If you are self-employed, rent out property, or have income that hasn’t been taxed, you may need to send HMRC a Self Assessment tax return. This guide covers who has to file, the deadlines for the current tax year, how to register and sign in, what a late return costs, and when it makes sense to hand the job to an accountant.

What is Self Assessment?

Self Assessment is how HMRC collects Income Tax from people whose tax is not fully handled through payroll. Most employees have tax deducted by their employer through PAYE, so they usually do not need to file. If you have other income, you report it on a Self Assessment tax return, and HMRC works out what you owe. Our Self Assessment tax return service covers every step if you would rather not do it yourself.

Who needs to file a Self Assessment tax return?

According to GOV.UK, you need to file a return if any of the following were true for you in the last tax year:

• you worked for yourself and your self-employed income was over £1,000, before any allowable expenses
• you were a member of a business partnership
• you sold or gave away something that had gone up in value, such as shares or property, and owed Capital Gains Tax on the gain
• you owed the High Income Child Benefit Charge and it was not being collected through PAYE
• you work under the IR35 off-payroll rules and repay a student or postgraduate loan

You may also need to file if you have rental income, tips and commission, savings interest, dividends, or foreign income. A non-UK resident with taxable UK income may also have to file. You must send a return whenever HMRC asks you to, even if you think you owe nothing. Landlords can read more on our tax advice for landlords page.

Self Assessment deadlines

Each tax year starts on 6 April and ends on 5 April. The dates below are the usual ones, and you can check the exact dates for each year on the GOV.UK deadlines page. The usual Self Assessment deadlines are 31 October for paper returns, 31 January for online returns and tax payment, and 31 July for the second payment on account.

Where you register after 5 October, HMRC sets your return deadline at three months from the date of its letter or email, although any tax is still due by the following 31 January. If you register for the first time, allow time for your UTR to arrive before you file.

How to register for Self Assessment

If you have never filed a return, you must register with HMRC first. Use the GOV.UK registration page to tell HMRC you need to file. Once you are registered, HMRC issues your Unique Taxpayer Reference (UTR), a 10-digit number that identifies you on its records. GOV.UK says it usually arrives by post about 15 days after you register, and can take longer if you live abroad. Registering after 5 October could lead to a penalty.

How to file your tax return online

1. Open the UK page to file your return and sign in. If you do not have sign-in details yet, you can create them.
2. You may be asked to prove your identity, usually with photo ID such as a passport or driving licence.
3. Have your UTR to hand. If you registered before but did not file last year, reactivate your account first, or your return may be delayed.
4. Work through each section that applies to you, such as employment, self-employment, property, savings and dividends. You do not have to finish in one sitting.
5. Check the calculation, submit the return and keep the confirmation.
6. Pay what you owe by 31 January. See how to pay your bill.

What if you cannot sign in or have lost your UTR?

For a lost UTR, GOV.UK suggests checking your personal tax account, the HMRC app, or older HMRC letters such as a notice to file or a payment reminder. If you still cannot find it, contact HMRC. For problems signing in, use the HMRC help page. Keep your UTR ready, because you will be asked for it.

Who cannot use the online service?

The online service is not available for partnership returns, for trusts and estates, or if you lived abroad as a non-resident. If so, you need a different route, and an accountant can handle it.

What to gather before you start

• your UTR and your sign-in details
• your National Insurance number
• P60 or P45 from any employer, and a P11D if you received benefits such as a company car
• records of your self-employed income and costs, which a bookkeeping service can keep tidy all year
• rental income and property expenses
• savings interest and dividend statements
• details of any assets you sold

Penalties for filing or paying late

A late return can cost you even if you owe no tax. The GOV.UK Self Assessment penalties page lists these late filing penalties:

• £100 straight away once the deadline has been missed
• daily charges of £10 that start three months after the deadline, up to a maximum of £900
• at six months, a further charge of 5% of the tax due or £300, whichever is higher
• at twelve months, the same again: 5% of the tax due or £300, whichever is higher

Late payment penalties add 5% of the unpaid tax at 30 days, at six months and at twelve months, plus interest on the amount owed.

Payments on account explained

Payments on account are advance payments towards your next bill, including Class 4 National Insurance if you are self-employed. There are two, each half of last year’s bill, due on 31 January and 31 July. You do not have to make them if last year’s bill was under £1,000, or if more than 80% of last year’s tax was paid some other way, for example through your tax code. If your tax turns out higher, you pay a balancing amount by the following 31 January, and if it is lower, you may get a refund. GOV.UK explains the calculation.

Making Tax Digital for Income Tax

Making Tax Digital for Income Tax is being phased in for sole traders and landlords by income level, starting with the highest earners and reaching smaller businesses in later years. It means keeping digital records and sending quarterly updates through compatible software. See GOV.UK’s eligibility guidance, or speak to our tax advisory team about what it means for you.

Claiming a Self Assessment tax refund

Sometimes a return shows you have paid too much tax. In that case, you may be able to get a tax refund, which HMRC also calls a repayment or rebate. GOV.UK has a page on Self Assessment refunds and a general tool to claim a tax refund that covers other situations too, such as job expenses, savings interest and income from abroad.

Two things are worth knowing. If you owe tax in the next 45 days, for example, a payment on account, HMRC may take your refund off that bill instead of paying it out. And after you claim, your online tax account may show the refund as pending, which means it has been created but still needs approval and payment. For timings, see HMRC’s guidance on reply times for any tax refund claim. If you are not sure whether you are owed money, our Self Assessment tax return service will check for you.

How to contact HMRC about Self Assessment

For general Self Assessment questions, the HMRC Self Assessment helpline is 0300 200 3310 from the UK and +44 161 931 9070 from abroad. The helpline is open from 8 am to 6 pm, Monday to Friday, and is closed on bank holidays. You can also use HMRC’s digital assistant, which can pass you to a webchat adviser if one is available.

If you prefer to write to HMRC, the HMRC Self Assessment address is Self Assessment, HM Revenue and Customs, BX9 1AS, United Kingdom. For technical problems with your online return, a lost user ID or error messages, use the HMRC online helpdesk instead. All the current contact details are on the GOV.UK Self Assessment contact page. If you would rather not spend time on hold, get in touch with us, and we can deal with HMRC for you.

What is a P800 tax calculation?

A P800 is a tax calculation letter from HMRC. HMRC sends it when you have paid too much or too little tax by 5 April, and it explains how to get a refund or pay what you owe. GOV.UK explains the P800 and what to do next. HMRC usually sends P800 letters to people who are employed or receive a pension, and common causes include being on the wrong tax code, having two jobs overlap in the same month, or starting a workplace pension.

If you file a Self Assessment return, HMRC adjusts your tax through that return, so you would not normally get a P800. If you receive one and it says you are due money, follow the steps on GOV.UK for claiming a refund. If it says you owe tax, see what to do if you owe tax. Not sure what your letter means? Our personal tax accountant team can explain it.

Frequently asked questions

Do I need to file if I owe no tax? Yes, if HMRC has told you to file or you fall into one of the groups above. A missed deadline can still bring a £100 penalty.

Can I file after the deadline? You can, but penalties start straight away and grow the longer you wait.

Can an accountant file my return? Yes. An accountant can prepare, check and submit it and deal with HMRC for you. See our personal tax accountant service.

Need help with your Self Assessment tax return in Luton?

If you would rather not deal with it alone, we can prepare and file your return, claim every allowable expense and make sure the deadlines are met. Call FSL Accountancy Limited on 01582 806111 or get in touch.

This article is general information only and does not take your personal circumstances into account. It reflects the rules as we understand them when it was written, and those rules can change. Because every situation is different, please speak to us or another qualified adviser before you act, or decide not to act, on anything you have read here. FSL Accountancy Limited cannot accept responsibility for any loss that results from relying on this article without taking advice on your own position first. If you would like help, get in touch with our team.