PSA for anyone near the VAT threshold: it's £90k in ANY rolling 12 months, not per tax year — and late registration gets backdated

Seen this bite a few people this summer, so a quick explainer.

The £90k VAT registration threshold is a rolling test: at the end of EVERY calendar month you look at your taxable turnover for the previous 12 months. It is not "per tax year", and that's exactly how people get caught — a strong spring plus a strong autumn can tip you over mid-year without any single year looking unusual.

Cross it and you have 30 days from the end of that month to notify HMRC. Registration takes effect from the first day of the SECOND month after you crossed. Example: rolling total passes £90k on 31 August → notify by 30 September → charging VAT from 1 October.

Separate trap: if you have reasonable grounds to expect more than £90k in the next 30 days ALONE (one big contract will do it), you must register immediately — the 12-month history is irrelevant.

If you miss it: penalty of 5%, 10% or 15% of the VAT due depending on lateness (minimum £50), and HMRC treats you as registered from the date you should have been — so you owe output VAT on all those sales even though you never charged your clients it. Sometimes you can invoice clients for it retrospectively if they're VAT-registered, but it's a mess.

Source: gov.uk — "Register for VAT" and VAT Notice 700/41 (late registration penalty).

If your rolling total is anywhere above ~£80k, put a monthly reminder in your calendar. Not advice, just the pattern — accountant who does the cleanup when it goes wrong.