When do you legally have to tell HMRC about your side income?
The most common question about side income is also one of the simplest to answer, yet it generates more anxiety than almost anything else about building a parallel income. People imagine a grey area. There is not one. There is a clear line, a clear deadline, and a clear process.
Here is the answer, without the noise, for employed professionals earning money alongside a salaried job.
Every deadline in one place
Free PDF: The Second Income Tax Pack 2026/27
The £1,000 rule, every 2026/27 deadline, what to set aside and how to report it.
The one-line answer
You must register for Self Assessment if your gross trading income exceeds £1,000 in a tax year.
Gross means total income before costs. The deadline to register is 5 October after the end of the tax year in which you crossed the line.
The dates that matter
The UK tax year runs from 6 April to 5 April. Income earned between 6 April 2025 and 5 April 2026 follows this timeline, and missing a date is what turns a simple obligation into a penalty.
Key deadlines for 2025 to 2026 income
What counts as trading
Not all money coming in is taxable. HMRC applies what it calls the badges of trade, and the practical distinction is whether you are running an activity for profit or simply selling things you already own.
Trading or not
Trading, so taxable
Not trading, not taxable
Selling your old coat on Vinted for £30 is not taxable, even if you do it fifty times. Buying coats at car boot sales to flip on Vinted is a trade from the very first sale. Intention is what separates the two.
The three thresholds people confuse
Most of the confusion around side income comes from mixing up three separate numbers that do three separate jobs.
Three thresholds, three different jobs
That last point matters most for employed professionals. Because your salary has already used your £12,570 personal allowance, your side profits are taxed from the first pound above the £1,000 trading allowance, at your marginal rate. There is no second tax-free band waiting for your side income.
"The platform reporting threshold is not a tax trigger. It simply means HMRC can now see what you earned. If your return matches, nothing happens. If it does not, the letter follows."
What happens if you do not declare
HMRC now receives seller data directly from eBay, Vinted, Etsy, Airbnb, and other platforms under digital reporting rules. If your declared income does not match their data, expect a nudge letter first and penalties second. Penalties scale with how deliberate the non-disclosure looks, and can reach 100% of the tax owed plus interest. The cost of getting this wrong is far higher than the cost of getting it right.
Earning on the side? Get the tax pack
Sixteen pages: the trading allowance, the registration test, the 2026/27 calendar, payments on account, Making Tax Digital and a twelve-point checklist. Nothing to buy.
Get the free tax pack →The employed professional's checklist
Related reading
- →The £1,000 trading allowance explained, and the new £3,000 reporting threshold
- →HMRC knows about your side hustle. Here is what platforms now report
- →Sole trader or limited company: which to choose for a UK side income
- →How to make a reversible bet: de-risking your first income experiment
Free calculators
Declaring a side income does not put your job at risk and does not require telling your employer. It is a form, a calculation, and a payment. None of this is tax advice, and your circumstances may differ, so confirm the current rules on GOV.UK or with an accountant. But treat the admin as part of the build, not a threat to it, and it stops being something to fear.
Build first. Leave second. Choose third. Start with the assessment →