A Self Assessment tax return is a form used to report income, gains, allowances, reliefs, and tax details to HMRC.
It is usually needed when tax has not been fully collected automatically, such as through PAYE. For example, you may need to complete a Self Assessment tax return if you are self-employed, receive rental income, earn untaxed income, receive dividends, have foreign income, or need to report capital gains.
HMRC uses the information in the return to calculate whether you owe tax, have already paid enough tax, or are due a refund.
For many people, Self Assessment sounds more complicated than it needs to. In simple terms, it is the way you tell HMRC about income that may not already be covered by your wages, pension, or other automatic tax deductions.
Self Assessments Explained
A Self Assessment tax return is a report you send to HMRC each year to declare taxable income and other information that affects your tax position.
Self Assessment is the system HMRC uses to collect Income Tax when tax has not already been deducted automatically. Tax is usually taken from wages and pensions through PAYE, but some people have income that HMRC needs to know about separately.
A Self Assessment tax return can include income, capital gains, allowances, reliefs, tax already paid, National Insurance where relevant, student loan repayments where relevant, and details that affect whether more tax is owed or a refund is due.
The return usually covers one tax year, which runs from 6 April to 5 April the following year.
How does Self Assessment work?
Self Assessment works by asking you to report your income and relevant tax details to HMRC.
If you need to complete a Self Assessment tax return, you usually register with HMRC, receive or use your Unique Taxpayer Reference, complete the return after the tax year ends, submit it by the deadline, and pay any tax owed.
This article explains what a Self Assessment tax return is and why it may apply. For a practical filing walkthrough, read our guide on how to do a Self Assessment tax return.
The main point to understand is that Self Assessment is not only for people who are self-employed. It can also apply to landlords, people with investment income, people with foreign income, company directors with dividends, and others with income that has not been fully taxed automatically.
Is Self Assessment the same as PAYE?
Self Assessment is not the same as PAYE.
PAYE stands for Pay As You Earn. It is the system used to deduct tax automatically from employment income and many pensions. If all your income is taxed correctly through PAYE, you may not need to complete a Self Assessment tax return.
Self Assessment is used when HMRC needs extra information about income, gains, reliefs, or tax charges that are not fully handled through PAYE.
For example, someone may have a job and pay tax through PAYE, but still need to complete a Self Assessment tax return because they also receive rental income, dividends, savings interest, self-employed income, foreign income, or capital gains.
This is why having tax deducted from your wages does not always mean Self Assessment will never apply.
Who needs to send a Self Assessment tax return?
Not everyone needs to send a Self Assessment tax return. You may need to complete one if HMRC asks you to, or if your income and tax position mean you need to report details yourself.
You may need to send a Self Assessment tax return if you:
- Are self-employed as a sole trader and earned more than £1,000 before tax relief
- Are a partner in a business partnership
- Receive rental income from property
- Have untaxed income from tips, commission, savings, investments, or dividends
- Have foreign income
- Need to report Capital Gains Tax
- Need to pay the High Income Child Benefit Charge outside PAYE
- Have been asked by HMRC to submit a return
The High Income Child Benefit Charge can apply where you or your partner receive Child Benefit and one of you has income over the threshold. For tax years from 2024/25 onwards, the charge starts where adjusted net income is over £60,000, with the full charge applying at £80,000 or more.
If you are unsure whether you need to file, it is better to check early rather than waiting until close to the deadline. You may need time to register, get access to your HMRC online account, gather records, or ask an accountant for help.
What income do you report on a Self Assessment tax return?
A Self Assessment tax return can include several types of income. It is not just a return for self-employed people.
Income reported through Self Assessment can include self-employed income, rental income, employment income where relevant, pension income, savings interest, dividends, partnership income, foreign income, capital gains, tips, commission, and other untaxed income.
Some people assume they only need to report income where no tax has been paid at all. That is not always the case. In some situations, income that has already had tax deducted may still need to be included so HMRC can calculate the full tax position correctly.
For example, someone with employment income and rental income may need to include both. The employment income helps HMRC understand how much tax has already been paid through PAYE, while the rental income may create an additional tax bill.
What information is included in a Self Assessment tax return?
A Self Assessment tax return includes the information HMRC needs to work out your tax position for the year.
The main Self Assessment tax return is known as the SA100. Depending on your income, you may also need extra sections or supplementary pages. For example, a sole trader may need self-employment pages, while a landlord may need property pages.
The return may include your personal details, Unique Taxpayer Reference, National Insurance number, income for the tax year, allowable expenses, tax reliefs, pension contributions, Gift Aid donations, tax already paid, student loan details where relevant, Capital Gains Tax details where relevant, and bank details if a repayment may be due.
If you are self-employed or have rental income, the return may also include income and expense figures. These figures should be supported by accurate records, such as invoices, receipts, bank statements, rental statements, or bookkeeping records.
What is a UTR number?
A UTR number is a Unique Taxpayer Reference.
It is a 10-digit number used by HMRC to identify you for Self Assessment. If you are filing a Self Assessment tax return for the first time, you usually need to register before HMRC issues your UTR.
You will normally need your UTR when completing your tax return, speaking to HMRC about Self Assessment, or authorising an accountant to act on your behalf.
It is worth keeping this number safe. If you leave it until close to the filing deadline and cannot find your UTR, it can delay the process.
When do you need to submit a Self Assessment tax return?
Self Assessment deadlines depend on the tax year and whether you file online or on paper.
For the 2025/26 tax year, the main deadlines are:
| Deadline | What it means |
| 5 October 2026 | Register for Self Assessment if required |
| 31 October 2026 | Paper tax return deadline |
| 30 December 2026 | Submit online if asking HMRC to collect tax through your tax code, where available |
| 31 January 2027 | Online tax return and payment deadline |
| 31 July 2027 | Second payment on account deadline, where payments on account apply |
The 31 January deadline is the date most online filers focus on. By then, HMRC must usually receive both the online return and any tax owed.
Filing early can make the process easier because you know what you owe sooner and have more time to plan payment.
For a complete breakdown of the upcoming dates, including registration, paper filing, online filing and payments on account, see our guide to the Self Assessment deadlines
What happens after you submit a Self Assessment tax return?
After you submit your Self Assessment tax return, HMRC uses the information you provide to calculate your tax position.
This may include Income Tax, Capital Gains Tax, Class 2 and Class 4 National Insurance for self-employed people where relevant, student loan repayments, payments on account, and whether a repayment is due.
Submitting the return and paying the tax are separate actions. You may submit your return and still need to pay the tax bill by the deadline. Alternatively, the return may show that you have paid too much tax and are due a refund.
This is why it is important to check the return before submitting. Incorrect income, missing expenses, wrong tax-paid figures, or missing reliefs can affect the final calculation.
What are payments on account?
Payments on account are advance payments towards your next Self Assessment tax bill.
They can apply to some people who pay tax through Self Assessment. They are usually split into two payments, one due by 31 January and the second due by 31 July.
Payments on account can surprise first-time filers because the January amount may include both the balancing payment for the year just filed and an advance payment towards the following year.
Not everyone has to make payments on account, but if they apply, they can affect how much needs to be paid and when.
What happens if you do not submit a Self Assessment tax return on time?
If you need to submit a Self Assessment tax return and miss the deadline, HMRC may charge penalties and interest.
Late filing penalties can start with an initial £100 penalty. Further penalties can apply if the return remains late for several months. Late payment can also lead to penalties and interest on the amount owed.
If HMRC has asked you to send a tax return, you should not ignore it, even if you believe no tax is due. You may still need to file the return or contact HMRC if you think Self Assessment no longer applies.
If the deadline has already passed, it is usually better to act as soon as possible rather than delay further.
Can you stop sending Self Assessment tax returns?
You may be able to stop sending Self Assessment tax returns if your circumstances change and you no longer meet the requirements.
For example, you may have stopped being self-employed, sold a rental property, stopped receiving untaxed income, or no longer need to report a tax charge.
However, you should not simply ignore a tax return request from HMRC. If HMRC expects a return and you do not send one, penalties may still apply. In many cases, you need to tell HMRC that you no longer need to complete a Self Assessment tax return.
If you are unsure, check before the deadline so it can be corrected in time.
How is Self Assessment changing under Making Tax Digital?
Making Tax Digital for Income Tax is being introduced in phases for some sole traders and landlords who are registered for Self Assessment.
It applies based on qualifying income from self-employment, property, or both. The current rollout is:
- over £50,000 qualifying income: from 6 April 2026
- over £30,000 qualifying income: from 6 April 2027
- over £20,000 qualifying income: from 6 April 2028
If Making Tax Digital applies to you, you may need to keep digital records, use compatible software, and send updates to HMRC more regularly.
This does not mean Self Assessment disappears for everyone. People who are exempt from Making Tax Digital must continue to report income and gains through a Self Assessment tax return.
For more detail, read our guide to understanding Making Tax Digital.
Do you need an accountant for a Self Assessment tax return?
You do not always need an accountant to complete a Self Assessment tax return. Some simple returns can be handled by the individual, especially where income is straightforward and records are clear.
However, accountant support can be useful if you are self-employed, a landlord, a company director, receive dividends, need to report capital gains, have foreign income, make payments on account, have unclear records, or are unsure what expenses or reliefs apply.
The value of an accountant is not just submitting the return. It is making sure the income, expenses, reliefs, and tax calculation are accurate before anything is sent to HMRC.
How Carliv Accountancy can help
At Carliv Accountancy, we offer self assessment tax return services in Middlesbrough, across the North East and UK, helping individuals, sole traders, landlords, company directors, and people with additional income to understand whether Self Assessment applies and prepare accurate tax returns.
We can help you check whether you need to file, register for Self Assessment, organise income and expense records, prepare self-employed or property income figures, check allowable expenses and reliefs, review payments on account, submit the tax return, and prepare for Making Tax Digital if it applies.
If you are unsure whether you need to submit a Self Assessment tax return, contact us today and can help you check what applies and prepare the return correctly.