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What does a Self Assessment form look like?

  • 4 min read
  • Last updated 28 Mar 2024

If you’re unsure what a Self Assessment form looks like, or how it should be filled in, then don’t worry! Tax forms aren’t as complicated as they first appear. As long as you’re prepared, organised and know what information you’ll need to provide, Self Assessment tax forms are easy to fill out.

So how do you know if you need to complete a Self Assessment form, and what information will you need to fill in? Here’s everything you need to know.

Do I have to do a tax return?

You might be thinking, ‘do I even need to worry about Self Assessment tax forms?’ 

Well, if you fall in to any of the following categories, then you’re legally required to complete a Self Assessment tax return by HMRC:

  • You’re self-employed
  • You earn more than £100,000 per year
  • You’re a landlord
  • You claim tax relief on your pension
  • You’ve made money from an investment
  • You earn money from side jobs alongside your main employment

If any of the above applies to you, then you must register with HMRC to declare your Self Assessment. Make sure that you do this online by 5th October to pay your tax bill for the tax year that has just ended. 

Once you’ve registered, HMRC will send you what’s known as a Unique Taxpayer Reference (UTR) number in the post. You will then use this to pay your tax return by 31st January. Be warned, if you fail to submit a tax return by HMRC’s deadlines, you’ll face an automatic £100 penalty, plus any additional penalties and interest on the tax you owe.

How do I fill in a Self Assessment form?

If you’ve never filled in a Self Assessment form before, it can look very scary and confusing. However, once you understand how the process works and have all the correct information you need, it’s relatively simple!

Here’s the information you need to have ready:

  • Your Unique Taxpayer Reference (UTR)
  • Your National Insurance number
  • Details of your untaxed income from the tax year
  • Records of your expenses
  • Contributions to charity or pensions that might be eligible for tax relief
  • Your P60 or other records showing how much income you received which you’ve already paid tax on.

How to fill in the main tax return section (SA100)

There are two sections to complete on a Self Assessment form. The main section is called the SA100, which deals with the following:


Both taxed and untaxed income from interest earned from bank and building society accounts and dividends from shares.

Pensions, annuities and state benefits: 

Including state pension contributions and any state benefits received, such as Jobseeker’s Allowance.

Other UK Income:

Any other form taxable income, not related to interest, dividends or on the supplementary pages.

Other pension contributions:

Any payments you made into a registered pension scheme, annuity contract, or employer’s scheme.

Charitable donations:

Here you enter the total of all Gift Aid donations you made to charities during the tax year.

Blind Person’s Allowance:

Confirm whether or not you are claiming Blind Person’s Allowance.

Student loan repayments:

Confirm whether or not you’re currently repaying your Student Loan and deductions made by an employer.

High income Child Benefit charge:

You’ll need to fill in this part of the form if you’re receiving Child Benefit and your income is more than £60,000.

Marriage Allowance:

You only need to complete this section if your income for the tax year was less than the Personal Allowance and you wish to transfer some of it to your wife/husband.

How to fill in the supplementary pages?

The supplementary pages are some of the most important parts of the Self Assessment form, especially if you’re declaring extra income from self-employed work, property or capital gains. 

Depending on what you’re declaring, you’ll need to fill in the following supplementary pages on the form. In these pages you’ll need to report on any untaxed income you’ve earned from these sources:

  • If you’re self-employed you’ll need to complete SA103
  • If you’re reporting property income, then fill in SA105
  • If you’re declaring capital gains, complete SA108

You’ll also be able to declare any allowable expenses in these sections, which will then be deducted from your tax bill by HMRC. 

Should I use an accountant to help me with my Self Assessment form?

The answer to this question depends entirely on your situation. Although it is possible to do your tax return yourself, the tax industry is notoriously complicated and the fines for making mistakes can be pretty hefty. 

As a general rule, the more simple your tax return, the easier it is to do it correctly on your own. If you want to take the plunge and fill out your Self Assessment form by yourself, then why not use one of our handy tax calculators here.

If you’re self-employed, it could be worthwhile to hire an accountant or use a one-off tax return service like TaxScouts!

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