When your VAT-taxable turnover exceeds £90,000 in any rolling twelve-month period, or when you expect to exceed it in the next thirty days alone. The rolling test is the one that catches people: it is not your tax year or your accounting year, it is any twelve consecutive months, so you have to check it monthly rather than annually. Once you cross, you have thirty days from the end of the month in which you crossed to register, and registration takes effect from the first day of the month after that. The forward-look test is different and bites immediately — if you sign a contract that alone will take you over £90,000 within thirty days, you must register at once. Late registration means paying over the VAT you should have charged, whether you charged it or not.