Self Assessment

Also called Tax return, SA100

The annual personal tax return for anyone whose tax is not fully collected through PAYE.

What it means

Self Assessment is how individuals report income that has not already been taxed at source — self-employment profits, dividends, rental income, and various other kinds — and pay the tax due.

The online filing deadline is 31 January following the end of the tax year on 5 April, with the balancing payment due the same day.

Why it matters

Directors taking dividends, sole traders and landlords all fall into it, and the deadline carries automatic penalties that escalate with time regardless of whether tax is owed.

Payments on account catch first-time filers hard: the first bill can be one and a half times what they expected.

What it looks like in practice

Register by 5 October following the tax year in which you started, or the penalty regime starts before you have filed anything.

Set aside tax monthly into a separate account. HMRC's Budget Payment Plan lets you pay in instalments through the year by direct debit.

What to watch out for

The January bill including the balancing payment for last year plus the first payment on account for this year.

Losing the login. The Government Gateway credentials and the UTR are needed every year and are routinely mislaid.

Where to get proper advice

GOV.UK for deadlines and registration. An accountant for anything beyond a simple return — the fee is usually less than the mistakes.