When someone tells you that you need to do an SA109, what they actually mean is that you need to do a full UK tax return.

UK Tax Return Explained: SA100, SA109 and What the Supplementary Pages Mean

Someone has told you that you need to do an SA109. Or an SA105. Or one of the other SA forms. And you have absolutely no idea what that means.

Here is the short answer: they mean you need to do a UK tax return.

The SA numbers refer to specific pages within a single document. Understanding how that document is structured makes the whole process far less confusing.

What Is a UK Tax Return?

A UK tax return is a single document submitted to HMRC that reports your income, gains and tax position for a given tax year. The technical name for this process is self assessment.

Despite what the various page numbers might suggest, you do not submit multiple separate forms. Everything goes to HMRC in one submission.

HMRC strongly prefers online filing. Sending a tax return by post is possible but creates problems. HMRC does not confirm receipt until the return has been fully processed, which means you have no way of knowing whether it has arrived safely. Filing online removes that uncertainty entirely.

The SA100: The Foundation of Every Tax Return

Every UK tax return starts with the SA100. This is the core document, eight pages long, that every taxpayer must complete regardless of their circumstances.

The SA100 covers your basic personal details, your income sources at a high level, and a series of questions that determine which supplementary pages you need to add. Think of it as the frame that holds everything else together.

Nobody submits just the SA100 and nothing else unless their tax affairs are very straightforward. For most people, the SA100 generates a list of additional pages that need completing based on their specific situation.

What Are the Supplementary Pages?

Supplementary pages are additional sections that attach to the SA100 based on your individual circumstances. You only complete the pages that are relevant to you.

The most common supplementary pages include the following. Employment income pages cover salary and benefits from your employer. Self employment pages cover income from running your own business. UK property pages cover rental income from UK property. The SA109 covers residence, remittance basis and other international tax matters.

Each supplementary page carries its own SA reference number, which is why people often refer to them by name. But it is important to understand that these pages do not stand alone. They always accompany the SA100 as part of a single complete tax return.

What Is the SA109 and Who Needs It?

The SA109 is the supplementary page that deals with residence and domicile. Anyone whose UK tax position is affected by their residency status needs to complete it.

This includes people who are leaving the UK and claiming non-residency, people who are arriving in the UK and need to establish their residency position, people claiming split year treatment in the year they depart or arrive, and people making claims under the remittance basis or the new Foreign Income and Gains regime.

When an adviser tells you that you need an SA109, what they mean is that your tax return requires the residency supplementary pages in addition to the core SA100. It is not a separate filing. It is one page within one document.

Why Does This Matter?

Understanding how the tax return fits together matters for two reasons.

First, it helps you understand what your adviser is actually doing on your behalf. A request to complete an SA109 is not a small or simple task. It means preparing a full tax return, establishing your residency position under the Statutory Residence Test, and making the appropriate claims and elections within that return. The SA109 is often the most technically complex part of the whole document.

Second, it helps you gather the right information. Your adviser needs your full income picture for the year, not just the income that relates to the specific supplementary page in question. HMRC receives and assesses the return as a whole, so every element needs to be correct.

Filing Deadlines

The UK tax year runs from 6 April to 5 April the following year. For online filing, the deadline is 31 January following the end of the tax year. For the 2024 to 2025 tax year, the online filing deadline is 31 January 2026.

Missing the deadline triggers an automatic penalty of £100, with further penalties applying if the return remains outstanding. Filing online before the deadline and on time avoids all of this.

Get Your Tax Return Right

If you have been told you need a specific supplementary page and you are not sure what that means for your overall tax position, the best thing to do is speak to someone who can assess your full situation rather than just one element of it.

At LSR Partners, we prepare UK tax returns for expats, non-residents and internationally mobile individuals. We handle everything from the SA100 through to the most complex supplementary pages, including the SA109 residency and remittance basis sections.

LSR Partners help you pay the right tax in the right place at the right time. Book a call with us at lsrpartners.com to discuss your situation.

This article is for general information purposes only and does not constitute tax advice. Your individual circumstances will affect your tax return requirements. Please contact us to discuss your specific position.