A Guide to Self-Assessment Online

HM Revenue & Customs (HMRC) has the onerous responsibility of running the UK’s tax system. Their key objectives are to collect the right amount of tax and to make it easy for taxpayers to pay what they owe.

The ongoing digitisation of the UK tax system presents a big opportunity for HMRC in terms of improving both accuracy and efficiency, which is why the Making Tax Digital (MTD) programme continues to roll out. MTD mandates the use of software to calculate and submit tax returns rather than using outdated paper-based systems or spreadsheets. MTD for VAT is already in place and Corporation Tax and Income Tax will follow, with MTD for Income Tax set to be phased in from April 2026.

But it was way back in July 2000 when online filing was first introduced for self-assessment. It is of course not mandatory to use the online service and you could still complete a paper SA100 form, but there are numerous benefits of using the self-assessment online service as well as accounting software to calculate and submit your self-assessment each tax year.

The challenges presented by the UK tax system, as well as ensuring compliance, can be something of a daunting prospect for self-employed people. But using HMRC’s online self-assessment service can make the process of meeting your tax obligations much easier and more efficient.

In this post we’ll cover online self-assessment with some tips and advice and how Gorilla Accounting and FreeAgent can help you.

What is Self-Assessment Online?

Self-assessment online is the system used by HMRC to calculate and collect income tax via their online portal for self-employed people and those that meet the criteria that mandates a self-assessment submission. You can check here if you’re unsure.

The self-assessment online system enables taxpayers to digitally complete and submit their self-assessment tax returns which includes reporting business income, expenses, and claiming deductions so the system has the necessary information to calculate tax liability.

What are The Benefits of Online Self-Assessment?

Convenient and Accessible

One of the main benefits of the online self-assessment service is the convenience. It is of course available 24/7 allowing you to handle your tax return at a time that suits you from the comfort of your own home or anywhere that has an internet connection without the need to visit an HMRC office or complete a paper tax return.


The online self-assessment portal has been live for almost a quarter of a century and has received a multitude of improvements and refinements over that time. HMRC wants the process of completing tax returns to be as easy and streamlined as possible for taxpayers so the system is designed to be as clear, intuitive and user-friendly as possible. There’s a host of features including prompts, instructions and guidance as well as calculators and interactive features to help taxpayers accurately complete and file their returns.

Helpful Automation with Real-Time Updates

The self-assessment online portal will use the information that you input – which underscores the importance of diligent bookkeeping – to automatically calculate your tax liability.  This helps to promote accuracy and reduce the chances of under or overpayment as well as any resulting penalties or fine..

The online self-assessment portal includes functionality including alerts and notifications to keep taxpayers abreast of any issues, deadlines or legislative changes. Users can also track the status of their submissions to help ensure everything is in hand.

Security is Paramount

In the digital age online security is very important, especially so with a service such as online self-assessment. It goes without saying that HMRC employs robust industry-standard security measures including a secure encrypted connection, secure sign in, identity verification, multi-factor authentication, automatic logout after 15 minutes of inactivity and a timestamp of your last login, as well as advanced data encryption technologies. You can be confident that HMRC has taken all necessary steps to safeguard your data against unauthorised access and misuse.

How to Submit your Self-Assessment Online

Register for the Self-Assessment Service

To complete and file your self-assessment online, you’ll need to use your Government Gateway user ID and password to sign in to the HMRC self-assessment portal. If you are registering for the first time, you’ll need to wait until your receive your Unique Taxpayer Reference (UTR) number and an activation code from HMRC.

Have Your Documents and Information Ready

Ensure that you have all the documentation needed prepared including records of expenses, income and deductions, receipts, pensions, state benefits, bank statements and P60 and P11D forms if applicable. You’ll also need your 10 digit  UTR number and National Insurance number.

Complete the Self-Assessment

Next you must fill in all required details in the main return and supplementary pages in order to complete the online self-assessment so your tax liability can be calculated. This includes your employment status, income from self-employment as well as extra income from other sources such as capital gains or from rental property.

Carefully Review and Submit

Once you’re confident that the self-assessment is completed, you should carefully review the accuracy of the information you’ve entered and make sure nothing has been missed. If you do find any omissions, discrepancies or inaccuracies they must be corrected before you submit and file your return. Once the submission has completed, you will receive email confirmation.

Complete The Payment

This is the final and least enjoyable part of the process. If you owe any tax, you can settle your liability via the secure HMRC payment portal. Various payment methods are available, however, including direct debit, cheque, bank transfer and debit or credit card.

Online Self-Assessment Tips and Hints

Preparation is Key

Implementing good bookkeeping practices will help to ensure a smooth completion of your online self-assessment. To that end it’s important to keep detailed and organised records of the financial activity of your business including invoices, expenses, bank records and any other supporting documentation covering the tax period to ensure you have all the relevant information you need to accurately complete and file your online self-assessment.

Start Early

Leaving things to the last minute is never a good idea and tax returns are no exception, so make an early start rather than leaving it late. By gathering all required documentation and getting your return underway in good time before the deadline, you give yourself breathing room if there are any issues to resolve and you also prevent a last minute rush which can increase the chance of errors and inaccuracies and subsequently receiving a fine or penalty from HMRC.

Look Out for Legislative Changes

The world of taxation is dynamic and changes to tax laws are a regular occurrence. Examples are changes to thresholds, deadlines, tax rates and tax-free allowances. It’s important that you keep abreast of any relevant changes to help ensure that you remain compliant.

Use Professional Help

If you are struggling to complete your online self-assessment or if you have any queries or are unsure about any elements of it, it’s a good idea to seek professional help from an expert such as an accountant. They will be able to offer advice and guidance based on your circumstances to help ensure a timely and accurate submission.

When you appoint Gorilla, your dedicated accountant will assist with the preparation of your annual self-assessment tax return as part of our service giving you peace of mind that your self-assessment will be expertly handled, as well as all your personal and business accounting needs.

You’ll also receive full access to FreeAgent accounting software which automatically calculates parts of your self-assessment in the background based upon the information you input during the tax year. To file your tax return, you just need to check the information, complete any extra details and FreeAgent then files your return directly with HMRC.

If you’re not a Gorilla client you can use our one-off self-assessment service so one of our expert accountants can prepare your self-assessment starting at just £200 + VAT. Fill out our Personal Tax Questionnaire to get the ball rolling.

Speak to an accountant today on 0330 024 0406 or request a callback if you need advice on self-assessment or if you have any queries about our accounting service and FreeAgent.