If you’re reading this, chances are you’ve just received a letter from HMRC mentioning Self Assessment, or someone told you that you need to “do a tax return” — and you have absolutely no idea what that means or where to start.
Don’t worry. You’re not alone, and it’s really not as scary as it sounds.
Self-assessment is just HMRC’s way of collecting tax from people whose income isn’t automatically taxed through their wages. If you’re employed, your employer takes tax out of your pay each month before you even see it. But if you’re self-employed, earn rental income, or have other untaxed earnings, you need to tell HMRC about it yourself — and that’s what Self Assessment is for.
This guide will walk you through everything step by step, so you can actually understand what you’re doing and why.
What is Self Assessment?
Self Assessment is a system that lets you report your income and work out how much tax you owe to HMRC. It’s called “self” assessment because you do the reporting — not your employer, not HMRC, but you.
At the end of each tax year (which runs from 6 April to 5 April the following year), you fill out a tax return. This is basically a form where you tell HMRC:
- How much you earned
- Where that money came from
- What expenses you can claim back
- Any other financial details they need to know
HMRC then uses that information to calculate how much Income Tax and National Insurance you owe — or sometimes, how much they owe you back.
The whole process happens online these days, and once you’ve done it once, it gets a lot easier the next time around.
Who Needs to Do a Self-Assessment?
This is the question most people ask first — and the answer is: you need to do one if your income isn’t already fully taxed at source.
Here’s the full list of people who typically need to file a Self Assessment tax return:
You’re self-employed or a freelancer
If you work for yourself — whether you’re a plumber, graphic designer, consultant, or run any kind of business as a sole trader — and your income is over £1,000 a year, you need to register for Self Assessment.
Even if you have a full-time job and do freelance work on the side, you still need to declare that extra income.
You’re a landlord
If you rent out a property and your annual rental income is more than £1,000 (after expenses), you must file a tax return. This applies whether you’re renting out one room in your house or managing a portfolio of properties.
You’re a company director
If you’re a director of a limited company and you take money out of the business — through salary, dividends, or loans — you need to file Self Assessment. This is true even if your salary is taxed through PAYE.
You earn over £100,000 a year
High earners have to file a tax return, even if all their income is taxed through. This is because once you earn over £100,000, you start losing your Personal Allowance (the amount you can earn tax-free), and HMRC wants to make sure everything is calculated correctly.
You have other untaxed income
This includes things like:
- Savings interest over £10,000
- Dividends from shares (if they’re above the dividend allowance)
- Foreign income
- Income from a trust or estate
- Tips and commission not already taxed
You’re claiming certain tax reliefs
If you want to claim tax relief on things like pension contributions, charitable donations through Gift Aid, or work expenses that your employer didn’t reimburse, you’ll need to file a Self Assessment.
HMRC has sent you a notice to file
Sometimes HMRC will send you a letter saying you need to complete a tax return — even if you don’t think you do. If you get one of these letters, don’t ignore it. Call HMRC and ask why, but until they confirm you don’t need to file, assume you do.
Still not sure? HMRC has a tool on GOV.UK called “Check if you need to send a Self Assessment tax return” — just search for it and answer the questions. It’ll tell you straight away.
When Do You Need to Register?
If this is your first time doing a self-assessment, you can’t just log in and fill out a tax return. You need to register with HMRC first.
The official deadline to register is 5 October — but that’s 5 October of the year after the tax year ended.
For example, if you became self-employed in June 2025 (which is in the 2025/26 tax year), you’d technically need to register by 5 October 2026. But here’s the thing: don’t wait that long.
Registration takes time — sometimes up to 3 weeks — because HMRC has to send you important codes and references by post. If you leave it until October, you’ll have less than 4 months to actually complete and file your tax return before the January deadline.
Do it as soon as you know you need to file. Seriously. It’ll save you a lot of stress later.
How to Register for Self Assessment
Registering is straightforward, and you can do it online in about 10 minutes. Here’s what you need to do:
Step 1: Go to GOV.UK and search for “Register for Self Assessment”
You’ll be taken to HMRC’s registration page. You’ll need to tell them why you’re registering — for example, because you’re self-employed, or you’re a landlord, or you’re a company director.
Step 2: Have your National Insurance number ready
You’ll need this to register. If you don’t know your National Insurance number, check your payslip, P60, or any letter from HMRC — it’s on most official documents.
Step 3: Fill out the online form
It’ll ask for your personal details, your address, and information about why you’re registering. It’s all pretty straightforward.
Step 4: Wait for your Unique Taxpayer Reference (UTR)
Once you’ve registered, HMRC will post you a 10-digit number called a UTR. This is your personal tax reference, and you’ll need it every time you deal with Self Assessment.
The UTR usually arrives within 10 days, but it can take up to 3 weeks. Don’t lose it — you’ll use it every year.
Step 5: Wait for your activation code
A few days after the UTR arrives, you’ll get another letter with an activation code. This lets you set up your online tax account on the Government Gateway (which is the website you use to actually file your tax return).
Once you have both the UTR and the activation code, you’re all set up and ready to file.
What Information Do You Need to Complete Your Tax Return?
Before you sit down to fill out your Self Assessment, gather everything you need. Being organised will make the whole thing much faster and less stressful.
Here’s what you’ll typically need:
Your National Insurance number and UTR
You’ll need both of these to log in.
Details of all your income
This includes:
- How much you earned from self-employment (your total sales or fees)
- Any salary from employment (you’ll get this from your P60 or payslips)
- Rental income from properties
- Dividends from shares
- Interest from savings accounts
- Any other income like pensions, trust income, or foreign earnings
Your business expenses (if you’re self-employed)
If you’re self-employed, you can deduct allowable business expenses from your income before tax is calculated. This is a big deal because it reduces how much tax you pay.
Allowable expenses include things like:
- Office costs (rent, bills, software, stationery)
- Travel and vehicle costs (petrol, train tickets, mileage)
- Equipment and tools
- Professional fees (accountant, legal advice, memberships)
- Marketing and advertising
- Phone and internet costs (if used for business)
Keep receipts for everything. HMRC might ask to see proof, and you’re legally required to keep records for 5 years after the tax return deadline.
Information about any tax you’ve already paid
If you’re employed as well as self-employed, you’ll have already paid some tax through PAYE. HMRC needs to know about this so they don’t tax you twice.
You’ll find this on your P60 (which your employer gives you at the end of the tax year) or on your final payslip.
Details of pension contributions and Gift Aid donations
If you’ve paid into a private pension or donated to charity through Gift Aid, you can claim tax relief on these. Make sure you have the amounts and dates.
Student loan information (if you have one)
If you’re repaying a student loan, you’ll need to tell HMRC about it so they can calculate the right repayment amount based on your income.
How to File Your Self Assessment Tax Return
Once you’ve got everything together, it’s time to actually fill out your tax return. Most people do this online, and honestly, it’s the easiest way.
Step 1: Log in to your Government Gateway account
Go to GOV.UK and search for “File your Self Assessment tax return.” You’ll be taken to the login page. Use your UTR and the password you set up when you registered.
Step 2: Select the right tax year
You’ll be asked which tax year you’re filing for. Make sure you pick the correct one. Remember, the 2025/26 tax year runs from 6 April 2025 to 5 April 2026 — and you’d file that return between 6 April 2026 and 31 January 2027.
Step 3: Fill in the main sections
The tax return is split into sections. You’ll only need to fill out the sections that apply to you — for example, if you’re not self-employed, you can skip the self-employment pages.
The main sections are:
- Personal details — your name, address, date of birth
- Employment — salary from any jobs you had
- Self-employment — your business income and expenses
- UK property — rental income and costs
- Savings and investments — interest and dividends
- Other income — anything else that doesn’t fit the above
- Reliefs and allowances — pension contributions, Gift Aid, etc.
Step 4: Check the calculations
As you fill in each section, the system will automatically calculate how much tax you owe (or how much you’re getting back). Double-check the figures before you submit — especially your income and expenses, as mistakes here can cost you money or trigger an HMRC inquiry.
Step 5: Submit your return
Once everything is filled in, you’ll get a summary page showing your total income, total tax, and any payments you need to make. Review it carefully, then hit submit.
You’ll get a confirmation message and a reference number — save this. It’s proof that you’ve filed.
What Are the Deadlines?
Missing a deadline will cost you money, so make sure you know when things are due.
Registration deadline: 5 October
If you’re registering for the first time, you technically need to do it by 5 October following the end of the tax year. But as we said earlier — don’t wait. Register as soon as you know you need to.
Paper filing deadline: 31 October
If you’re filing on paper (which hardly anyone does anymore), your tax return must reach HMRC by 31 October.
Online filing deadline: 31 January
This is the big one. Your online tax return must be submitted by midnight on 31 January following the end of the tax year.
For example, the 2025/26 tax year ends on 5 April 2026. You have until 31 January 2027 to file the return online.
Payment deadline: 31 January
Any tax you owe is also due by 31 January. If you owe a lot of tax (usually over £1,000), you might also need to make a second payment on 31 July — this is called a “payment on account.”
What happens if you miss the deadline?
HMRC charges automatic penalties:
- £100 fine if your return is one day late
- £10 per day (up to £900) if it’s more than 3 months late
- More fines and interest if it’s 6 or 12 months late
On top of that, you’ll be charged interest on any tax you owe but haven’t paid.
Don’t risk it. File on time.
How Much Tax Will You Pay?
The amount of tax you pay depends on how much you earn. Here’s how it works for the 2025/26 tax year:
Personal Allowance
Everyone gets a Personal Allowance — an amount you can earn before you pay any tax at all. For most people, this is £12,570.
So if you earn £15,000 in a year, only £2,430 of it is taxable (£15,000 minus £12,570).
Income Tax rates
Once you’ve used up your Personal Allowance, you pay tax at these rates:
- 20% (Basic rate) on income between £12,571 and £50,270
- 40% (Higher rate) on income between £50,271 and £125,140
- 45% (Additional rate) on income over £125,140
National Insurance (if you’re self-employed)
Self-employed people also pay National Insurance. There are two types:
- Class 2 NI: A flat weekly amount (currently £3.45 a week) if your profits are over £12,570
- Class 4 NI: 6% on profits between £12,570 and £50,270, then 2% on anything above that
If you’re employed as well as self-employed, the system gets a bit more complicated — but the online tax return will calculate it all for you automatically.
Can You Pay in Instalments?
Yes. If you owe more than £1,000 in tax, HMRC assumes you’ll pay it in instalments rather than all at once.
These are called payments on account, and here’s how they work:
- You pay half of your estimated tax bill by 31 January
- You pay the other half by 31 July
- Then, when you file your next tax return, HMRC works out if you paid too much or too little and either refunds you or asks for the balance
If you’re struggling to pay, HMRC does offer payment plans. Call them and explain your situation — they’d rather set up a plan than chase you for unpaid tax later.
Do You Need an Accountant?
You don’t have to use an accountant, but a lot of people do — and for good reason.
If your tax situation is simple (for example, you’re self-employed with straightforward income and expenses), you can absolutely do it yourself using the online system. It’s designed to be user-friendly, and it does the maths for you.
But if any of these apply, an accountant might be worth the cost:
- You run a limited company
- You have multiple income streams
- You’re not confident with numbers or paperwork
- You want to make sure you’re claiming all the expenses you’re allowed
- You’ve received a letter from HMRC and don’t understand it
A good accountant will make sure your return is accurate, help you claim everything you’re entitled to, and potentially save you more in tax than they cost in fees.
If you’re unsure, many accountants offer a free initial consultation — so it’s worth asking.
Common Mistakes to Avoid
Here are the mistakes we see all the time — and how to avoid them:
1. Forgetting to declare all your income
If you earned it, declare it. This includes side income, rental income, foreign income, and even the cash job you did for a mate. HMRC has ways of finding out, and undeclared income can lead to penalties or even prosecution.
2. Claiming expenses you’re not allowed
Just because you bought it doesn’t mean you can claim it as a business expense. The expense has to be “wholly and exclusively” for business use. That means:
- You can claim a laptop you use only for work
- You can’t claim a laptop you use for Netflix and work
- You can claim travel to meet clients
- You can’t claim your daily commute to your office
3. Not keeping records
HMRC expects you to keep records of all your income and expenses for at least 5 years. If they ask to see proof and you don’t have it, you could face penalties.
4. Missing the deadline
Even one day late triggers a £100 fine. Set a reminder for mid-January and get it done early.
5. Not registering on time
If you’re self-employed and you don’t register for Self Assessment by the deadline, HMRC can fine you — even if you don’t owe any tax.
What Happens After You File?
Once you’ve submitted your tax return, HMRC will process it and send you a tax calculation. This tells you exactly how much you owe (or if you’re getting a refund).
If you owe money, make sure you pay by 31 January. You can pay online, by bank transfer, or even by card.
If HMRC owes you money (because you’ve overpaid tax), they’ll usually refund you within a few weeks. The money goes straight into your bank account.
And that’s it. You’re done for another year.
Final Thoughts
Self Assessment sounds intimidating, but once you’ve done it once, it becomes routine. The key is to:
- Register early
- Keep good records throughout the year
- Don’t leave it until the last minute
- Ask for help if you need it
And remember — thousands of people file Self Assessment tax returns every year, and most of them have no accounting background at all. You can do this.
If you’re still feeling stuck, or if you’d rather hand the whole thing over to someone who does this for a living, get in touch with us. We help people with Self Assessment every single day, and we’d be happy to take it off your hands.
Need Help With Your Self Assessment?
At Applegrow Financial Advisors, we specialise in helping individuals and small businesses with their tax returns. Whether you’re self-employed, a landlord, or a company director, we can handle your Self Assessment from start to finish — so you don’t have to worry about deadlines, penalties, or paperwork.
Contact us today to find out how we can help.
