You must register for VAT once you realise that your VAT taxable turnover will exceed £85,000 in the next 30 days.
Registration must be done before the end of this 30-day period. The effective date of your registration is the date you realised, not the day your turnover exceeded the threshold. For example, on May 1, you realise that your monthly VAT taxable turnover will exceed the threshold. You must register before May 30. Your effective registration date is May 1.
If you have already exceeded the VAT threshold in the last 12 months, you must register before the end of the month. The registration date is the first day of the second month after the threshold is exceeded. For example, between July 10, 2019 and July 9, 2020, your VAT turnover was £100,000. This is the first time it has exceeded the VAT threshold. You must register by August 30, 2020. Your effective registration date is September 1, 2020.
You can also temporarily exceed VAT by requesting an exception.
You can request an exception if your taxable turnover temporarily exceeds the threshold. You must present evidence to HMRC that you believe your VAT business volume will not exceed the £83,000 deregistration threshold in the next 12 months. HMRC will consider your exception and write a confirmation if you get one. Otherwise, they will automatically register you for VAT.