Probably, once the side income exceeds the trading allowance of £1,000 gross. Below that you can ignore it entirely; above it, you either claim the allowance against the income or deduct actual expenses, whichever gives the better result, and file a return. Note it is £1,000 gross, not profit — turnover of £1,500 with £700 of costs is above the line despite £800 of profit. There is a separate £1,000 property allowance which works the same way for rental income. Your PAYE income does not shelter the side income; it sits on top and is taxed at your marginal rate, which means a higher rate taxpayer with a side hustle is paying 40% on it from the first pound. Worth knowing before you spend it.