Side hustles have completely changed the way people in the UK earn money. Whether you’re selling on Vinted, doing freelance design work, driving for a delivery app, or running a small online shop, the extra income is brilliant — but it does come with responsibilities. One of the biggest is filing a Self Assessment tax return.
If this is your first year earning outside of a traditional PAYE job, the whole process can feel overwhelming. The good news is it’s far more manageable than it looks, especially once you understand the basics and have the right tools in place.
Who Needs to File a Self Assessment Tax Return?
HMRC requires you to file a Self Assessment tax return if you earn more than £1,000 from self-employment or side income in a tax year. This threshold is known as the trading allowance, and it’s designed to give hobbyists and very small earners some breathing room. But once you cross it, you’re required to register with HMRC and file annually.
This catches a lot of side hustlers off guard. You might assume that because your main income is taxed through PAYE, you don’t need to worry about Self Assessment. That’s not the case. Any additional income — whether it’s from freelancing, selling handmade goods, renting out a room, or completing paid surveys — needs to be declared separately.
The registration deadline for new self-employed earners is 5 October following the end of the relevant tax year. Miss it and you risk a penalty, so it’s worth getting registered as soon as you start earning consistently.
What Counts as Side Hustle Income?
Almost everything counts. Here are some of the most common sources side hustlers need to declare:
- Freelance services (writing, design, photography, coding)
- Selling items online for profit (not just clearing out old belongings, but buying and reselling)
- Income from platforms like Fiverr, Upwork, Etsy or eBay
- Delivery and driving work through apps
- Renting out a room or property on Airbnb
- Paid social media content or brand partnerships
- Tutoring or coaching services
The key distinction HMRC makes is between selling personal possessions occasionally (which generally doesn’t count) and operating a trading activity for profit (which does). If you’re regularly buying things to resell, or consistently offering a service in exchange for payment, that’s trading income and it needs to be declared.
Understanding What You Can Deduct
One of the most valuable things a side hustler can do is understand allowable expenses. These are legitimate business costs you can deduct from your income before calculating your tax bill, which can significantly reduce what you owe.
Common allowable expenses include:
- Equipment and tools used for your business (laptop, camera, specialist tools)
- A proportion of your home bills if you work from home
- Marketing costs such as website hosting or paid advertising
- Professional subscriptions or memberships relevant to your work
- Travel costs directly related to your business activity
- Software and apps used to run your side hustle
Keep receipts for everything. Even if you’re not sure whether something qualifies, hold onto the evidence and check later. Good record keeping throughout the year makes filing your return much easier and means you won’t miss deductions you’re entitled to.
The Deadlines You Need to Know
Self Assessment runs on a fixed annual cycle. The UK tax year runs from 6 April to 5 April the following year. Once the tax year ends, you have until:
- 31 October to file a paper return
- 31 January to file online and pay any tax owed
Most people file online, which gives you several extra months. The 31 January deadline is also when your payment is due, so you need to have both your return filed and your bill settled by that date to avoid penalties and interest.
If this is your first year filing, HMRC will also ask you to make payments on account — advance payments toward your next year’s tax bill. This surprises a lot of new filers because it effectively means paying one and a half years’ worth of tax in your first January. Planning ahead for this cash flow impact is important.
Making the Process Easier
The biggest mistake side hustlers make is leaving everything to January. Scrambling to find receipts, reconstruct your income, and figure out what you owe in the final days before the deadline is stressful and increases the risk of errors.
A much better approach is to track your income and expenses throughout the year. Even a simple spreadsheet updated monthly saves you hours when it comes to filing time.
For those who want something more structured, dedicated self-assessment software can make the process significantly smoother. Tools like Pie Tax are built specifically for the UK market and walk you through the filing process step by step, helping you make sure you’re not missing deductions or making mistakes that could attract an HMRC inquiry. If you’re filing for the first time or find the HMRC portal confusing, having purpose-built software in your corner makes a real difference.
Don’t Let Tax Stop You Hustling
The tax side of running a side hustle is genuinely manageable once you’ve got a system in place. Register with HMRC as soon as you cross the £1,000 threshold, keep records throughout the year, understand what you can deduct, and give yourself plenty of time before the January deadline.
The worst thing you can do is ignore it. HMRC has become increasingly effective at identifying undeclared income, particularly from online platforms, and the penalties for late filing and payment add up quickly.
But handled properly, Self Assessment is just another part of running a successful side hustle — and one that gets easier every year once you know what you’re doing.
