How to complete your Self Assessment tax return

A version of this article was originally published in April 2023.

If you’re new to Self Assessments, submitting a tax return can seem daunting at first. So we’ve put together some key information and top tips to help make the task less taxing.

What is a Self Assessment tax return?

Employees’ tax is usually deducted automatically from their wages. But income that HMRC doesn't yet know about usually has to be declared.

This is done through Self Assessment, a system used by HMRC to collect Income Tax and claim tax relief for individuals and sole traders.

Do I need to do a Self Assessment?

There are many reasons for needing to complete a Self Assessment tax return. Freelancers and small businesses need to file them if they operate as a sole trader and generate gross sales of £1,000 or more per tax year (which runs from 6 April to 5 April the next year).

You also need to submit a Self Assessment if you're a partner in a business partnership. And you may need to file a tax return to tell HMRC about untaxed income such as:

Income from renting out a property
Foreign income
Dividends.

HMRC has a useful tool to help you check if you need to submit a Self Assessment.

Deadlines for Self Assessment tax returns

As a business owner, you have a lot to keep track of and it can be hard to stay on top of tax deadlines. But to avoid getting fined, you need to meet the deadlines for registering for Self Assessment, filing your tax return and paying your Income Tax.


6 April	Start of the new tax year
5 October	Deadline for registering for Self Assessment
31 October	Deadline for paper Self Assessment tax returns
30 December	Deadline to file should you want HMRC to collect tax owed via PAYE
31 January	Deadline for online Self Assessment tax returns
31 January	Deadline to pay the tax you owe for the previous tax year

How to register for Self Assessment

You need to register for Self Assessment by 5 October in your second tax year after setting up your business, and you can do this in a couple of different ways.

Register for your first Self Assessment

While you can also do it by post, the easiest way to register is online through your business tax account. You’ll need a Government Gateway user ID to do this, and you can create one if you don’t have one already.

After you register, you’ll receive a letter with your Unique Taxpayer Reference (UTR). Make sure you keep this safe – you’ll need your UTR to file your tax return.

Re-register if you’ve submitted a Self Assessment before

If you previously submitted an online Self Assessment but didn’t do so last year, you’ll need to re-register online using form CWF1. You’ll need to use the UTR you were given the last time you registered.

Top tips for completing your Self Assessment

Do your research so you don’t miss out

Before starting your Self Assessment, do some research to find ways to save time and pay the right amount of tax.

HMRC’s website is a good place to start. It has plenty of support for Self Assessment tax returns, including videos, webinars, toolkits and how-to guides. HMRC also has a Self Assessment helpline (0300 200 3310 – available Monday to Friday, 8am to 6pm).

You might be asked some security questions, so make sure your details are up to date in your personal tax account.

Claim your rightful tax allowances and reliefs

Taxpayers can claim several tax allowances and reliefs. HMRC has a detailed list of expenses that can be claimed, as well as Income Tax reliefs and personal allowances.

If your expenses total less than £1,000 per tax year, you may be better off claiming your annual trading allowance instead of allowable expenses.

Consider using simplified expenses

‘Simplified expenses’ is an HMRC-approved way of claiming business expenses using flat rates rather than actual costs, which can help save you money and cut back the time you spend filing your Self Assessment.

However, not everyone can use simplified expenses, as they can only be used by sole traders and business partnerships for costs related to vehicles, working from home and living on your business premises.

And if you are eligible, you should use HMRC’s simplified expenses checker to make sure you won’t end up paying more tax by using this method.

Have all the right information to hand

When you complete your Self Assessment tax return, you’ll need your Unique Taxpayer Reference (UTR), your National Insurance number, records of earnings from all taxable sources, details of expenses and your P60 if you’re also employed and get paid via PAYE.

It’s important to have access to all the relevant information when you begin completing your Self Assessment tax return, as having to go and find hard-copy invoices, bank statements and receipts will make the process take much longer.

And once you’ve submitted your return, you should keep all your records stored safely for at least six years in case HMRC audits your submission.

Give yourself enough time

People often rush because they don’t give themselves enough time to complete their Self Assessment tax return. But the more you rush, the more likely you are to make mistakes. More serious errors can lead to a penalty if HMRC believes they’re the result of you being careless.

Leave plenty of time to complete the return thoroughly, paying due care and attention when entering data and ticking boxes. Once you’ve entered all your data into your Self Assessment tax return, double-check before submitting it to HMRC.

File your Self Assessment return early

Why put off completing and filing your Self Assessment? You don’t have to wait for the deadline. In fact, you can complete your return for the previous period from the day the new tax year begins.

There are several advantages to doing this. Firstly, you’ll know it’s out of the way and will be able to focus on the parts of your business that you enjoy most. Secondly, it can help you manage your financial planning by finding out how much you owe sooner. Finally, if you’re owed a refund you’ll get it earlier.

Use third-party Self Assessment software

Given that the process of completing and returning your tax return can be quite complicated, it’s worth remembering you can always use third-party Self Assessment software that makes completing and filing your tax return quicker and easier, while reducing the likelihood of mistakes.

How to fix Self Assessment mistakes

Many people make the same mistakes every year, but it's important to correct them quickly, or avoid them entirely.

If you make a mistake when completing your Self Assessment, you could end up paying the wrong amount of tax. Fortunately, it's possible to amend a tax return to fix any errors.

If you submitted your tax return online, you can make amendments by logging into your account. If you sent a paper return in the post, you’ll need to download a new form and post the pages that need amending. And if you used third-party software, you should contact the software provider for guidance – if they’re unable to help, contact HMRC directly.

Any changes must be made within a year of the tax return deadline. If you miss this deadline, you’ll need to contact HMRC to find out whether you can still fix any mistakes.