A Self Assessment tax return is the annual HMRC filing used to report income that is not fully taxed at source. For the 2025/26 tax year, paper returns are due by 31 October 2026, while online returns and any tax owed are due by 31 January 2027. A missed filing deadline triggers an immediate £100 penalty.
Accounting practices getting it right in the first place is very important; otherwise, it creates unnecessary problems for you and your client. This guide sets out the 2026 rules, dates and penalties in plain terms.
What is a Self Assessment Tax Return?
A Self Assessment tax return is an annual HMRC filing in which a taxpayer reports income that is not fully taxed at source, so HMRC can calculate Income Tax, Class 4 National Insurance and any Capital Gains Tax due.
Taxes owed by salaried people in the UK are deducted automatically from their salaries via PAYE (Pay as you Earn) before they get it in their hands. However, for your self-employed clients, the HMRC will not know how much tax they owe, so they must tell HMRC. Self-assessment is done for this reason.
Every self-employed person or person who receives any other income must submit a self assessment tax return online every year. The information provided in the tax return will be used to calculate the overall tax bill and determine whether you need a refund. If your clients are new to this, how to register for self-assessment with HMRC is the first step.
Who needs to file a Self Assessment tax return?
A client must file if they are self-employed and earned over £1,000, are a business partner, had total income above £150,000, owe Capital Gains Tax, or must pay the High Income Child Benefit Charge. Untaxed income, such as rent or foreign income, can also trigger a return.
There is a wide range of people who are required to do self assessment tax returns, including:
| Trigger | Threshold or rule |
| Self-employed sole trader | Gross income over £1,000 (the trading allowance) |
| Business partner | Any partner in a business partnership |
| High income | Total income over £150,000 |
| High Income Child Benefit Charge | You or your partner earn over £60,000 and claim Child Benefit (tapered to £80,000) |
| Capital Gains Tax | Gains above the £3,000 annual exempt amount |
| Untaxed income | Rent, commissions, dividends, savings interest, foreign income (check the gov.uk list) |
How can accountants keep Self Assessment tax returns accurate?
Accuracy comes from five habits: collecting documents early, knowing allowable expenses, using MTD-compatible software such as Xero or QuickBooks, tracking deadlines, and checking each return against current HMRC rules before it is submitted.
Maintaining accuracy is important to comply with HMRC regulations and avoid penalties.
Gather the Documents Well in Advance
Make it a priority to collect the relevant documents for self-assessment.
- Income records (employment, self-employment, rental income, etc.)
- Expense receipts and invoices
- P60, P45, or P11D forms (if applicable)
- Bank statements and investment income records
- Pension contributions and charitable donation receipts
Know the Allowable Expenses
Get to know the allowable expenses in detail. Some of the common ones are as follows:
- Travel costs (business-related)
- Office supplies
- Staff costs, professional fees, advertising and training courses
Clients with gross trading income under £1,000 can use the £1,000 trading allowance instead of claiming expenses. Check whether this suits each client before choosing a method.
Use Digital Tools
Use the best accounting software like Xero or QuickBooks to automate calculations and reduce human errors. Since April 2026, clients above the MTD threshold also need software that is compatible with Making Tax Digital for Income Tax.
Continuous Tracking of Deadlines
Maintaining track of the self-assessment return deadline is important so that you can submit before it and avoid penalties. You will be able to alert your clients and provide them with a list of required documents.
Offer All-Round Support
Provide year-round guidance to clients on tax-saving strategies and record-keeping best practices. Large practices can scale this support through tax outsourcing services. Before selecting any outsourcing service, make sure to analyse their case studies; it will aid you in making better decisions.
Maintain Compliance with HMRC
Keep up with the latest HMRC rules and guidance, including Making Tax Digital (MTD) requirements, and make sure clients hear about any changes affecting their self-assessment in good time.
What are the Self Assessment tax return deadlines for 2025/26?
For the 2025/26 tax year (6 April 2025 to 5 April 2026), new clients must register by 5 October 2026, paper returns are due by 31 October 2026, and online returns plus any tax owed are due by 31 January 2027. Payments on account fall due on 31 January and 31 July.
| Date | What is due |
| 5 October 2026 | Register for Self Assessment (first-time clients) |
| 31 October 2026 | Paper tax return for 2025/26 |
| 31 January 2027 | Online tax return for 2025/26 and payment of any tax owed |
| 31 January 2027 | First payment on account for 2026/27 |
| 31 July 2027 | Second payment on account for 2026/27 |
To reduce the stress of complying with deadlines, accounting practices can take the help of the Tax Return Preparation Guide.
We understand that self assessment tax returns are complex, and doing them for multiple clients can be time-consuming. Hence, you should get tax outsourcing support from reputed service providers such as Corient to save time and minimise errors.
Dates can change; please check the government website for the current dates.
What are the penalties for filing a Self Assessment tax return late?
HMRC charges an automatic £100 penalty the day after the filing deadline, even if no tax is owed. After 3 months, daily penalties of £10 apply for up to 90 days (maximum £900), followed by 5% or £300 charges at 6 and 12 months, whichever is greater.
| Time after deadline | Late filing penalty |
| 1 day | £100 fixed penalty |
| 3 months | £10 per day for up to 90 days (maximum £900) |
| 6 months | Further 5% of tax due or £300, whichever is greater |
| 12 months | Further 5% of tax due or £300, whichever is greater |
Late payment is penalised separately: 5% of the unpaid tax at 30 days, 6 months and 12 months, plus interest. Check HMRC’s penalty page for the latest figures.
How does Making Tax Digital for Income Tax change Self Assessment in 2026?
From 6 April 2026, sole traders and landlords with qualifying income above £50,000 must keep digital records, send quarterly updates through compatible software and still submit a final return by 31 January. Making Tax Digital for Income Tax (MTD for ITSA) is HMRC’s software-based replacement for annual-only reporting.
| Start date | Qualifying income | Based on tax year |
| 6 April 2026 | Over £50,000 | 2024/25 |
| 6 April 2027 | Over £30,000 | 2025/26 |
| 6 April 2028 | Over £20,000 | 2026/27 |
HMRC’s policy paper expects around 780,000 people to join from April 2026 and a further 970,000 from April 2027. HMRC’s latest estimate is about 1,077,000 additional sole traders and landlords joining in April 2027. The legal framework is set out in HMRC’s Self Assessment legal framework manual.
For practices, the workload shifts from one annual return to quarterly updates. ACCA warns that the February quarterly reporting deadline falls shortly after the 31 January Self Assessment peak. Corient’s Making Tax Digital service supports practices preparing clients for this.
How do you file a Self Assessment tax return online?
Register the client with HMRC, wait for the 10-digit Unique Taxpayer Reference (UTR), activate the online service, then complete and submit the return through HMRC’s online service or compatible software before 31 January.
Self-assessed tax returns online are quite beneficial for both HMRC and your clients. HMRC is moving taxpayers to digital reporting through Making Tax Digital. If your clients have not registered for online tax returns, you must first register them. Registration might take some time, so it is important to register well in advance so as not to clash with the tax filing dates.
Online self-assessment has multiple benefits, including improved accuracy of digital records, the ability to send digital records directly to HMRC, and wider productivity gains for your clients’ businesses. HMRC reported that 97.25% of 2024/25 returns were filed online, compared with 316,000 paper returns.
Once you have registered your client, HMRC will send a Unique Taxpayer Reference number. Follow the instructions accordingly to finish setting up the account. Once that is done, you can use the HMRC self assessment tax return online services to submit the tax return.
What do you need to complete a Self Assessment tax return?
You need the client’s 10-digit UTR, National Insurance number, income and expense records, and statements for savings, investments, rent, pensions and charity gifts. Having these ready before sign-in avoids delays.
To complete a self assessment tax return successfully, you will require all the paperwork to contain accurate information. This information is as follows:
- 10-digit Unique Taxpayer Reference (UTR)
- National Insurance Number
- P11D, which shows any benefits and expenses
- Summary of any rental income and expenses
- Savings and investment statements showing how much you have earned in interest and other income like dividends
- Documents detailing your self-employment income, including receipts, bank statements and accounts
- Documents showing any contributions to charities or pensions that might be eligible for tax relief
How can large practices manage Self Assessment at scale?
Larger practices handle Self Assessment at scale by keeping preparation and review separate, using a consistent process to collect client information, and bringing in outside help during the busy seasonal peaks. That frees up partners to focus on advisory work and cuts down on filing mistakes across hundreds of clients.
- Segment clients early: flag who is now inside MTD for Income Tax and who joins in April 2027.
- Standardise document requests: use one checklist per client type (sole trader, landlord, partner).
- Add a review layer: a second check on each return before submission, supported by Sherlock, Corient’s error-detection AI.
- Outsource the peak: Corient’s Personal Tax Outsourcing and Tax Outsourcing Support teams are trained in UK standards and HMRC rules.
Frequently Asked Questions (FAQ)
How does Making Tax Digital (MTD) affect self-assessment?
MTD for Income Tax went live on 6 April 2026 for sole traders and landlords earning over £50,000. In practice, you keep your records digitally and send HMRC an update every quarter using compatible software. The final return is still due by 31 January. Not everyone’s caught yet, though. The limit falls to £30,000 in April 2027 and £20,000 in April 2028.
What is a self assessment tax return?
Basically, it’s how you tell HMRC about income that didn’t have tax taken off through PAYE. Think self-employment profits, rent, dividends or money earned abroad. HMRC takes those figures and works out your Income Tax, Class 4 National Insurance and any Capital Gains Tax. Sometimes it turns out you’re owed a refund instead.
Who needs to file a self assessment tax return?
Your client has to file if they’re self-employed and made more than £1,000, or if they’re a partner in a business. The same goes if their total income went over £150,000 or they owe Capital Gains Tax. Income above £60,000 can also trigger the High Income Child Benefit Charge, which means a return too. And anyone with untaxed income, like rent, savings interest, dividends or foreign earnings, needs to file.
How do I file my self assessment tax return online in the UK?
You’ll need to be registered with HMRC and have your Self Assessment service activated using your UTR. Get your income records, expense receipts and any P60 or P11D forms together, then log in to your HMRC online account. Work through each section and check the calculation before you submit. For 2025/26 the deadline is 31 January 2027, which is also when any tax you owe has to be paid. Keep your records digital as well.
How do I submit the SA1 Self Assessment form online?
Log in to your Government Gateway account and open the Self Assessment registration service. It asks for your full name, date of birth, National Insurance number and address, plus your reason for registering, such as rental income or Capital Gains Tax. Once you’ve submitted it, you wait for your 10-digit UTR to arrive in the post. That can take up to 10 working days. When it turns up, activate the online service.
Conclusion
Self-assessment of tax returns is an important task that your client expects you to do flawlessly, so we understand the pressure on you. HMRC’s online self-assessment and Making Tax Digital for Income Tax have changed how returns are prepared, but you still face the challenge of dealing with multiple self-assessments, which is time-consuming. This situation can lead to human errors in filing, which can lead to penalties and legal issues for your clients.
An outsourcing partner can absorb multiple assessments at once. Our accountants are well-versed in the UK’s accounting standards and HMRC regulations and offer tech-savvy accounting services. Describe your problems or doubts on our website contact form, and our executive will contact you as soon as possible.
