMRC’s £100 fine for missing the tax deadline.
Common First-Time Filer Mistakes
- Leaving registration until September or October and getting caught out by UTR postal delays
- Not keeping records of income and expenses from day one, making the return far harder to complete accurately
- Forgetting to declare all sources of income, not just the “main” one — for example, a small amount of rental income alongside employment
- Assuming Self Assessment is only for the self-employed and missing the deadline as a first-time landlord
- Not budgeting for payments on account, which can catch new filers by surprise in year two
Record-Keeping From the Start
Good record-keeping makes your first return far less stressful and gives you a solid foundation for every year after. Keep invoices, receipts, bank statements and any correspondence relevant to your income and expenses as you go, rather than trying to reconstruct a year’s activity in January.
How Felix Accountants Can Help
We help first-time filers register correctly, understand what they can and can’t claim, and get their first return submitted well ahead of the deadline — taking the guesswork out of a process that only gets easier the second time around.
Frequently Asked Questions
How long does it take to get a UTR number?
HMRC typically posts your Unique Taxpayer Reference within around 10 working days if you’re in the UK, or up to 21 working days if you’re abroad. It’s sensible to register well before the 5 October deadline to allow for this.
Do I need to register for Self Assessment if I’m already employed and pay tax through PAYE?
Yes, if you have additional untaxed income — such as self-employment earnings, rental income, or significant dividends — on top of your PAYE employment. Self Assessment and PAYE aren’t mutually exclusive.
What if I register late?
You can still register after 5 October, but if this causes you to also miss the filing deadline, you may face penalties. It’s best to register as soon as you realise you need to, rather than waiting.
Can I file my first return on paper instead of online?
Yes, but the paper deadline (31 October) is earlier than the online deadline (31 January), so most first-time filers find it easier to register for online filing.