The SA100 form: how to use it to file your Self Assessment tax return

If you’re submitting by post, the SA100 form is the main document you need to fill out when doing your Self Assessment tax return.

You fill out the same information whether you file an online or a paper tax return, so it’s completely up to you how you want to file. Just remember, the deadline to file and submit your tax return by post is October 31st following the end of the tax year.

For example:

• If you’re filing for the 2024/2025 tax year
• You have to register for Self Assessment by 5th October 2025
• You have until midnight on 31st October 2026 to submit your paper tax return

If you’re not sure that you’ll need to submit and file a Self Assessment, then check out this guide on who needs to file a tax return.

How do I fill the SA100 form?

You’ll find all the documents you’ll need to print and fill out on HMRC’s website. 

There, you’ll be able to pick the year you want to file for. Once you choose what year you need, you can download the SA100 tax return form to your computer. 

Remember, if you want the form for the 2024/2025 tax year, you click on 2025. If you want it for the tax year before that, you can choose 2024, and so on.

If you want to fill in tax return forms published before 2019, you can find them on the National Archive.

Once you’ve picked your year, all you have to do is:

• Print it
• Fill it in
• Submit it

Easy! Don’t forget to attach any supplementary forms as well, depending on what your situation is. 

If you file a Self Assessment tax return every year, then HMRC might also send you your SA100 tax return to fill out via post. However, we recommend not cutting it too close to the deadline. 

HMRC have instructions that will be helpful for you to reach before you begin filling in the form. It’s titled “How to fill your tax return” and it’s just underneath the SA100 form. You can download it and read it online or print it out if that’s easier for you.

This guide will explain what each item on the SA100 form means and make it easier for you to fill in.

How do I submit it?

You’ll have to send it by post to HMRC, so allow some time for it to arrive there – HMRC will fine you if you miss the deadline. The sooner, the better. 

In most cases you’ll also need to download, print, fill, and submit additional forms, depending on your tax situation and the type of income. These are called supplementary forms. Some of the forms you may need to fill out include the:

• SA101 – less common types of income, deductions and tax reliefs (SEIS, EIS, VCT)
• SA102 –  employment income 
• SA103 – self-employment income 
• SA105 – UK property income
• SA106 – foreign income and gains
• SA108 – capital gains and losses

You can find the full list here.

Attach the supplementary forms you use to your main SA100 form.

Do I need to use it if I file my tax return online?

Nope. This method is only for when you submit your paper Self Assessment tax return, which you send via post. If you’re filing online, then you don’t need to fill in or post anything – everything is done online 😅

You’re able to file your tax return directly with HMRC on their website. 

Even easier, we can file your tax return online for you.