How to submit your Self Assessment tax return to HMRC

Whether you’re a sole trader, small business owner, or freelancer, you probably dread your Self Assessment tax return.

And while the process can be much easier with good accounting software, the deadlines throughout 2025 and into 2026 are circled in stark red pen on calendars up and down the UK.

But if you’re new to Self Assessment tax returns, you’re probably feeling a lot of pressure to get it right. To help you out, this guide will break down everything you need to know — including what a Self Assessment tax return is, how to submit one step by step, and the most common pitfalls to watch out for on the way.

What is a Self Assessment tax return?

Put simply, a Self Assessment tax return is how individuals in the UK report their income to HMRC when it’s not automatically taxed, like it would be through Pay As You Earn (PAYE) for most employees.

Through the tax return, you’ll need to report your income and then HMRC will calculate how much tax — and sometimes National Insurance Contributions (NICs) — you owe. You can also claim any allowable expenses or reliefs to help reduce your overall tax bill.

Who has to file a tax return?

In the UK, you must file a tax return if any of the following apply to you:

• You’re self-employed or a sole trader: if you run your own business or work as a freelancer.
• You’re a company director: even if you don’t take a salary, you must still file a return as a director.
• You earn income from property: if you receive rental income as a landlord/landlady.
• You earn over a certain amount: if your income is above the personal allowance threshold (£12,570 in the 2024-2025 tax year) and hasn’t already been deducted, you’ll need to file a tax return.

You can find the full list on the government’s Self Assessment tax return page.

Another common question among small business owners is “How much can I make before I have to file a tax return?”

Luckily, the answer to this one is simple — £1,000. That means as soon as your revenue (the amount your business has earned in a tax year) surpasses this amount, you’ll need to submit a Self Assessment tax return.

What you need before you start

Before starting your Self Assessment tax return, it’s a good idea to get all your documents and details in order. Having everything ready ahead of time will help make the process smoother and avoid costly mistakes.

Here’s what you’ll need to have ready before you begin:

• Your Unique Taxpayer Reference (UTR): a 10-digit number HMRC gives you when you register for Self Assessment. This can be found in letters from HMRC or on your online account.
• Government Gateway login: gives you access to your personal tax account. If you haven’t already, you’ll need to create an account on the government website.
• Your National Insurance number: HMRC will use this to track your NI contributions.
• Income records: you’ll need documentation for self-employment income (invoices, bank statements, etc.), employment income (P60 or P45), rental income, dividends or interest from savings and/or investments, or any other untaxed income.
• Expenses records (if self-employed): proof of allowable expenses, such as travel, office costs, or software subscriptions.
• Details of tax reliefs or allowances: this includes pension contributions, charitable donations, or things like Seed Enterprise Investment Scheme (SEIS) or Enterprise Investment Scheme (EIS) investments.

How to file a Self Assessment tax return

Filing your first Self Assessment tax return might feel daunting at first, but it gets easier once you know what to expect. Here’s a step-by-step rundown of everything you need to do —  from first registration to paying your bill.

1. Register for Self Assessment

The very first step is to register with HMRC. Remember, you’ll need to do this by 5 October following the end of the tax year in which you earned income. Once registered, you’ll receive your unique taxpayer reference (UTR).

2. Log in to your HMRC account

For this, you’ll need to go to the HMRC login page and sign in using your Government Gateway ID. From there, you’ll be able to access your personal tax account and the Self Assessment section.

3. Fill in your income and expenses

Next, you’ll need to enter details of your self-employment income and allowable expenses. If you’re using accounting software, you may be able to import this data directly.

Make sure to include all relevant income for the tax year, and only claim expenses that are exclusively for business use, such as travel, office equipment, and professional services.

You should also ensure everything is accurate. HMRC may ask for evidence, and incorrect claims could lead to penalties. Therefore, you should keep digital records throughout the year to make this process easier.

4. Add any additional income (if applicable)

If you have income from other sources besides self-employment, you’ll need to include it too. This includes:

• Employment income (if you had a job alongside self-employment)
• Rental income
• Dividends and/or interest from savings or investments
• Foreign income

5. Review and submit

Before hitting that submit button, you should double-check everything first — especially your income, expenses, and tax reliefs. Once you’re confident that all the details are accurate, you can submit your return.

6. Pay your tax bill

Once submitted, HMRC will tell you how much you owe. You’ll need to pay this by 31 January. You will also need to make a second “payment on account” by 31 July, which is a payment made towards your next tax bill. However, you’ll be exempt from POA if the amount of tax you owed last year was under £1,000.

There are several ways you can pay your tax bill, including:

• Your online bank account
• Online or telephone banking (Faster Payments)
• Clearing House Automated Payment System (CHAPS)
• Debit or corporate credit card online
• Your local bank or building society (you will need a paying-in slip from HMRC to do this)

You can find more information on paying your tax bill on the government website.

How to claim expenses

Claiming the right expenses is absolutely vital. Otherwise, you’ll end up paying more tax than you have to. But how do you know what counts as allowable and what doesn’t?

Luckily, HMRC have made things a little easier by listing the kind of expenses you can claim back. These include:

• Office costs: stationery, rent, computer software, utility bills, etc.
• Travel expenses: fuel, vehicle insurance, parking, breakdown cover, etc.
• Clothing expenses: uniforms, protective clothing, and costumes for actors and entertainers.
• Staff expenses: employee salaries, bonuses, pensions, staff benefits, etc.
• Reselling goods: goods for resale (stock), raw materials, and direct costs from producing goods.
• Legal and financial costs: when you need to hire an accountant, solicitor, surveyor, or architect for business reasons.

So, how do you actually claim back on these expenses?

It’s simple — all you have to do is include them in your Self Assessment tax return under the self-employment section. When filling it out, you’ll either need to enter your total allowable expenses as a lump sum or break them down into categories.

Just make sure you only claim business-related expenses, keep records of your costs, and be prepared to provide proof if HMRC asks to see it during a review or audit.

For any expense that’s split between business and personal use (for example, the running costs of a car you use for business-related meetings and picking up groceries), you’ll need to work out what share of the use is for business, and then claim for that. If it’s 50/50, for example, you can claim half of those costs as a business expense.

Common mistakes to avoid

The last thing you want to do is make any errors on your tax return, as this could result in unwanted penalties. So to help you avoid the unnecessary costs and stress, here are some common mistakes to steer clear of.

❌ Missing the deadline: late submissions will result in penalties, even if you don’t owe any tax. Make sure you set reminders well in advance and aim to file as early as possible.

❌ Not registering in time: you must register for Self Assessment by 5 October. Miss it, and you could face more fines. Therefore, you should register as soon as you know you need to file.

❌ Forgetting other income sources: only including your self-employment income? You could be underreporting. To avoid this, record all income streams throughout the year and cross-check them before submitting.

❌ Claiming unallowable expenses: only claim for wholly and exclusively business-related costs. Claiming personal expenses will only cause problems, so make sure you separate these from your business expenses.

❌ Not keeping records: you need to keep receipts, invoices and bank statements for at least five years after the filing deadline. Use accounting software or a simple spreadsheet to track income and expenses as you go.

What happens after you submit?

Once you’ve submitted your tax return, you’ll receive confirmation from HMRC. From there, HMRC will review your submission and calculate how much tax you owe.

After this, you’ll receive a tax calculation showing how much you owe and how much you may be due to receive as a refund. This will be available in your online HMRC account, and you’ll also get a statement of this calculation.

It’s at this stage where you’ll need to pay the tax you owe. Remember that you can refer to the government’s page on paying your Self Assessment tax bill for different payment options.

It’s also important to note that HMRC might contact you for more information. In this case, you should keep hold of your records to provide proof of expenses or income.

Conclusion

No one enjoys the Self Assessment process, and that’s never going to change. It’s a tedious process of box-ticking that culminates in you having to pay the government some of your profits.

However, keeping records and planning ahead can make it a lot less stressful. The key is staying organised throughout the year so that filling out the dreaded form is merely a minor task. Plus, using accounting software and seeking professional help can simplify the process even further.

Just remember that the more you plan ahead and stay on top of your finances, the smoother tax time will be. And if you’re ever in doubt, don’t hesitate to reach out to HMRC or a professional accountant for help. Taking these steps will help make your Self Assessment experience as stress-free as possible.