How to Do a Self Assessment: Step-by-Step Complete Guide

Self Assessment can feel intimidating, especially if it's your first time or if your income has become more complex. But once you understand how it works and what His Majesty's Revenue and Customs (HMRC) expects, it becomes a clear and manageable process.

In the UK, Self Assessment is how HMRC collects tax from people whose income isn't fully taxed through PAYE. This includes sole traders, freelancers, contractors, landlords, and anyone with other untaxed income. From 2026, the process will also start to change due to the rollout of Making Tax Digital for Income Tax.

This guide explains how to do a Self Assessment step by step, what you need to prepare, how to submit your return, and what is changing from 2026 onwards. 

Whether you're filing for the first time or getting ready for Making Tax Digital (MTD), this guide will help you stay compliant and avoid surprises.

Key points

• Deadlines are strict, and missing them is costly ⏰
  You must register for Self Assessment by 5 October following the end of the tax year in which you qualify, and submit your return and pay any tax due by 31 January. Missing these dates can trigger automatic penalties, even if you owe no tax.
• Good preparation makes the whole process easier 📋
  Getting organised early by keeping clear records of income and allowable expenses reduces mistakes, speeds up filing, and helps ensure you only pay the tax you actually owe.
• Making Tax Digital will change how many people report tax from 2026 💻
  If your income from self-employment or property is above the thresholds, you'll need to keep digital records and submit quarterly updates, even though you'll still pay tax annually.
• ANNA can simplify your tax admin 🚀
  ANNA automatically tracks your income and expenses, calculates your tax in real time, and helps you stay on top of your tax obligations, so you can spend less time on paperwork and avoid unexpected tax bills.

What is Self Assessment?

Self Assessment is the system HMRC uses to calculate Income Tax and National Insurance for people who don't have all their tax deducted automatically.

If you're employed under PAYE, your employer deducts tax before paying you. However, if you earn money outside PAYE, you're responsible for reporting that income and paying the correct tax yourself.

You can do this by completing a Self Assessment tax return for each tax year. The UK tax year runs from 6 April to 5 April the following year. You're required to submit your return and pay any tax owed by 31 January after the end of the relevant tax year.

Self Assessment isn't just about paying tax, though. It's also how you claim allowable expenses, tax reliefs, and deductions that reduce your tax bill. 

Who needs to file a Self Assessment?

You usually need to file a Self Assessment tax return if any of the following apply:

• You're self-employed and earned more than £1,000 in the tax year.
• You're a partner in a business partnership.
• You receive rental income from a property.
• You earn untaxed income, such as dividends or interest.
• You're a company director and receive income outside PAYE.
• You need to pay the High Income Child Benefit Charge.
• You have foreign income that is taxable in the UK.

HMRC will often tell you if they expect a return from you, but it's still your responsibility to check. If you're unsure, it's better to confirm early than to risk missing a deadline.

Key Self Assessment deadlines

Deadlines matter, and missing them can lead to automatic penalties.

For most people, the main dates are:

• 5 October, following the end of the tax year – The date by which you have to register for Self Assessment if it's your first time
• 31 January – Due date for submitting your online tax return and paying any tax due

For example, for the 2025 to 2026 tax year, the deadline to file online and pay is 31 January 2027.

From 2026 onwards, some people also have quarterly reporting deadlines under Making Tax Digital.

How to do a Self Assessment: 6 main steps to follow

Filing a Self Assessment is much easier when you know what to expect. Follow the steps below to register, prepare your information, and submit your tax return.

Step 1: Register for Self Assessment

If you have never filed a Self Assessment before, you need to register with HMRC. You should register as soon as you know you need to file. 

When you register, HMRC will issue you with a Unique Taxpayer Reference (UTR). This is a ten-digit number that identifies you for tax purposes.

HMRC sends your UTR by post, which can take time. Keep in mind that you can't submit your tax return without it.

Once registered, you'll also need a Government Gateway account to access HMRC's online services. You can set one up on the HMRC website using your business details and a valid email address. This will allow you to file taxes, manage VAT, and handle payroll digitally.

Step 2: Get organised early

Preparation is the most important part of Self Assessment. Before you start completing your tax return, you should have:

• Your UTR and National Insurance number
• Records of all income for the tax year
• Bank statements
• Receipts and invoices for business expenses
• P60 or P45 form if you had PAYE income
• Details of dividends, interest or rental income

If you're self-employed, you need to work out your total business income and subtract allowable expenses to calculate your profit. Tax is paid on profit, not turnover.

Pro tip: 

Using a business account and bookkeeping tools like those offered by ANNA can make this much easier by automatically categorising transactions and storing receipts as you go.

Step 3: Understand allowable expenses

Allowable expenses are costs that are exclusively for business use. These reduce your taxable profit and, therefore, your tax bill.

Common examples include:

• Office supplies and software
• Business travel and mileage
• Professional fees and insurance
• Use of home for business
• Marketing and advertising costs

Personal expenses aren't allowable, even if you use them partly for work; if an expense is partly personal and partly business, you can only claim the business portion.

Step 4: Log in and start your return

Once your records are ready, log in to HMRC using your Government Gateway account and select Self Assessment.

The online form is divided into sections and will guide you through each step. Keep in mind that you don't have to complete it in one sitting; you can save and finish it at any time.

You'll enter details about:

• Employment income
• Self-employment income and expenses
• Property income
• Savings and dividends
• Pensions and benefits
• Tax reliefs and adjustments

While you are entering this information, HMRC's system will calculate your estimated tax bill.

Step 5: Review and submit

Before submitting, review everything carefully. Ensure that income figures match your records and that expenses are accurate.

Once submitted, you'll receive confirmation from HMRC. At this point, your tax bill becomes final. However, if you spot a mistake after submission, you can usually amend your return online within a set time period (usually 12 to 15 months).

Step 6: Pay your tax

You can pay using online banking, debit card, or direct debit.

If your last Self Assessment tax bill was over £1,000 and less than 80% was already collected through PAYE, HMRC will ask you to make payments on account – advance payments towards next year's tax, due in January and July.

If you cannot afford to pay in full, contact HMRC as soon as possible. Not paying the bill results in penalties and interest, but payment plans are sometimes available.

What is changing in 2026 with Making Tax Digital?

From April 2026, Self Assessment changed for many people due to the introduction of Making Tax Digital for Income Tax Self Assessment (MTD for ITSA).

This is a new way to report income and expenses as they arise, rather than only at the end of the tax year.

Who will be affected?

MTD for ITSA will be introduced in phases:

• From 6 April 2026 – This first phase applies to those with a combined income from self-employment and property over £50,000.
• From 6 April 2027 – The next phase covers those with income over £30,000.
• From 6 April 2028 – The final phase applies to those with income over £20,000.

These thresholds refer to total income before expenses.

If your income is below the threshold, you can continue with the current Self Assessment process. HMRC hasn't set a mandatory date for those below the £20,000 threshold to join MTD for ITSA, and this group can continue to use the usual method until further notice.

What will be different?

If MTD applies to you, you'll need to:

• Keep digital records of income and expenses
• Use MTD-compatible software
• Submit four quarterly updates to HMRC each year
• Submit a final year-end declaration, similar to the current Self Assessment return

Keep in mind that quarterly updates don't mean quarterly payments. They are summaries, not final tax calculations, and you'll still pay tax by 31 January as usual.

Why this matters now

Preparing early is important. Moving to digital records and regular reporting can feel like a big shift if you're used to doing everything once a year.

Tools like ANNA’s Auto Accountant are designed to support this transition by keeping records digitally, tracking income in real time, and helping you stay prepared for MTD without changing how you work overnight.

Tips and common Self Assessment problems to be aware of

A little planning goes a long way with Self Assessment. Here are some tips that should make this task easier:

Keep your records organised

Under MTD for ITSA, keeping your business records digitally organised will become a legal requirement, not just a useful practice. This means all income, expenses, and receipts must be stored in a way that can be reported through compatible software.

Even beyond compliance, digital records make it easier to access and explain figures if HMRC requests clarification. HMRC can request supporting documents for up to five years (six for companies) after the 31 January deadline, so organised digital storage will help you stay prepared and avoid issues.

Do not forget income

It's easy to overlook small income sources, especially from side work or irregular payments. All taxable income must be included, regardless of how minor it may seem. Missing income is one of the most common reasons HMRC challenges a return.

Set money aside for tax as you earn

Putting funds into a separate savings pot throughout the year helps avoid a large bill shock in January. This is especially useful if your income varies month to month.

File early and check your access

Don't leave your return until January. Filing early avoids last-minute stress, penalties, and technical issues. Log in to your Government Gateway well before the deadline to make sure passwords and security details are working.

Stay aware of Making Tax Digital changes

As HMRC moves towards digital reporting, check whether MTD will apply to you in future tax years. Getting everything in order early makes the transition smoother and prevents any rushed decisions that could lead to mistakes later.

How ANNA helps with Self Assessment and tax

Self Assessment is much easier when your finances are organised all year round, not just in January. That is where ANNA comes in.

ANNA is designed for freelancers, contractors, and small business owners who want to spend less time on admin and more time running their business. With ANNA, your business account and bookkeeping work together, so income and expenses are automatically tracked as you go.

You can use ANNA+Taxes for your business to:

• Automatically categorise transactions as income or expense and match them to receipts. This makes your bookkeeping accurate and saves hours of manual work.
• Store receipts digitally by snapping a photo in the app or uploading them to your dashboard. Quotations and invoices can also be stored and linked to matching transactions.
• Track income and expenses in real time by automatically categorising transactions as they happen, so you always know exactly where your business stands.
• Get an estimate of your tax bill as you earn with ANNA’s Auto Accountant, which automatically calculates tax estimates and helps you set aside money in smart pots.

Get started with ANNA today, and handle your Self Assessment with ease.