Self Assessment for Side Hustles: What You Need to Know

If you earn money outside your PAYE job, you almost certainly need to complete a Self Assessment tax return — even if you already pay tax through your employer.

When you must register

You must register for Self Assessment if your self-employed income exceeds £1,000 in a tax year (the trading allowance). Register by 5 October following the tax year end.

What you'll pay

• Income tax on self-employed profits at your marginal rate
• Class 4 National Insurance: 6% on profits £12,570–£50,270, 2% above
• Class 2 NI: £3.45/week if profits exceed £12,570 (voluntary below)

Expenses you can deduct

Equipment, software, home office costs (proportional), travel, professional subscriptions, and marketing spend can all reduce your taxable profit.

The deadlines, and what each one actually is

Four dates matter, and missing the first causes the most trouble because it is the one people do not know exists:

• 5 October following the end of the tax year: register for Self Assessment if this is your first year.
• 31 October: deadline for paper returns.
• 31 January: deadline for online returns and for paying any tax owed.
• 31 July: second payment on account, if you are required to make them.

Registration is not instant. HMRC issues a Unique Taxpayer Reference by post, and you cannot file without it, so leaving registration until January is how people end up filing late through no fault of the return itself.

Payments on account catch almost everyone once

If your Self Assessment bill exceeds £1,000, HMRC generally requires payments on account towards the following year. In your first year that means paying your bill plus half of it again on 31 January, and the other half on 31 July.

A £2,000 bill therefore produces a £3,000 payment in January. This is not a penalty and it is not an error, it is prepayment of next year's tax, but it is a genuinely unpleasant surprise if you budgeted only for the £2,000. If your income has fallen you can apply to reduce the payments, though underestimating deliberately attracts interest.

Working out the home office deduction

Two methods are available. Simplified expenses gives a flat monthly amount based on hours worked from home, requires no records, and suits most people with modest side income. The apportionment method claims a proportion of actual household costs based on rooms used and time spent, which produces a larger figure but needs bills and a defensible calculation.

For a side hustle run a few evenings a week, the flat rate is almost always the sensible choice. The apportionment method starts to pay for its record keeping when you work from home most of the week.

Keep records as you go

You must keep records for at least five years after the 31 January filing deadline. The practical version of this is a separate bank account for the side income and a habit of photographing receipts when you get them.

Reconstructing a year of expenses from card statements in late January is the single most reliable way to overpay tax, because the costs you cannot evidence are the ones you do not claim.

Penalties for filing late

A late return triggers an automatic £100 penalty even if you owe no tax at all, with further penalties and interest accruing the longer it goes unfiled. Filing on time while unable to pay is a substantially better position than not filing, and HMRC will discuss instalment arrangements for tax you cannot pay immediately.

Estimate what you will owe

Work out your combined tax bill with our employed & self-employed tax calculator, or the self-employed calculator if you have no PAYE income. The National Insurance guide covers the Class 2 and Class 4 position in more detail.

General information, not tax advice. Thresholds and rates change between tax years and HMRC guidance is definitive. An accountant usually costs less than the errors they prevent once income becomes meaningful.