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How to Complete Your Self Assessment Tax Return: A Step-by-Step Guide
What Is a Self Assessment Tax Return?
Many people believe that tax is automatically taken care of through PAYE, but that isn’t true for everyone. If you earn income that HMRC cannot fully tax at source, you’ll usually need to report it through a Self Assessment tax return.
This enables HMRC to calculate the tax you owe based on various sources of income, including self-employment profits, rental income, dividends, savings and investment income, capital gains, and foreign income. When completing your return, HMRC also considers any allowable expenses, tax reliefs, available allowances, and National Insurance contributions to determine your final tax liability.
Do You Need to File a Self Assessment Tax Return?
Not everyone needs to complete a Self Assessment tax return. However, if you receive income that HMRC cannot automatically tax through PAYE, you may be required to report it yourself. This commonly applies to self-employed individuals, business partners, landlords, investors, and people with other sources of untaxed income.
You will usually need to file a Self Assessment tax return if any of the following apply during the tax year:
- You were self-employed as a sole trader and earned more than £1,000 before expenses.
- You were a partner in a business partnership.
- You had Capital Gains Tax to pay after selling or disposing of an asset.
- You were liable for the High Income Child Benefit Charge.
- You received untaxed income, such as rental income, dividends, savings interest, tips, commission, or foreign income.
- HMRC specifically asked you to submit a tax return.
If you’re filing a Self Assessment return for the first time, you’ll also need to register with HMRC. The registration deadline is 5 October following the end of the tax year. For example, if you need to report income for the 2025/26 tax year, which ends on 5 April 2026, you must register by 5 October 2026.
Remember, filing requirements are based on your individual circumstances, not just your employment status. Even if most of your income is taxed through PAYE, additional income from property, investments, or other sources could still mean you need to submit a Self Assessment tax return.
What Information Do You Need Before Starting Your HMRC Self Assessment?
A successful Self Assessment tax return starts with good preparation. Gathering the right income records, tax documents, and supporting evidence beforehand can save time, improve accuracy, and help you avoid costly mistakes when filing with HMRC.
Your UTR Number and HMRC Self Assessment Login Details
Before you can complete your Self Assessment tax return, you’ll need your Unique Taxpayer Reference (UTR) and a Government Gateway account. These details allow you to access HMRC’s online filing system, submit your return, and manage your tax affairs securely.
Income Records You’ll Need
Gather records of all income received during the tax year, including self-employment profits, employment income, rental income, dividends, savings interest, foreign income, and any other taxable earnings. Accurate records help ensure your Self Assessment tax return is completed correctly.
Business Expenses, Deductions and Tax Reliefs
If you’re claiming deductions, keep details of any allowable business expenses, pension contributions, Gift Aid donations, and other tax reliefs. Claiming eligible expenses and reliefs can help reduce your overall tax bill.
Documents That Support Your Self Assessment Tax Return
Supporting documents such as invoices, receipts, bank statements, P60S, P45S, P11Ds (if applicable), dividend vouchers, and rental records can help verify the figures you enter on your Self Assessment tax return and make it easier to respond to any HMRC queries.
Key HMRC Self Assessment Deadlines You Must Know
Missing a Self Assessment deadline can be costly. Late registration, filing, or payment can all result in penalties and interest charges from HMRC. The good news is that staying on top of a few key dates can help you avoid unnecessary costs and keep your tax affairs running smoothly. Here are the deadlines every taxpayer should know before completing their return.
First Self Assessment Registration Deadline
If you’re filing a Self Assessment tax return for the first time, you must register with HMRC by 5 October following the end of the tax year in which you became liable to file.
Self Assessment Tax Return Submission Deadlines
The deadline for online Self Assessment tax returns is 31 January following the end of the tax year. If you choose to file a paper return, you’ll need to submit it much earlier. (HMRC must receive your paper tax return by 11:59 pm on 31 October 2026)
Self Assessment Tax Payment Deadlines
You must usually pay any Income Tax and National Insurance due by 31 January. Paying on time helps you avoid interest charges and potential HMRC penalties.
Payments on Account Deadlines
If you’re required to make Payments on Account, these advance tax payments are typically due in two instalments: 31 January and 31 July. Missing either deadline may result in additional interest.
How to Complete a Self Assessment Tax Return Online
Completing a Self Assessment tax return may seem daunting at first, but the process is far more straightforward when broken down into manageable steps. By following the correct process and checking your information carefully, you can submit an accurate return, avoid unnecessary penalties, and pay only the tax you owe.
1. Sign In Using Your HMRC Self Assessment Login
Start by logging into your HMRC online account using your Government Gateway ID and password. If you’re filing for the first time, make sure you’ve registered for Self Assessment and received your UTR number.
2. Choose the Correct Tax Year
Select the tax year you’re filing for. Make sure all income and expenses relate to the correct tax year to avoid errors in your tax calculation.
3. Enter Employment and PAYE Income
If you’ve been employed during the tax year, enter details from your P60, P45, or other employment records. This allows HMRC to take any tax already paid through PAYE into account.
4. Report Self-Employment Income
Declare all income earned from self-employment and provide details of any allowable business expenses. This helps HMRC calculate your taxable profit accurately.
5. Declare Property and Rental Income
If you receive income from renting out property, you’ll need to report your rental earnings and any allowable property expenses for the tax year.
6. Add Dividends, Savings, and Other Income
Include any additional income sources, such as dividends, savings interest, foreign income, tips, commissions, or investment income that may be taxable.
7. Claim Allowable Expenses and Reliefs
Review any tax reliefs and deductions you may be entitled to claim, including business expenses, pension contributions, and Gift Aid donations. Claiming eligible reliefs can reduce your tax bill.
8. Review Your Tax Calculation
Before submitting, carefully check all figures and review HMRC’s tax calculation. This is your opportunity to correct any mistakes before they become costly.
9. Submit Your Self Assessment Tax Return
Once you’re satisfied that everything is correct, submit your Self Assessment tax return online. Keep a copy of your submission confirmation and tax calculation for your records.
How Your Self Assessment Tax Bill Is Calculated
Wondering why your Self Assessment tax bill is sometimes much higher than expected? You’re not alone. Many taxpayers focus on submitting their tax return but pay little attention to how HMRC actually calculates the amount due.
As a result, unexpected tax bills, overlooked National Insurance contributions, and surprise Payments on Account can catch people off guard. Understanding the components behind your tax calculation can help you budget more effectively, avoid surprises, and file your Self Assessment return with confidence.
Income Tax
This is the largest part of your Self Assessment tax bill. HMRC calculates this based on your taxable income, which may include self-employment profits, rental income, dividends, savings interest, and other taxable earnings. After applying any available allowances and tax reliefs, the remaining amount is taxed using the relevant Income Tax bands.
National Insurance Contributions (NICs)
If you’re self-employed, your tax bill may also include National Insurance Contributions (NICs). These contributions help fund certain state benefits and are calculated separately from Income Tax. The amount you pay typically depends on your taxable profits for the tax year.
Payments on Account
One of the most misunderstood parts of Self Assessment is Payments on Account. If your tax bill exceeds certain thresholds, HMRC may ask you to make advance payments towards next year’s tax liability. This often surprises first-time filers because they may end up paying part of their future tax bill alongside their current year’s liability.
Balancing Payment
After HMRC compares your actual tax liability with any Payments on Account you’ve already made, it calculates what’s known as a Balancing Payment. If you’ve underpaid during the year, you’ll need to pay the difference. If you’ve overpaid, you may be entitled to a refund. This final adjustment ensures you pay the correct amount of tax based on your actual income and circumstances.
How to Pay Your Self Assessment Tax Bill
You’ve completed your Self Assessment tax return. Now comes the part many taxpayers worry about most: paying the bill. The good news is that HMRC offers several payment methods, allowing you to choose the option that best suits your circumstances. Whichever method you use, paying on time is essential to avoid interest charges and potential penalties.
Online Banking
Online banking is one of the most popular ways to pay a Self Assessment tax bill. Simply make a bank transfer from your personal or business bank account to HMRC using the payment details provided in your online account.
CHAPS and Faster Payments
If you’re making a last-minute payment, Faster Payments and CHAPS can be useful options. These electronic bank transfer methods are typically quicker than standard bank payments, helping ensure your payment reaches HMRC before the deadline.
Direct Debit
Direct Debit can be a convenient option for taxpayers who prefer a more automated approach. Once set up, HMRC can collect the amount due directly from your bank account, reducing the risk of forgetting an important payment deadline.
Debit or Corporate Credit Card
HMRC also allows Self Assessment tax payments by debit card and certain corporate credit cards online. This can be a helpful option if you want a fast and convenient way to settle your tax liability.
Using the Correct Payment Reference
No matter how you choose to pay, using the correct payment reference is crucial. Your 10-digit Unique Taxpayer Reference (UTR), followed by the letter K, tells HMRC which taxpayer the payment belongs to. Entering an incorrect reference could delay the allocation of your payment and potentially lead to unnecessary interest or payment reminders.
Top tip: Don’t wait until the deadline to pay. Leaving it to the last minute can create unnecessary stress, especially if there are banking delays or payment processing issues. Planning helps ensure you pay your Self Assessment tax bill on time and in full.
Common Mistakes of Filing a Self Assessment Tax Return
Think the hard part is over once you’ve completed your Self Assessment tax return? Think again. Many taxpayers end up paying more tax than necessary, receiving HMRC penalties, or facing avoidable compliance issues because of simple mistakes. The good news is that most of these errors are easy to avoid if you know what to look out for.
Missing Sources of Income
One of the most common Self Assessment mistakes is failing to declare all taxable income. Rental income, dividends, savings interest, foreign income, tips, and freelance earnings can sometimes be overlooked, resulting in an inaccurate tax return and potential HMRC enquiries.
Claiming Incorrect Expenses
While claiming allowable expenses can reduce your tax bill, claiming personal costs or non-qualifying expenses can create problems. Before submitting your Self Assessment tax return, make sure every expense is wholly and exclusively related to your business activities.
Missing Deadlines
Late registration, late filing, or late payment can all lead to penalties and interest charges. Keeping track of key Self Assessment deadlines is one of the simplest ways to avoid unnecessary costs and stay compliant with HMRC requirements.
Using Incorrect Figures
Entering estimates, transposing numbers, or relying on incomplete records can result in errors within your tax return. Always check your figures against supporting documents such as invoices, bank statements, P60s, and dividend vouchers before submitting.
Forgetting Payments on Account
Many taxpayers are surprised when their Self Assessment tax bill is higher than expected because they overlook Payments on Account. These are advance payments towards your next tax bill, which may be due alongside your current year’s tax. Checking your tax calculation carefully can help you avoid unexpected costs and plan your cash flow more effectively.
What Happens If You Submit a Self Assessment Tax Return Late?
Think missing a Self Assessment deadline is no big deal? It can quickly become an expensive mistake. A late tax return or overdue payment may trigger HMRC penalties, interest charges, and potentially increased scrutiny of your tax affairs. The longer the delay, the greater the financial consequences, making it important to stay ahead of every deadline.
Late Filing Penalties
HMRC charges penalties for late Self Assessment tax returns, even if you have little or no tax to pay. You’ll usually receive an initial £100 penalty as soon as the filing deadline is missed. If the return remains outstanding for more than three months, additional penalties of £10 per day may apply, up to £900. Further charges can be added after six and twelve months, amounting to 5% of the tax due or £300, whichever is greater. Filing on time is the simplest way to avoid these unnecessary costs.
Interest on Late Payments
Submitting your tax return is only part of the process. If you pay your Self Assessment tax bill late, HMRC will also charge interest on the outstanding balance until it is paid in full. Separate late payment penalties may apply if tax remains unpaid for extended periods.
HMRC Compliance Checks
Late, incomplete, or inaccurate returns can increase the likelihood of an HMRC compliance check. During a review, HMRC may ask for records, receipts, and evidence to support the figures reported on your tax return. Keeping accurate records and filing on time can help reduce the risk of unnecessary scrutiny.
Self Assessment Record-Keeping Requirements
Think of your records as your safety net. Whether you’re claiming expenses, reporting income, or responding to an HMRC query, having the right documents makes Self Assessment less stressful and helps ensure you pay only the tax you actually owe.
What Records Should Be Kept?
A good rule of thumb is to keep evidence for every figure you enter on your tax return. This includes invoices, receipts, bank statements, payroll records, rental income details, dividend vouchers, and documents relating to tax reliefs or allowances. The more organised your records are, the easier it is to complete your return accurately and confidently.
How Long Should Records Be Retained?
Don’t throw away your paperwork too soon. HMRC generally requires Self Assessment records to be kept for at least five years after the 31 January submission deadline for the relevant tax year. Keeping records for the required period can protect you if HMRC asks questions later and gives you peace of mind that you’re fully compliant.
Making Tax Digital and Future Changes to HMRC Self Assessment
Self Assessment is becoming more digital, and that’s good news for many taxpayers. Making Tax Digital (MTD) for Income Tax is designed to reduce errors, simplify record-keeping, and help individuals stay on top of their tax obligations throughout the year rather than rushing everything at the last minute.
Who Will Need to Use Making Tax Digital?
Making Tax Digital for Income Tax is being introduced in stages for sole traders and landlords based on their qualifying income (better to explain what qualifying income is) from self-employment and property. If your qualifying income exceeded £50,000 in the 2024/25 tax year, you should be using MTD from 6 April 2026. The threshold falls to £30,000 from April 2027 and £20,000 from April 2028.
For many taxpayers, this change offers a chance to modernise their finances, gain better visibility over income and expenses, and avoid the last-minute stress often associated with annual tax returns.
Digital Record-Keeping Requirements
Under MTD, you’ll need to keep digital records of your income and expenses using compatible software. Digital record-keeping helps reduce manual errors, keeps important information organised, and makes it easier to track your tax position throughout the year.
The real advantage is confidence. With accurate records maintained year-round, preparing tax submissions becomes quicker, simpler, and far less overwhelming.
Quarterly Updates and Final Declaration
Instead of relying solely on a single annual submission, MTD requires taxpayers within scope to send quarterly updates to HMRC using compatible software. These updates provide HMRC with a summary of income and expenses throughout the tax year. At the end of the year, you’ll then submit a Final Declaration to confirm your income, claim any reliefs or allowances, and finalise your tax position.
While this may sound like extra work at first, many taxpayers find that regular updates make tax management more manageable. Rather than facing one large Self Assessment task each January, you can spread the workload across the year, making it easier to stay organised and avoid surprises when your tax bill becomes due.
When Should You Get Help With Your Self Assessment Tax Return?
Many people start their Self Assessment thinking it’ll take an hour or two, only to find themselves second-guessing figures, worrying about HMRC rules, and wondering if they’ve missed something important. If that sounds familiar, it may be time to get professional support.
Multiple Sources of Income
The more income streams you have, the easier it is to miss something. Whether you’re self-employed, receive dividends, earn rental income, or have investments, keeping track of different tax rules can quickly become overwhelming.
Rental Properties and Landlord Income
Owning property comes with its own tax challenges. From claiming the right expenses to understanding what can and can’t be deducted, many landlords seek advice simply for the peace of mind that they’re getting it right.
Selling Assets and Capital Gains
Sold a property, shares, or another valuable asset? Calculating Capital Gains Tax isn’t always straightforward. A mistake could mean paying more tax than necessary or attracting unwanted attention from HMRC.
Higher Income and Complex Tax Affairs
As your income grows, your tax affairs can become more complicated. Additional tax rules, reliefs, allowances, and reporting requirements may apply. Professional advice can help you stay compliant while identifying opportunities to improve tax efficiency.
Avoiding Costly Self Assessment Mistakes
Let’s be honest, most people don’t worry about completing a tax return. They worry about getting it wrong. Missing income, claiming incorrect expenses, or overlooking a deadline can lead to penalties, interest, and stress that far outweigh the cost of getting expert help.
Feeling overwhelmed by Self Assessment? You’re not alone. Whether you’re a sole trader, landlord, company director, or growing business owner, the team at More Than Accountants can help you stay compliant, minimise tax liabilities, and file with confidence. Get in touch today for expert support tailored to your circumstances.
Frequently Asked Questions About Self Assessment
How Do I Find My UTR Number?
Your UTR (Unique Taxpayer Reference) can be found on HMRC letters, previous tax returns, payment reminders, or within your HMRC online account.
Can I Amend a Submitted Self Assessment Tax Return?
Yes. If you’ve made a mistake, you can usually amend your Self Assessment tax return after submission and update the incorrect information.
What If I Made a Mistake?
Don’t panic. You can correct most errors by amending your tax return or contacting HMRC. The sooner you act, the easier it is to resolve the issue.
Do I Need to File If I Made No Profit?
Usually, yes. If you’re registered for Self Assessment, HMRC may still require a tax return, even if your business made no profit or incurred a loss.
Can HMRC Issue a Tax Refund?
Yes. You may be entitled to a Self Assessment tax refund if you’ve overpaid tax, claimed additional reliefs, or paid more through Payments on Account than your final tax bill required.
Not sure what applies to you? More Than Accountants can help you complete your Self Assessment correctly, identify available tax reliefs, and ensure you stay compliant with HMRC requirements.