A complete guide to SA100: Everything you need to know

Self Assessment is the process HMRC uses to collect Income Tax from individuals whose tax isn't automatically deducted. If you are self-employed, a landlord, a company director, or have other untaxed income, you may need to complete a Self Assessment tax return each year. Understanding who needs to file, what information to include, and the key deadlines can help you stay compliant and avoid unnecessary penalties.

Read on to learn everything you need to know about the SA100 Self Assessment tax return.

What is Self Assessment?

Self Assessment is the system used by HM Revenue and Customs (HMRC) to collect Income Tax. While tax is usually deducted automatically from wages and pensions, individuals with income that HMRC does not tax at source, such as profits from self-employment, rental income, or other taxable earnings, must report it by filing a Self Assessment tax return.

What is SA100?

The SA100 is the main tax return form used by individuals to report their income and calculate the tax they owe to HMRC. It is a key part of the Self Assessment process and is designed for taxpayers whose tax affairs cannot be fully captured through the Pay As You Earn (PAYE) system.

Who needs to file an SA100?

Below is a summary of individuals who may need to submit an SA100 form:

• Sole traders with an annual trading income of more than £1,000

• Landlords with annual rental income of more than £1,000

• Partners who receive income from business partnerships

• Individuals with foreign income or other untaxed personal income that must be declared to HMRC

• Individuals liable to the Capital Gains Tax on the disposal of an asset that has increased in value

• Individuals subject to the High Income Child Benefit Charge who do not settle this liability through PAYE

• Off-payroll workers repaying a student loan or postgraduate loan

Note: If you're within the scope of Making Tax Digital (MTD) for Income Tax, you'll need to maintain digital records, submit quarterly updates to HMRC, and file your annual tax return. From April 2026, MTD for Income Tax applies to sole traders and landlords with qualifying income over £50,000.

How to register for Self Assessment

Before filing your tax return, you'll need to register with HMRC. It's worth starting the process early, as registration can take a few weeks.

Follow the steps below to register for Self Assessment and receive your reference number.

1. Complete the online registration through the HMRC registration service. Follow the instructions based on your circumstances, whether you're a sole trader, registering a partnership, or reporting untaxed income.

2. HMRC will send you a 10-digit Unique Taxpayer Reference (UTR). This usually takes around 10 working days.

3. Create a Government Gateway account using the activation code sent along with your UTR. You'll use this account to access HMRC's online services.

4. Once your account is set up, you can file your tax return online. Sign in to your Government Gateway account to start, complete, and submit your Self Assessment tax return.

Note: It can take up to 20 days from registration to receive both your UTR and activation code. If you're registering close to the filing deadline, make sure to allow enough time for the process to be completed.

What are the Self Assessment deadlines?

Listed below are the key dates to remember for your Self Assessment filing:

What are the penalties for missing Self Assessment deadlines?

HMRC imposes penalties for late Self Assessment tax returns and delayed tax payments. Understanding these charges can help you avoid unnecessary costs.

Late filing penalties 

• 1 day late: £100 fixed penalty

• 3 months late: £10 per day, up to a maximum of £900

• 6 months late: An additional penalty of 5% of the tax due or £300, whichever is greater

• 12 months late: A further penalty of 5% of the tax due or £300, whichever is greater

Late payment penalties 

If you fail to pay your tax on time, HMRC may charge penalties of 5% of the unpaid tax at:

• 30 days overdue

• 6 months overdue

• 12 months overdue

In addition to these penalties, interest will accrue on the outstanding amount until it is paid in full.

What are the supplementary forms?

In some cases, taxpayers may need to submit supplementary pages based on the types of income they receive.

• SA101 for less common income, deductions, tax reliefs, and losses

• SA102 for employment income

• SA103S/F for self-employment income

• SA104S/F for business partnership income

• SA105 for UK property income

• SA106 for foreign income and gains

• SA108 for capital gains

• SA109 for non-UK residency or dual residency status

How Zoho Books helps with Self Assessment filing

Zoho Books simplifies Self Assessment filing with built-in tax return generation and direct HMRC filing. You can preview supported forms—SA100, SA102, SA103S (with pre-filled values for sole traders), SA105, and SA110—in the required HMRC format before submission, access a detailed tax breakdown (SA302), and file directly with HMRC. Plus, our cloud accounting software offers a free plan to help you stay on top of your Self Assessment obligations.Self Assessment filing doesn't have to involve juggling multiple tools or manual processes. With the right accounting solution, managing your finances and staying compliant can be hassle-free. Start early, stay up to date with the latest tax

Frequently asked questions

How is SA100 different from MTD for Income Tax?

The table below helps you understand the difference between MTD for Income Tax and Self Assessment.

What needs to be included in the SA100 form?

The SA100 tax return form includes your personal details, income, tax reliefs, and any other information HMRC needs to calculate your tax bill. It includes the following sections:

• Personal information

• Student loans repayments

• UK interest and dividends

• UK pensions and state benefits

• Other UK income

• Tax reliefs

• Charitable giving

• High Income Child Benefit Charge

• Marriage Allowance

• Blind Person's Allowance

• Tax refund details

• Declaration

• Winter Fuel Payment

You only need to complete the sections that are relevant to your income and circumstances.

What are payments on account?

Payments on account are advance payments towards your next tax bill, required when your Self Assessment bill exceeds £1,000. Your liability is split into two equal instalments, each half of the prior year's bill, due on 31 January and 31 July. If your actual tax for the year turns out to be higher, a balancing payment is due the following 31 January.

What information should I have ready while filing Self Assessment?

Listed below is the required information you should keep ready while filing your Self Assessment tax return:

• National Insurance Number (NINO) and Unique Taxpayer Reference (UTR)

• Details of untaxed income and employment income (including P60, P45, or P11D)

• Records of your self-employed income and allowable expenses

• Pension statements

• Records of capital gains

• Records of overseas income

• Details of eligible tax relief

• Bank account details (for any repayment due)

• Student or postgraduate loan repayment details (if applicable)

For more details, check HMRC's official website.

How to file a Self Assessment tax return?

Submit your tax return online:

You can submit your Self Assessment tax return online. According to HMRC, 97% of taxpayers already file their returns this way.

Submit a paper tax return:

If you're filing a paper return, you can download the SA100 form from HMRC or contact HMRC to request a copy by post.

If your circumstances require additional information, you may also need to complete one or more supplementary pages. You can find the supplementary forms here.

If you need help with your return, you can refer to HMRC's guidance, seek advice from a tax professional, or contact HMRC directly.

Note: You should only use the Short Tax Return (SA200) if HMRC has sent one to you.

Submit using accounting software:

You can also submit your Self Assessment tax return using commercial accounting software, such as Zoho Books.