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Published 03 April 2024
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What Self-Employed Taxpayers Should Know About the SA100 Self-Assessment Tax Form

The SA100 form is filled in by those who are self-employed or have more complex finances and need to report their income to HM Revenue & Customs (HMRC) and calculate their tax bill. You may more commonly hear form SA100 referred to as a tax return or self-assessment form.

Most workers do not need to fill out a self-assessment form because their income tax is automatically deducted from their wages by an employer.

Those who are self-employed, however, pay tax differently and are required to fill out the SA100 form, either online or by downloading it and posting it to HMRC.

You may also have to fill out a self-assessment form if you have income from other sources or if you need to claim any tax reliefs or allowances.Here, we explain everything you need to know about the SA100 form.

What is the SA100?

The SA100 is the main form needed for filling out a self-assessment tax return. It is completed by people who have, in the last tax year: 

  • been a self-employed worker earning more than £1,000 as a sole trader
  • served as a partner in a business partnership
  • earned taxable income in excess of £100,000
  • earned income that was not taxed first, such as income from a rental property
  • paid capital gains tax due to the sale of an asset that has increased in value
  • paid extra tax through the child benefit system

» MORE: Who Has To File for Self Assessment?

How to complete the SA100

There are several different ways you can complete the SA100 form. You can do it online, you can download it and post it back, or you can ask someone else to complete it for you, such as an accountant, a friend or relative. In certain circumstances, such as filing for a partnership or trust, you can submit a return through commercial software.

How to pay tax owed through self-assessment

When you fill out an SA100 you will need to show how much you have earned in the previous tax year. Depending on your circumstances and reasons for filling out the form, you may also need to give details of any other income you have received.

HMRC will then calculate how much income tax and national insurance you need to pay, and it will give you details of how and when you need to pay this by. This can usually be done via a bank transfer or with a debit card.

What are the deadlines for filing a SA100 and paying tax?

There are strict deadlines for filing a SA100. If you miss a deadline you may be charged a penalty fee. Penalties start at £100 for tax returns that are up to three months late but increase over time.

If you are sending a paper tax return, you will need to get it to HMRC by midnight on 31 October following the end of the tax year.

You can also send an online tax return, which needs to be sent to HMRC by midnight on 31 January following the end of the tax year.

However, if you are sending an online return and have not sent one before, you will need to register with the HMRC government gateway system by 5 October. In order to do this, you will need to create an account on the website and you will then be sent a unique pin number which can take up to seven days. When you receive it you can enter this online and your account will be created. You’ll also be sent your Unique Taxpayer Reference (UTR), which you’ll need to complete the return.

It’s important to leave yourself enough time to complete the form and send it back before the deadline. If you are fined for paying your tax return late you may be able to appeal against the fine if you can give a reasonable excuse for the delay.

What information do you need for the SA100?

You’ll need to include all details of income on your SA100. This will be different depending on the reason for completing the form.

You’ll also need to include personal details including your national insurance number and UTR.

When filing the return there is a table for entering your business expenses from the tax year. This can include money spent on renting an office, insurance, or travel costs. The total amount of these expenses is listed on the form and an amount will be deducted. This is because you do not have to pay tax on certain expenses.

» MORE: Reducing Your Tax Bill With Business Expenses

Where to find help with your taxes?

You can find a range of helpsheets on the HMRC website and you can also call it directly if you have a question. There are a number of free and independent charities such as Citizens Advice where you can ask for help with your tax return.

If you have decided to seek help from an accountant, they can fill the form out for you but there will be a charge.

Who has to complete supplementary pages?

In some circumstances, you may need to fill out what are called “supplementary pages” in your tax return. The following situations are the most common reasons for filling these pages out, and the sections required.

  • SA102: Employees or company directors
  • SA103S or SA103F: Self-employment
  • SA104S or SA104F: Business partners
  • SA105: Those with UK property income
  • SA106: Earning a foreign income
  • SA108: Capital gains
  • SA109: Non-UK residents or dual residents

Image source: Getty Images

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