Self Assessment Tax Return: A Complete Guide

Anyone within the UK who earns a type of income that isn’t fully taxed via PAYE will need to complete a self assessment tax returnWhether you’re self-employed, a landlord, or freelancer, or simply have extra income, you might need to inform HM Revenue and Customs (HMRC) about your earnings.

Whilst tax returns can appear complex, they become much easier when you understand how they workBy knowing who needs to complete them, what information to have ready, and when to submit them can help take away any stress you may feel.

Below we’ll explain how the system works and give some useful top tips.

What is a Self Assessment Tax Return?

A self assessment tax return is a form that HMRC use to gather information about your income, expenditure, and other financial informationThis allows them to work out how much Income Tax you owe.

Some individuals will need to work out how much taxable income they have and declare it, rather than having it automatically taken out of their earnings like employees.

The tax year in the UK runs from the 6th April to 5th April the following yearAnyone who is eligible will have to declare their financial information and pay any outstanding tax by the relevant deadlines.

Who Needs to Complete a Tax Return?

Not everyone will need to complete a tax return, but some individuals will need to register for Self Assessment to declare their income.

• Self-employed and earned more than £1,000 (before allowable expenses)
• Have rental income
• Earned income from freelancing or side hustle
• Have investment or savings income to declare
• Have certain foreign income
• Have to pay Capital Gains Tax on any taxable profit
• A company director with income to declare

For example, if you work full time but earn some money from freelancing as a graphic designer, your employer might take tax from your salaryHowever, you may still need to declare your extra income.

Always check HMRC’s latest guidance to see if you need to complete a tax return.

How to Register for Self Assessment

If you need to complete a tax return for the first time, then you will need to register with HMRCOnce registered, you will be given a Unique Taxpayer Reference (UTR)You can complete this on the official GOV.UK websiteYou may also have to create or log into your Government Gateway.

Make sure you store your Unique Taxpayer Reference and login details so you can use them when completing your return.

How to Complete a Self Assessment Tax Return

Having all your information to hand will really help when completing your returnFollow the steps below.

Gather your Information

Before you start, you’ll want to gather all your documentsThis could include:

• Bank statements and business expenditure
• Employment income and P60s
• Freelance/self-employed income
• Rental income
• Receipts for allowable business expenditure
• Pension contributions and any other relevant tax relief

This will help ensure you don’t miss anything important.

Log Into Your HMRC Account

Go to the official HMRC website and log in using your Government Gateway detailsChoose the option to complete your tax return and select the tax year.

Complete Your Income and Expenditure

Enter accurate information about your taxable incomeIf you’re self-employed, enter your business income and allowable expenditure.

For example, if you’re a freelance writer who earned £20,000 during your tax year, you may be able to claim back certain business expenses such as some software subscriptions and office costs.

Double Check and Submit

Carefully go through each section of your return and ensure all your income, expenditure, and personal information is correct.

Once you’ve submitted your form, keep a copy of your confirmationHMRC will then calculate how much tax you owe.

Important Tax Return Deadlines

It’s important that you meet your deadlines with HMRC to avoid any penalties.

For the 2025-26 tax year, the deadlines are:

Requirement Deadline First Registration 5th October 2026 Paper return 31st October 2026 Online return 31st January 2027 Outstanding Tax 31st January 2027

If you’re completing a tax return for the first time and have missed the deadline for registering with HMRC, then get in touch with them as soon as possibleYou may find your registration letter has a different deadline for completing your return.

If you complete your online return after the deadline, you’ll usually be charged an initial £100 fine, even if you don’t have any tax to payThere may be further fines if you’re late.

Check the latest deadlines and fines on the official HMRC tax return deadlines page.

Common Mistakes

Avoid making any mistakes when completing your returnCommon mistakes include:

• Not declaring all your income: Declare all relevant taxable incomes including side jobs and rental properties.
• Incorrect expenditure: Only claim allowable expenditure.
• Late submissions: Complete your return before the deadline to avoid any unnecessary fines.
• Not keeping records: Save your receipts, invoices, and bank statements.
• Incorrect personal information: Check your National Insurance number and other information.

If your finances are complicated, consider getting help from a qualified accountant or tax adviser.

Final Thoughts

Completing a tax return doesn’t have to be a daunting taskIf you prepare properly, keep accurate records, and understand what HMRC require, then you can feel confident.

Whether you’re self-employed, have rental income, or other taxable incomes, understanding how to complete your return will help avoid common mistakes and unnecessary fines.

Start preparing early, check the deadlines, and double-check your informationThis simple process will help make your annual tax obligations much easier.