The self assessment tax return deadline for side hustlers in the UK is 31 January each year for online filing. This applies to anyone who has earned taxable income from self-employment, freelancing, or a side hustle alongside employment or other sources.
Missing this deadline can lead to automatic penalties, so understanding what must be declared and how to prepare is essential for early-stage founders, e-commerce sellers and anyone running a side business.
Who needs to file a Self Assessment tax return
If you have income from self-employment that exceeds certain levels, or if you have untaxed income from selling online, freelancing, or other side activities, you are usually required to register for Self Assessment with HMRC. Employees who receive additional income that has not been taxed through PAYE must also consider filing.
Side hustlers who sell on platforms such as Etsy, eBay or Shopify, provide services via Upwork, or run any small business activity should check whether their total income requires a return. Requirements can change, so always verify your obligations on GOV.UK.
The 31 January online filing deadline explained
For the tax year ending 5 April, the online Self Assessment deadline is 31 January of the following year. For example, the return covering 6 April 2023 to 5 April 2024 must be filed by 31 January 2025. Paper returns have an earlier deadline of 31 October but most side hustlers choose to file online.
Filing by 31 January allows HMRC to calculate any tax due automatically. Late submission after this date triggers penalties regardless of whether tax is owed. Setting calendar reminders in October helps ensure you gather records well before the January cut-off.
What income must be declared on your return
You must declare all income from self-employment, side hustles, freelance work, online sales, rental income, and any other taxable sources. This includes turnover from e-commerce stores before deducting allowable expenses. Even small amounts may need to be reported if they take you over the thresholds that require registration.
Keep clear records of all sales, including platform fees, PayPal or Stripe transactions, and any cash payments. Income from your main job reported through PAYE does not usually need repeating, but any untaxed side income must be included. Check current rules on GOV.UK because thresholds and requirements change regularly.
Practical steps to prepare and file on time
- Register for Self Assessment on GOV.UK as soon as you realise you need to file; this can take up to several weeks for your unique taxpayer reference to arrive.
- Gather records throughout the year: keep digital copies of sales reports, bank statements, expense receipts and mileage logs.
- Calculate your income and allowable business expenses. Examples include platform selling fees, packaging, internet costs, and a proportion of home office expenses if you work from home.
- Use HMRC’s free online Self Assessment service or compatible commercial software to complete the return.
- Submit the return and pay any tax due by 31 January to avoid penalties.
Start preparing in early January at the latest. Many side hustlers find it helpful to review their records monthly rather than leaving everything until the new year.
Penalties for late filing and late payment
HMRC imposes an automatic £100 penalty if your Self Assessment return is filed even one day after 31 January. Further daily penalties can apply after three months, followed by percentage-based penalties after six and twelve months. These increase the longer the delay continues.
Separate penalties and interest apply if tax is paid after the 31 January deadline. The exact amounts and rates change over time, so always check the latest guidance directly on GOV.UK or contact HMRC for your specific situation. Early registration and good record-keeping reduce the risk of unexpected penalties.
Common pitfalls for e-commerce side hustlers
Many side hustlers underestimate the need to declare platform sales or forget to separate personal and business transactions. Others overlook allowable expenses such as a proportion of phone bills, website costs or stock purchases, which can increase the tax bill unnecessarily.
Keeping accurate records from day one avoids rushed calculations in January. Consider using simple bookkeeping tools or spreadsheets to track income and expenses monthly. Understanding cash flow helps you set aside money for tax as you earn rather than facing a large bill in January.
Getting support as a UK startup founder
Running a side hustle while building toward full-time self-employment can be complex. Practical guidance on tax, bookkeeping and cash flow makes a real difference. Loomworks in Liversedge, West Yorkshire, offers selected early-stage founders free desks, a registered address and mentoring with no fees or equity taken.
Whether you use free HMRC tools, community resources or professional advice, staying organised and filing by 31 January protects your business from unnecessary penalties and keeps your focus on growth.
Frequently asked questions
Do I need to file a Self Assessment if my side hustle earned very little?
It depends on the total income and whether it exceeds the current registration thresholds. Even small earnings may require a return if they are not taxed elsewhere. Check the latest rules on GOV.UK each year because the limits change.
What happens if I miss the 31 January Self Assessment deadline?
You will usually receive an automatic £100 penalty. Additional daily fines, percentage penalties and interest on late tax can follow. Filing as soon as possible after the deadline limits further charges.
Can I claim business expenses on my side hustle tax return?
Yes, you can usually deduct allowable expenses that are wholly and exclusively for business use. Common examples include selling fees, packaging, advertising and a share of home running costs. Keep evidence of all claims.
When do I pay the tax I owe from my side hustle?
Any tax due must be paid by 31 January alongside filing your return. If you owe significant amounts you may also need to make payments on account the following year. Always check current HMRC guidance.
How do I register for Self Assessment as a new side hustler?
Visit GOV.UK and complete the online registration form. You will receive a Unique Taxpayer Reference (UTR) by post. Register as soon as you know you need to file so the reference arrives in good time.
Related reading: What business expenses can self-employed UK founders claim?, How does corporation tax work for new limited companies?, How to build a simple cash flow forecast for your startup. Browse all tax & finance guides or apply to Loomworks.