The January 31st deadline for submitting self-assessment tax returns in the UK has just passed. If you're among the millions required to complete these annual tax returns, it's important to understand the rules and deadlines to avoid penalties. This guide explains everything you need to know.

What is Self-Assessment?

Self-Assessment is the system HM Revenue and Customs uses to collect income tax in the UK. You must complete an annual Self Assessment tax return if you have income outside of regular employment or pensions, such as self-employment, property rental, investments, etc.

You calculate how much Tax you owe based on your income and capital gains. This self-assessed amount must be reported and paid to HMRC by the deadline each year. Failure to file tax returns or late payment of any tax due can result in financial penalties.


Who Needs to Send a Tax Return?

You must complete a tax return if in the past year:

  • You were self-employed as a sole trader with profits over £1,000.

  • You were a partner in a business partnership.

  • Your employment income exceeded £100,000.

  • You had capital gains from selling assets like property or shares.

  • You needed to pay the High-Income Child Benefit Charge

  • You received untaxed income like rental profits or commission.

  • You receive income from foreign sources, such as pension, interest, or dividends.

You may also opt to send a return voluntarily to claim tax reliefs or report extra income. If HMRC sends you a notice to file a tax return, you must complete it. You'll be fined if you don't.


Registration & Sending Your Return

If it's your first time, you must register for Self-Assessment by October 5th. You can do this online via the government gateway.

"If you are self-employed, you should register online and file Form CWF1. If you are not self-employed and must register for Self-Assessment, you should file form SA01." 

Once registered, you can file online or request a paper return from HMRC. Most choose the online method as it's faster and more convenient. As a self-employed person, you'll need to keep records like bank statements and receipts to calculate your income and taxes accurately.



There are important Self-Assessment deadlines to be aware of:

Registering: October 5th

Filing Paper Returns: October 31st

Filing Online Returns: January 31st

Paying Tax Owed: January 31st

You must pay a penalty for missing the deadline of January 31st, and interest accrues on late tax bills.


What if Your Income Changes?

Suppose your final income for the tax year is sometimes known after the January filing deadline. For example, if you're waiting on investment dividends or a business valuation. In this case, you provide estimated "provisional" figures when filing your return. You must submit an amended return within 12 months of the deadline with the final confirmed income amounts. Your tax bill will be recalculated at that stage, and you'll pay any additional tax owed (or receive a refund).



Late filing penalties start at £100 if your return is up to 3 months overdue. Additional penalties apply for payments over three months late, too. The penalties can quickly escalate into large sums, so file and pay on time.

You may be able to appeal the penalty if you have a reasonable excuse for lateness.


Amending Your Return

If you realise you've made a mistake, you can submit an amended return to correct it online or via an updated paper return. You have 12 months from the filing deadline to do this without needing approval from HMRC. If it's an older return or a later amendment, you must write to HMRC explaining the changes required.


When Might You No Longer Need to File a Return?

Conversely, if your situation changes and you no longer have additional untaxed income, you may not need to complete the Self-Assessment process. Scenarios where you might not need to file include:

  • Ceasing self-employment

  • No longer renting out property.

  • Income falling below the £100,000 threshold.

  • No longer need to pay the High-Income Child Benefit Charge

If you don't need to file a return, you must contact HMRC to inform them and receive formal confirmation. This prevents penalties for non-submission in case HMRC disagrees with your status change. You can notify them through an online form, their digital chatbot, or post/phone if needed. Include identifying details like your National Insurance and Unique Taxpayer Reference numbers. Once HMRC confirms you're exempt in future years, keep the letter on file in case any issues later arise.


Submitting a Return for Someone Who Has Died

If a person passes away and HMRC requests a return, someone close to them must complete the forms on their behalf. This could be a surviving spouse, family member or estate executor. Details of the deceased's income and assets will be needed to calculate taxes owed. For example, paperwork about their:

  • Bank/savings accounts.

  • Employment/pensions

  • Investment dividends

  • Rental income

  • Business records

If you need help finding all the necessary records, contact HMRC's Bereavement helpline for assistance before the deadline.


Getting Help with Self-Assessment

If you are unfamiliar with tax law and guidance and struggling to understand self-assessment and meet your obligations, a number of resources can help:

  • Hire an accountant or tax adviser.

  • Use HMRC's webinars and online guidance.

  • Contact HMRC's helpline with queries.

  • Access help sheets explaining key processes.

This annual task can be smooth if you prepare methodically. Give yourself time, keep accurate records, and don't hesitate to seek assistance. The secret is getting organised nicely and early!