Self Assessment Online: Step-by-Step Guide to Completing Your HMRC Self Assessment

Understanding how to complete your self assessment online is essential for staying compliant with HMRC and avoiding unnecessary penalties. Whether you are self-employed, a company director, or earning untaxed income from other sources, knowing the correct self assessment steps will make the process far more manageable. Many taxpayers find the HMRC self assessment system confusing at first, especially when trying to gather documents, report income accurately, and ensure everything is filed correctly. This guide breaks down exactly how to file self assessment in a simple, structured way so you can complete your return confidently and on time.

Do I Need to Complete A Self Assessment?

• Your Taxable income was OVER £150 000
• You received a P800 from HMRC saying you didn’t pay enough tax last year
• Sole Proprietor income over £1000.00
• Rental income of over £2500.00
• Untaxed income (like tips or commission) over £2500.00
• Savings or Investment Income over £10,000
• Income from selling shares or second homes
• You’re a director of a company
• Claiming child benefit and your (or your partner’s) income exceeds £50 000.00
• Earning international income, or reside outside of the UK, but earning in the UK
• You’re a trustee of a trust or registered pension scheme
• Your State Pension was your only source of income and was more than your personal allowance (£12,570)

Now that you know whether or not this is something you should be doing, it is worth confirming your position in more detail before proceeding. Our dedicated guide on whether you need to file a self assessment tax return covers a wider range of individual circumstances to help you confirm your obligation before starting. Let’s now take you through the steps to complete your Self-Assessment online.

Step 1: Organise Your Documents

Pre-planning prevents failure, right? So ensure that you are logging in with all the necessary documentation already compiled so you don’t have to scramble around trying to collect documents and information while you are attempting to submit everything under self assessment online.

What will you need for your Self-Assessment using self assessment online?

You will need the following documents and information to complete your online self-assessment: 

• Your ten-digit Unique Taxpayer Reference (UTR)
• Your National Insurance number
• Details of your untaxed income from the tax year (including income from self-employment, dividends and interest on shares)
• Records of any expenses relating to self-employment.
• Any contributions to charity or pensions that might be eligible for tax relief (If applicable)

P60 or other records showing how much income you received that you’ve already paid tax on (via PAYE).

Step 2 – Register for Your Self-Assessment

If this is your first time doing a self-assessment, you will have to register for self-assessment on the gov.uk. Before registering, it is worth confirming your obligation in fullour guide on who must send in a tax return covers every category of taxpayer required to file under HMRC rules, helping you ensure registration is the right next step for your specific situation.

There are three categories of registration. 

• Self-Employed

If you are a sole proprietor and earning an income over £1000, you are required to registered for self assessment. 

• Non Self-Employed
• Registering a partner or partnership

It is equally important to know what happens at the other end of the process. If your circumstances change and you no longer meet the criteria for self-assessment, you must notify HMRC. Our guide on what to do if you no longer need to submit a tax return explains how to deregister correctly and avoid unnecessary filing obligations going forward.

Step 3 – Fill In The Online Self-Assessment Form

There are two sections in the self-assessment form. The first section (main section) deals with: 

• Income (taxed and untaxed) in the form of dividends and interest.
• Contributions to your Pension Fund
• Donations made to charity.
• Benefits (State Pension, Child Benefit and Blind Person’s Allowance).

• Office costs, for example stationery or phone bills
• Travel costs, for example fuel, parking, train or bus fares
• Clothing expenses, for example uniforms
• Staff costs, for example salaries or subcontractor costs
• Things you buy to sell on, for example stock or raw materials
• Financial costs, for example insurance or bank charges
• Costs of your business premises, for example heating, lighting, business rates and maintenance
• Advertising or marketing, for example website costs
• Training courses related to your business, for example refresher courses
• Interest on loans
• Bank, credit card and other financial charges
• Accountancy, legal and other professional costs.
• Vehicle and travel expenses

If you are claiming expenses it is important for you to keep a detailed record of all your expenses for up to 5 years.

It is also worth knowing that submitting your return is not always the final step. If you realise after submission that you have made an error or omitted income, HMRC allows amendments within a set timeframe. Our guide on changing your tax return explains how to make corrections after filing and what deadlines apply for amending a submitted return.

Step 4 – Pay Your Self Assessment

When you’ve successfully submitted your self assessment, HMRC will notify you what your bill is (tax and national insurance if applicable). You can pay in whatever way is most comfortable for you. Some of the options available are: 

• Online Banking.
• CHAPS (Clearing House Automated Payment System).
• Corporate Credit Card.
• Debit Card.
• At your Local Bank.
• At your local building society.
• Send a cheque.
• Arrange Bank Transfer.
• Direct Debit.

However, it is essential to note the deadline (31 January). HMRC must have received the payment by the deadline. If you’re paying on the deadline or very close to it, it is recommended that you use instant payment methods to avoid penalties. One of the most effective ways to avoid this pressure is filing your tax return early, which gives you more time to plan payments, correct any errors, and avoid the risks that come with last-minute submissions.

If you’re close to or past the 31 January deadline, filing late can trigger automatic penalties. We explain how these work and how to limit exposure in our guide to handling late filing penalties from HMRC.

If you’re having trouble with your self assessment you can reach out to the CIGMA Accounting team.

Frequently Asked Questions About Completing Self Assessment Online in the UK

To do a self assessment, you register with HMRC, gather your income and expense records, complete the online tax return, and submit it before the deadline. You then calculate and pay any tax owed based on your total taxable income.

The main steps include registering for self assessment, collecting financial records, completing the SA100 form, calculating tax due, and submitting the return online. Accurate bookkeeping throughout the year makes the process much easier.

In 2026, key self-assessment advice includes keeping digital records, tracking all income sources, claiming all allowable expenses, and submitting returns early. This helps reduce errors and avoid HMRC penalties.

You must complete a self assessment if you are self-employed, a landlord, a company director receiving dividends, or have untaxed income above HMRC thresholds. It ensures all income is properly reported and taxed.

Missing the deadline results in automatic penalties, starting with a fixed fine and increasing over time. Interest is also charged on unpaid tax, making timely submission essential to avoid unnecessary costs.

You can simplify self assessment by using accounting software, maintaining monthly records, separating business and personal finances, and seeking professional advice where needed. This reduces stress and improves accuracy.

Make Filing Your Self Assessment Simple and Accurate

Accurate self assessment accounting is essential for HMRC compliance. We help UK taxpayers understand how to do a self assessment, provide practical self-assessment advice, and ensure correct reporting to reduce errors and improve filing accuracy.

Trusted guidance from London-based accountants, focused on accuracy, clarity, and compliance.