When submitting your Income Tax Self Assessment, there are often more details needed than you might expect. This is because there are many ways to generate “income” beyond simple, contractual employment.

Traditionally, many of these alternative modes of income would correspond to physically separate forms which must be filled in individually when you file Self Assessment. Therefore, it’s important that you keep track of what does or does not apply to you.

Luckily, however, MTD-compatible Self Assessment software allows for a convenient solution: simply fill in a short questionnaire and the correct forms will be identified and filed automatically when you submit your tax return online.

In this article, we check out some common supplementary pages and their meanings, and explain how Self Assessment software can help you save time and hassle by filing your return online!

SA100: Main Tax Form

Before diving into supplementary forms, let’s first establish what goes into the most essential Self Assessment tax return: the SA100. The SA100 form, otherwise known as the “main tax return”, is the baseline report for your Income Tax Self Assessment.

In the SA100, you declare a range of information relating to some types of income, tax relief, student loans and allowances. However, the list is not exhaustive. Therefore, any taxable income not covered in the SA100 must usually be declared through additional forms (the supplementary pages).

One example of this is self-employment, which is not covered in the SA100 (although it covers income from other means such as pensions, dividends and interest). It’s important to read through the forms and work out what is relevant to you – or simply use an online service such as AbraTax, which automatically works it out for you.

SA102: Employment

The employment page (SA102) refers primarily to employees and company directors. If you are employed full-time, part-time, or even casually employed, then you should provide these details. You should also include any income received from any foreign employment while residing in the UK.

This form does not include self-employment (see SA103S below), otherwise known as sole trading. However, if you run your own limited company and are paid as a director, then that would be regarded as “employment” in terms of SA102. (More details can be found on the HMRC website.)

SA103S: Self-employment (short)

The “S” in SA103S refers to “short”. When it comes to declaring details of self-employment, there are two supplementary forms to consider: “short” and “full”. As the most straightforward option, the short form is for small businesses turning over less than £85,000. If you’re a sole trader, or self-employed small business owner, turning over more than £1,000 per year, then you need to submit this information to HMRC.

Fortunately, AbraTax supports the self-employment short form, enabling self-employed small business owners to submit the relevant paperwork directly and online with ease. No paperwork, no hassle!

SA105: UK Property

This supplementary page (SA105) is pretty self-explanatory: use this form to record income derived from UK property rental. This means that, if you receive rent from property you own or lease out in the UK, it must be declared – if the income exceeds £1,000.

There are some technical details to bear in mind here. For instance, if you provide meals or other services, you might have to record this income under “Self-employment”. Furthermore, income from furnished holiday lettings in both the UK and the EEA (European Economic Area) might also need to be declared here.

As always, the specifics can be found on the HMRC website.

Self Assessment software: the simple solution

If your head is spinning with all the details you need to juggle, then you’re in luck! Online tax return software such as AbraTax can help you submit your ITSA (Income Tax Self Assessment) for free. By following a few simple steps, AbraTax will take care of the supplementary nuances and file your return online. No separate forms and no second-guessing.

Filing Self Assessment online is a great option because you don’t need to wade through any paper forms: after providing the relevant details to AbraTax, we work out exactly what to send electronically to HMRC on your behalf. Everything is accessible through your dashboard, and you can submit your returns at a click.


It’s vital not to miss any of the relevant income tax forms when you file Self Assessment. Thankfully, AbraTax provides a free Self Assessment software, allowing you to quickly and easily file tax with supplementary pages (SA102, SA103S, SA105) automatically included where needed. We believe AbraTax provides the best Self Assessment software for individuals (and sole traders) out there – sign up now to benefit for free today!

Disclaimer: We aim to offer educational articles on our blog, focusing on tax-related topics. However, it's important to note that over time, the relevancy of this content might diminish, and we cannot guarantee accuracy. While these articles serve as a tool for enhancing tax knowledge, they are not a replacement for expert advice in accounting, taxation, or legal matters, given the unique nature of each individual's situation. Should you require personalized assistance, we encourage contacting HM Revenue and Customs (HMRC).