Self-assessment and administration

Follow a return through its life cycle and the administration rules arrange themselves. WHO files: self-assessment is need-triggered, not universal — PAYE already collects most employees' tax in real time, so a return is required only where PAYE can't capture the picture (self-employment, significant untaxed income, the child benefit charge, and similar). Crossing the higher-rate threshold is not, by itself, a trigger. WHEN: an online return and the balancing payment share one deadline — 31 January after the tax year ends; 31 October is the PAPER filing deadline only, and the exam plants it as a distractor for online filers. HOW the money flows: where last year's liability was significant, two payments on account fall due, each half of the PRIOR year's bill — not an estimate of the current year — with the balancing payment (or repayment) truing things up the following January; and note that payments on account cover income tax and Class 4 NICs, never capital gains tax. WHAT goes wrong: the penalty regimes are separate and behave differently. The initial late-FILING penalty is a fixed amount from the first day late — it applies even where no tax is owed, because it punishes the late return, not late money. Late-PAYMENT penalties and interest are the tax-geared ones. Finally, HOW LONG exposure lasts: HMRC has a fixed window (twelve months from actual delivery of an on-time return) to open a routine enquiry; after that the return is normally final — but 'normally' is load-bearing, because discovery powers survive where fraud or carelessness emerges. Precision to carry into the exam: fixed versus tax-geared penalties, filing versus payment, prior-year versus current-year — every distractor on this node blurs one of those pairs.