UK Self Assessment Tax Return: How to complete your tax return
The mere mention of "Self Assessment" can bring a sense of dread for many. That annual task of compiling figures, navigating forms, and meeting deadlines often feels daunting. But what if you could approach your tax return with confidence and clarity? At Plouta, we're committed to demystifying finance, helping you protect your money, save effectively, and build a future of financial independence.
This ultimate guide is designed to walk you through the UK Self Assessment process step-by-step. We’ll cover who needs to file, what information you’ll need, how to complete the return, key tips to avoid common pitfalls, and upcoming changes like Making Tax Digital for Income Tax Self Assessment (MTD for ITSA).
What you will learn in this guide: ⤵
Who Needs to File: Clear criteria to determine if you must complete a Self Assessment tax return.
Getting Started: How to register for Self Assessment if it's your first time.
Information Gathering: A comprehensive checklist of documents and records you'll need.
Key Deadlines: Understanding the crucial dates for registration, filing, and payment.
Navigating the Return: A practical overview of completing the online form, section by section.
Expenses & Reliefs: Tips on claiming what you're entitled to.
Common Mistakes & Penalties: How to avoid errors and the consequences of being late.
Making Tax Digital (MTD) for ITSA: An important update on upcoming changes.
Practical Tips for a Stress-Free Filing Experience.
Do You Need to File a Self Assessment Tax Return?
Not everyone needs to file a tax return. If your only income is from employment (where tax is deducted via PAYE) and you have no other significant untaxed income, you probably don't. However, you must send a tax return if, in the previous tax year and any of the following applied:
You were self-employed as a sole trader and earned more than £1,000 (before taking off anything you can claim tax relief on).
You were a partner in a business partnership.
You earned £100,000 or more in total taxable income.
You had untaxed income from renting out a property of £2,500 or more a year (after allowable expenses). For income below this, you may need to contact HMRC.
You had taxable income from savings or investments of £10,000 or more (before tax).
You needed to pay Capital Gains Tax on profits from selling assets like shares or a second home.
You were a company director (unless it was for a non-profit organisation and you didn't get any pay or benefits).
Your income (or your partner's) was over £60,000 and you claimed Child Benefit (you may need to pay the High Income Child Benefit Charge). (Note: This threshold increased from £50k, and the taper now extends to £80k, from April 2024).
You lived abroad but had a UK income.
You received dividends from shares and the dividend income was over your Dividend Allowance and not taxed at source.
HMRC has sent you a notice (form SA316) telling you to file a tax return.
You want to claim certain tax reliefs or expenses (e.g., tax relief on pension contributions if you're a higher-rate taxpayer and your scheme doesn't provide full relief at source).
If you're unsure, use the checker tool on the GOV.UK website or contact HMRC.
Step 1: Registering for Self Assessment (If You Haven't Already)
If it's your first time filing, or you haven't filed in a while, you need to register for Self Assessment.
How to Register: You can usually register online via the GOV.UK website. The process varies slightly depending on whether you're registering as self-employed, not self-employed, or as a partner.
Unique Taxpayer Reference (UTR): Once registered, HMRC will send you a 10-digit UTR. Keep this safe – you’ll need it for filing your return and any correspondence with HMRC. This can take a few weeks to arrive.
Activation Code: For online filing, you'll also set up a Government Gateway account and will receive an activation code by post for your Self Assessment online service.
Registration Deadline: You must register by 5th October after the end of the tax year you need to send a return for. The registration deadline is 5 October 2025. Don't miss this, or you could face penalties.
Step 2: Gather Your Information – The Essential Checklist
Good preparation is half the battle. Before you start your tax return, gather all relevant information for the tax year (6 April to 5 April):
Your UTR Number
National Insurance Number
Income from Employment: P60 (summary of pay and tax for the year from each employer), P45 (if you left a job during the tax year), P11D (for taxable benefits in kind).
Income from Self-Employment:
Records of all your business income.
A detailed list of all your allowable business expenses (e.g., office costs, travel, stock, staff costs). Keep receipts and invoices.
Income from Property:
Rental income received.
Allowable expenses (e.g., letting agent fees, repairs, mortgage interest relief note restrictions on this for residential landlords).
Savings and Investment Income:
Interest received from bank and building society accounts (even if below your Personal Savings Allowance, it may need to be declared if you complete a return for other reasons).
Dividend income from shares (P600 if provided by your registrar, or dividend vouchers).
Pension Income: P60 from your pension provider(s), details of any State Pension received.
Capital Gains: Details of assets sold (e.g., shares, second homes), including purchase and sale prices, dates, and associated costs (e.g., stamp duty, broker fees, legal fees).
Overseas Income: Details of any foreign income (this can be complex, and you may need to refer to double-taxation agreements).
Charitable Donations: If you made Gift Aid donations and are a higher or additional rate taxpayer, you can claim further tax relief.
Pension Contributions: Details of contributions to private pensions, especially if you want to claim higher-rate tax relief not given at source.
Student Loan Details: If applicable.
Child Benefit Details: If you or your partner earn over £60,000 and receive Child Benefit, you'll need the details to calculate the High Income Child Benefit Charge.
Plouta Tip: Keep meticulous records throughout the year. Using accounting software or a dedicated spreadsheet can save you a huge amount of time and stress when it comes to filing.
Step 3: Understanding Key Deadlines
Missing deadlines can result in penalties, so note these carefully:
Register for Self Assessment: 5th October after the end of the tax year.
Paper Tax Return Filing Deadline: Midnight 31st October after the end of the tax year.
Online Tax Return Filing Deadline: Midnight 31st January after the end of the tax year.
Pay the Tax You Owe (including first Payment on Account): Midnight 31st January after the end of the tax year.
Second Payment on Account Deadline: Midnight 31st July.
Most people file online now as it's quicker, provides instant confirmation, and often calculates your tax liability automatically.
Step 4: Completing Your Tax Return – A Section-by-Section Overview
HMRC's online Self Assessment service is designed to guide you through the process. It tailors the sections you need to complete based on your initial answers about your circumstances.
Log In: Use your Government Gateway ID and password.
Tailor Your Return: You'll be asked a series of questions to determine which sections (supplementary pages) you need to fill in (e.g., for employment, self-employment, property income, capital gains).
Enter Your Details: Carefully input the information you've gathered for each relevant section.
Personal Details: Check these are correct.
Employment Pages (SA102): Information from your P60/P45/P11D.
Self-Employment Pages (SA103S - short, or SA103F - full): Your business income and allowable expenses.
Property Pages (SA105): UK property income and expenses.
Capital Gains Pages (SA108): Details of any assets sold.
Foreign Income Pages (SA106): If you have income from overseas.
Pension Details: Contributions made, income received.
Review Each Section: Double-check all figures for accuracy before moving on. The online system often has built-in checks and prompts.
View Your Calculation: Once you've completed all relevant sections, the system will calculate your tax and National Insurance liability (or refund due). Review this carefully.
Save or Print a Copy: Keep a copy of your submitted return and the tax calculation for your records.
Plouta Tip: Don't leave it until the last minute! Starting early gives you time to gather information, clarify any doubts, and avoid the stress of a looming deadline. You can save your progress online and return to it later.
Step 5: Claiming Allowable Expenses and Reliefs
This is a crucial area where you can legitimately reduce your tax bill.
Self-Employed: Claim for allowable business expenses like office costs (e.g., a portion of home utility bills if you work from home), travel, stationery, professional fees, training, stock, marketing, etc.
Landlords: Claim for allowable expenses like letting agent fees, repairs and maintenance (not improvements), insurance, service charges, etc. (Mortgage interest relief is now given as a basic rate tax reduction).
Other Reliefs:
Pension Contributions: Tax relief is given on contributions up to certain limits.
Gift Aid Donations: If you're a higher/additional rate taxpayer, you can claim the extra relief.
Marriage Allowance: If eligible (see earlier).
Keep thorough records and receipts for all expenses claimed. If unsure what you can claim, check HMRC's guidance or consult an accountant.
Step 6: Calculating Your Tax and National Insurance
If you file online by the deadline, HMRC's system calculates your tax and National Insurance contributions (Class 2 and Class 4 for self-employed) automatically based on the figures you provide.
Payments on Account: If your Self Assessment tax bill is over £1,000 and less than 80% of your income was taxed at source (e.g., through PAYE), you'll usually need to make "Payments on Account." These are advance payments towards your next year's tax bill, each typically being 50% of your previous year's bill.
The first is due by 31 January.
The second is due by 31 July.
Step 7: Filing Your Return and Paying Your Bill
Filing Online: Once you're happy with your return, submit it online. You'll get an instant submission receipt.
Paying Your Tax Bill: You can pay HMRC in several ways:
Online via debit card or bank transfer (CHAPS, Faster Payments, Bacs).
At your bank or building society (if they still offer this and you have a paying-in slip from HMRC).
By Direct Debit (allow time to set this up).
Through your tax code (if you owe less than £3,000 and meet certain conditions, HMRC might be able to collect it through your wages/pension from the next tax year, but you need to file early for this).
Make sure your payment reaches HMRC by the deadline (31 January) to avoid interest and penalties.
Common Mistakes to Avoid When Filing Your Self Assessment
Missing Deadlines: For registration, filing, or payment.
Incorrect Figures: Double-check all entries, especially transposing numbers.
Forgetting Income Sources: Declare all taxable income (e.g., casual earnings, interest).
Over-claiming or Under-claiming Expenses: Only claim for allowable business expenses and keep records.
Not Tailoring Your Return Correctly: Failing to declare a source of income because you didn't tick the right box at the start.
Calculation Errors (if filing by paper): Online filing largely eliminates this.
Not Keeping Records: HMRC can ask to see records for several years.
Penalties: What Happens if You're Late?
HMRC charges penalties for late filing and late payment:
Late Filing:
£100 initial penalty if your return is up to 3 months late.
Further penalties if it's later, potentially reaching £1,600 or more.
Late Payment: Interest is charged on unpaid tax from the due date. Further penalties apply if tax remains unpaid after 30 days, 6 months, and 12 months.
Record Keeping: How Long to Keep Your Documents
You must keep records (e.g., bank statements, receipts, invoices, P60s) for at least:
5 years after the 31 January submission deadline if you're self-employed or have property income.
22 months after the end of the tax year if you're not self-employed and don't have property income.
Important Update: Making Tax Digital for Income Tax Self Assessment (MTD for ITSA)
MTD for ITSA is a new system being introduced by HMRC that will change how self-employed individuals and landlords report their income and expenses.
What it means: You'll need to keep digital records and use MTD-compatible software to send quarterly updates of your income and expenses to HMRC, followed by an end-of-period statement and a final declaration.
Rollout (as of June 2025):
From April 2026: For self-employed individuals and landlords with an annual business or property income above £50,000.
From April 2027: For those with an annual business or property income above £30,000.
Action to take: If this applies to you, start researching MTD-compatible software and consider how you'll adapt your record-keeping.
This is a significant change, so stay informed via the GOV.UK website.
Where to Get Help with Your Self Assessment
GOV.UK Website: The primary source for guidance, forms, and online services.
HMRC Helplines: You can call HMRC for help, though expect queues near deadlines.
Accountant or Tax Advisor: If your affairs are complex, or you lack confidence, a professional can prepare and file your return for you (for a fee). This can often save you tax and stress in the long run.
Charities: Organisations like TaxAid and Tax Help for Older People offer free help to those on lower incomes.
Quick Takeaway Tips for a Stress-Free Self Assessment
Register Early: If you need to.
Keep Good Records: All year round.
Start Early: Don't wait until January.
Use HMRC's Online Service: It's secure and calculates for you.
Double-Check Everything: Before you hit submit.
File and Pay on Time: To avoid penalties.
Understand MTD for ITSA: If it will affect you soon.
Ask for Help if Needed: Don't struggle alone.
Conclusion: Taking Charge of Your Tax Return
Completing your Self Assessment tax return doesn't have to be an annual ordeal. With good organisation, an understanding of what's required, and by starting in good time, you can manage it effectively. Using HMRC's online services and being aware of allowable expenses and reliefs can make the process smoother and ensure you pay the right amount of tax.
By taking control of your tax affairs, you are also taking another important step towards managing your overall finances effectively, a key part of achieving long-term financial security and freedom.
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Disclaimer: This article provides general information about UK Self Assessment tax returns for the tax year ending 5 April 2025 (with online filing due by 31 January 2026), based on guidance available as of June 2025. Tax laws and HMRC procedures can change. This information does not constitute tax or financial advice. Always refer to the official GOV.UK website for the latest guidance or consult with a qualified accountant or tax advisor if you are unsure about your specific tax situation.