Registering and Completing Self-Assessment with HMRC

Completing self-assessment with HMRC is an important responsibility for individuals, involving reporting personal income, expenses, and other financial details to determine tax liabilities or claim tax refunds. Individuals can fulfill their tax obligations and ensure compliance with HMRC regulations by accurately completing the self-assessment process.

To help you get started with your self-assessment tax returns, we have put together a guide on registering and completing your self-assessment. 

Who Needs to Register for Self-Assessment?

You must register for a self-assessment tax return if any of the following apply for the last tax year (6th April – 5th April):

  • You were self-employed and earned more than £1,000
  • You were a partner in a business partnership 
  • You earned £100,000 or above 

However, you should also send a self-assessment tax return if you have any untaxed income, such as;

  • Tips and commission 
  • Money from renting out a property 
  • Income from investments or dividends 
  • Foreign Income 
  • Support payments or Covid-19 grant

If you’re still unsure, you can check if you need to send a self-assessment tax return using the handy online tool on the Gov UK website. 

How to Register for Self Assessment

Once you have determined if you need to send a self-assessment tax return, you must ensure you are registered. You should also be aware that there are different ways to register for self-assessment, as follows:

Self-employed Individuals

If you are self-employed or a sole trader, you must register for both self-assessment and Class 2 National Insurance by October 5th in your business’s second tax year. You can do both by registering through your business tax account. You will then receive a Unique Taxpayer Reference (UTR) by post within ten working days and a subsequent letter reminding you to complete your self-assessment tax return. 

If you have previously registered but have not sent a self-assessment return, you can register online using form CWF1, and you will need your UTR number. 

Once registered, you can submit your self-assessment return online using a paper form or commercial software. 

Not-self Employed

If you are not self-employed but still need to submit a self-assessment tax return, you need to register by the 5th of October to do so. You can register by filling out and returning the SA1 form for self-assessment, either online or by post. Then you will receive your Unique Taxpayer Reference (UTR), which you can use to sign in to your personal tax account

For Partnerships and LLPs

If you are in a partnership, you must register your partnership, and this is also different if you are a limited liability partner. Once you have done this, you will receive a UTR for your partnership which you will need when submitting tax returns. 

Documents and Information Needed for Registration

When registering for self-assessment, you will need the following information to hand:

  • National Insurance Number
  • Full Name (and any previous names)
  • Current Address (and when you moved in)
  • Date of Birth
  • Gender
  • Contact Number
  • Email Address

You may also be asked if you have registered for self-assessment previously. 

How Long Does Registration Take?

Registering for self-assessment is quick and easy online. However, you will be waiting up to 10 work days to receive your UTR. This is even longer if you are based abroad, and you should expect to wait around 21 working days. 

Understanding HMRC Self Assessment

Self-assessment tax returns are forms individuals complete to report their income, expenses, and other financial details for calculating their tax liability. They are required for self-employed individuals, company directors, landlords, and those with additional sources of income. Filing deadlines must be met, and accurate reporting is essential to avoid penalties. Seeking professional assistance can simplify the process, but here are a few more points to help you understand your HMRC self-assessment. 

Types of Expenses to Include In the Tax Return

Costs that you can claim as allowable expenses on your HMRC self-assessment include:

  • Travel Costs
  • Office Costs 
  • Clothing Expenses
  • Staff Costs 
  • Stock or Raw Materials 
  • Financial Costs 
  • Business Premises Costs 
  • Marketing & Advertising 
  • Training Costs 

If you’re unsure whether one of your business costs is an allowable expense, you can contact the HMRC using the self-assessment helpline

Deadlines and Penalties for Late Filing

You must register for self-assessment by the 5th of October. However, there are different deadlines for filing your tax returns, as follows:

  • Paper Tax Returns: Midnight 31st October
  • Online Tax Returns: Midnight 31st January 
  • Pay Tax Owed: Midnight 31st January 

If you miss your tax return deadline, you will be charged a penalty fine, starting at £100 for the first 3 months. The penalty fee will be raised if your self-assessment tax return is still not returned at this time, and interest may also be charged. 

How to Complete Your Self-Assessment Tax Return

The self-assessment tax return form is available online, and you can fill it out electronically or on paper by calling the HMRC to request an SA100 form. The form will ask for details about your income, including employment income, self-employment income, rental income, and any other sources of income you have. You’ll also need to provide information on deductible expenses and allowances you’re eligible for. 

You must ensure you file your tax returns by the deadlines stated above and ensure you keep a copy for your records. 

Gathering and Organising Financial Information

Before filling out your tax return, you’ll need to gather important financial information such as records of income, expenses, and any relevant documents like bank statements or receipts. This will help you accurately report your financial situation.

How to Calculate and Report Taxable Profits

Calculating and reporting taxable profits involves determining the income generated from business activities and deducting allowable expenses. Start by calculating the total revenue earned during the tax year. Then, subtract allowable expenses such as rent, utilities, supplies, and employee wages. The resulting figure represents the taxable profit, which should be reported accurately on tax returns to determine the amount of tax owed.

Your self-assessment tax returns must be accurate, and so we recommend utilising the services of a professional accountant to help you calculate your taxable profits. 

Completing Supplementary Sections and Additional Forms

You may need to complete some additional sections, known as supplementary sections, for certain types of income. These include:

  • Employees or Company Directors
  • Self-Employment
  • Business Partnerships
  • UK Property Income
  • Foreign Income or Gains
  • Capital Gains
  • Non-UK Residents or Dual Residents

If any of the above apply to your self-assessment tax return, you can find the relevant forms using the self-assessment guidance on the government website.

Submitting the Self-Assessment Tax Return

Once you have submitted your self-assessment tax return, you can make an amendment if you have completed it incorrectly. You can do this either online or by filling in a paper form up to 12 months after the deadline for the initial return.


Completing self-assessment with HMRC is crucial for individuals to fulfill their tax obligations and maintain compliance. Accurate reporting of income, expenses, and other financial details ensures a fair calculation of tax liabilities or refunds. Meeting filing deadlines and seeking professional guidance, when needed, can simplify the process and help avoid penalties. 

We hope our guide has helped improve your understanding of how to register and complete your self-assessment tax return. 


News & Articles FAQs

How to contact HMRC Self Assessment?

You can contact the HMRC regarding your self-assessment tax return using their online technical support. Otherwise, you can call the HMRC.

How do I amend a submitted Self Assessment tax return?

You can amend your self-assessment tax return online or by resubmitting paper forms. You can do this up to 12 months after the return date.

Do I need an accountant to complete my Self Assessment tax return?

You may feel confident in your ability to complete a self-assessment tax return. However, a trained accountant is able to offer you peace of mind there will be no errors or inaccuracies that the HMRC could flag. They are also there to offer advice on any financial challenges you may face in your business.

Do HMRC check all Self Assessments?

All self-assessment tax returns are checked by the HMRC, either manually or automatically. The HMRC can also randomly select your tax return for a compliance check and will alert you to any inaccuracies or suspicions.