Filling in a Self Assessment tax return online can seem complicated, especially if you are filing for the first time. From gathering your income details to claiming allowable expenses, small mistakes can lead to incorrect tax calculations or delays.
The process becomes easier when you know what information you need and which sections to complete.
If you are wondering how to fill in your Self Assessment tax return online, following a few simple steps can help you complete your return accurately and avoid common mistakes.
The following tips for Self Assessment tax return can also help you prepare your information, check your figures and submit your return with confidence.
Gather all your information before you start
Before logging into your HMRC account, collect all the information you need. Having your documents ready can make the process quicker and reduce the risk of missing income or expenses.
You may need:
- Your Unique Taxpayer Reference (UTR)
- National Insurance number
- P60 or P45
- P11D, if applicable
- Details of self-employment income
- Bank and savings interest
- Dividend income
- Rental income
- Pension income
- Details of taxable benefits
- Records of allowable business expenses
- Details of pension contributions and other tax reliefs
Check which tax year you are filing for
Always check the tax year before entering your figures. A Self Assessment return covers a specific tax year, which runs from 6 April to 5 April of the following year.
Using figures from the wrong tax year can result in an incorrect return.
Keep your income records accurate
Enter all taxable income that applies to you. This could include money from employment, self-employment, property, investments, pensions or other sources.
If you have more than one source of income, check each record carefully before entering the figures.
Do not rely on estimates if you have the actual figures available.
Report your self-employment income correctly
If you are self-employed, you will need to provide details of your business income and expenses.
Keep records of:
- Total business income
- Sales and other business receipts
- Allowable expenses
- Business mileage, where applicable
- Equipment and other qualifying costs
- Other business-related payments
Make sure you enter business figures in the correct section of your tax return.
Claim allowable expenses you are entitled to
Allowable expenses can reduce your taxable profit, so it is important not to overlook legitimate business costs.
Depending on your circumstances, these may include:
- Office costs
- Business insurance
- Professional fees
- Travel expenses
- Advertising costs
- Business phone and internet costs
- Equipment and software
- Certain costs of working from home
Only claim expenses that are allowed under HMRC rules and relate to your business.
Check your property income
If you receive rental income, you may need to report it on your Self Assessment tax return.
Keep records of your rental income and eligible expenses. If you own more than one property, make sure you have accurate figures for each property and understand which expenses can be claimed.
Do not forget other sources of income
One common mistake is reporting your main income but forgetting additional income.
Check whether you received income from:
- Savings interest
- Dividends
- Investments
- Pensions
- Property
- Foreign income
- Other taxable sources
Review your bank statements and financial records to help identify income you may have missed.
Use the correct supplementary pages
Not everyone needs to complete the same sections of a Self Assessment tax return.
For example:
- SA100 – Main Self Assessment tax return
- SA103 – Self-employment income
- SA105 – UK property income
- SA106 – Foreign income
- SA108 – Capital gains
Only complete the supplementary pages that apply to your circumstances.
Check your tax reliefs
You may be entitled to certain tax reliefs or deductions. These can reduce the amount of tax you need to pay.
Depending on your circumstances, check whether you need to report:
- Pension contributions
- Gift Aid donations
- Certain employment expenses
- Marriage Allowance
- Other eligible tax reliefs
Keep supporting records for any relief you claim.
Check your National Insurance details
If you are self-employed, your Self Assessment calculation may include National Insurance contributions.
Check that your business income and other details have been entered correctly so HMRC can calculate the amount due.
Check payments on account
If your tax bill is high enough, you may need to make payments on account towards your next tax bill.
When reviewing your Self Assessment calculation, check whether payments on account have been included. This can help you understand why the amount you need to pay may be higher than expected.
Review your tax calculation carefully
Do not submit your return without checking the final calculation.
Review:
- Total income
- Allowable expenses
- Taxable income
- Tax deductions
- Tax reliefs
- National Insurance
- Payments on account
- Total tax due
If something does not look right, go back and check the information you entered.
Submit your return before the deadline
Once you have checked everything, submit your tax return online.
For the 2025/26 tax year, the online Self Assessment filing deadline is 31 January 2027. Any tax owed for the year is generally also due by 31 January 2027.
Avoid leaving your return until the last minute. Filing early gives you more time to correct mistakes and plan for your tax payment.
Check your HMRC account after filing
Submitting your return does not mean you should forget about it.
Log into your HMRC account to check your tax calculation, payment information and any messages from HMRC. This can help you identify if further action is required.
Do not guess if you are unsure
If you are unsure about a figure, expense or section of your tax return, do not simply enter a guess.
Check the relevant HMRC guidance or speak to a qualified accountant or tax professional. Getting advice can help you avoid errors that could result in additional tax, penalties or interest.
At 3E’S, we specialise in making taxes easy and error-free for everyone, and our experts would be delighted to help with Self Assessment online. Reach out to our expert team to know more about our well-loved and affordable services today!
Final words
Hopefully, you are clearer now on how to get started with Self Assessment tax returns. Need further assistance? At 3E’S, we specialise in making taxes easy and error-free for everyone, and our experts would be delighted to help with Self Assessment online. Reach out to our expert team to know more about our well-loved and affordable services today!
Frequently Asked Questions
What information do I need to complete my Self Assessment tax return?
You may need your UTR, National Insurance number, employment income, self-employment records, property income, savings and investment income, expenses and details of any relevant tax reliefs.
What happens if I make a mistake on my Self Assessment tax return?
If you find an error after submitting your return, you may be able to amend it within the applicable amendment period. Check the latest HMRC guidance to understand the rules that apply to your return.
Can I submit my Self Assessment tax return early?
Yes. You do not need to wait until the deadline. Filing early can help you understand your tax bill sooner and give you more time to prepare for payment.
What happens after I submit my Self Assessment tax return?
HMRC processes your return and provides your tax calculation and payment information. You should check your HMRC account for any updates, messages or amounts that you need to pay.
Should I get help with my Self Assessment tax return?
If your tax affairs involve multiple income sources, property, foreign income, capital gains or other complex situations, professional advice can help you complete your return correctly.
