What is a SA100?

SA100 is a Self Assessment tax return and is used to calculate how much tax you owe from the earnings you declare on the form. The SA100 is only meant for individuals and supplementary pages are added to the SA100 so you can declare different forms of income.

Filling in a SA100 tax return doesn’t always mean you will owe tax, in some situations you could be due a tax rebate.

Who has to fill in a SA100 tax return?

Anyone who is self employed needs to complete SA100 but there are other circumstances that mean you legally need to complete a tax return.

Common reasons for SA100 criteria include:

• UK landlord – Having rental income from property will mean a tax return needs to be submitted telling the tax office your income. This has to be done whether you have made a profit or loss.
• UK non resident tax payer – If you have income in the UK and pay tax you will typically have to declare your income on a Self Assessment tax return.
• UK non resident landlord – In most cases if you are non resident and have rental income in the UK a tax return is required. Depending on whether you have paid tax on your rental income you might be entitled to a tax rebate on the tax paid.
• Work expenses – Having work expenses of over £2500 in any one tax year. For example if you use your own car for business use, and your total mileage claim is for £2500 or more, you will normally have to claim your mileage tax relief by completing a tax return.

This is not an exhaustive list and we have only covered some of the more common scenarios that require SA100 registration.

How do I apply for SA100?

SA100 forms are not always issued automatically meaning if you meet the self assessment criteria you will need to complete the SA1 self assessment registration form.

The SA1 is an HMRC form that allows you to notify them that you need to complete a self assessment tax return. If you are setting up as self employed you should complete form CWF1 instead of the SA1.

You can complete and submit your SA1 online or you can post it to HMRC directly.

SA100 UTR number

After you have successfully submitted your SA1 the tax office will send you a unique tax reference number (UTR number). It is made up of ten digits and you need to use it when completing your tax return and communicating with HMRC.

What do I need to enter on my SA100?

To complete your SA100 you will need your UTR number and standard personal information. You will usually have to complete supplementary pages depending on the reason why you need to complete a tax return.

For example if you have PAYE income you will need to complete SA102 which requests your pay and tax details from your employment income. In this example you should use the figures from your P60 and or P45 to complete the SA102 section of your tax return.

How do I submit my SA100 tax return?

The most common way to submit a SA100 tax return is online by using the facility provided by HMRC or by a third party accountancy software provider.

When you have submitted your tax return online HMRC should update your self assessment record with the information from your return in around 48 working hours.

There is an option to print off your tax return so you can complete it manually and then post it to the tax office.

Post your SA100:

If you live in the UK you can post your SA100 to:

OR if you live outside the UK send it to: