You must register for UK VAT when your rolling 12-month taxable turnover exceeds £90,000, or when you reasonably expect a single 30-day period to exceed it. Below that, voluntary registration may still be worth it.

The threshold is checked monthly on a rolling basis, not in line with your accounting year. Get this wrong and HMRC backdates the registration plus the VAT owed plus penalties.

How the threshold actually works

There are two tests you must apply every month.

The historical test: at the end of every month, add up your taxable turnover for the previous 12 months. If it exceeds £90,000, you must register — application due within 30 days of the end of that month, with the effective registration date the first day of the second month after.

The forward test: if at any point you can reasonably foresee that taxable turnover in the next 30 days alone will exceed £90,000 (e.g. you''ve just won a big contract), you must register immediately. Effective date is the day you became aware.

"Taxable turnover" means standard-rated, reduced-rated, and zero-rated sales. It does not include exempt supplies (most insurance, some education, some healthcare). It does include sales abroad that would have been UK VAT-taxable if made here.

Why voluntary registration can make sense

Below the threshold, you can register voluntarily. Most businesses don''t — but in two cases it''s a clear win:

1. Most of your customers are VAT-registered businesses themselves. The VAT you charge them is fully recoverable on their own return, so it costs them nothing — and you get to reclaim VAT on your own costs (laptops, software, premises). Net cash gain.

2. You have significant input VAT in proportion to your output VAT — typically capital-heavy startups not yet selling much, or businesses with high zero-rated outputs (most food retailers).

The argument against voluntary registration: paperwork, slightly more complex pricing, and an additional point of failure (late VAT returns get penalties of their own).

The mechanics of registering

• Apply online through the HMRC portal or in software. Most agents apply on the client''s behalf.
• HMRC needs business details, expected turnover, the effective date you''re registering from, and bank account details for repayments.
• You''ll be issued a VAT registration number (typically within 2-4 weeks) and a unique HMRC online account for VAT.
• Pick your accounting scheme on the application: standard, cash accounting, annual accounting, or flat rate (eligibility varies).
• Once registered, you start charging VAT from the effective date. You must invoice VAT-inclusively from day one — even if your VAT number hasn''t been issued yet, you bill plus 20% (or whichever rate applies) and reissue the invoices with the number once it arrives.

Common mistakes

• Missing the rolling-12 test. People check at year-end, not monthly. By the time their accountant catches it at year-end, they''re backdated three months and owe VAT on sales they didn''t charge it on.
• Confusing turnover with profit. The threshold is gross VAT-taxable sales, not net profit. A break-even business with £100,000 of turnover must register.
• Forgetting about MTD. Once you''re VAT-registered, you must use Making Tax Digital-compatible software for VAT records and returns. Spreadsheets-only doesn''t work — you need bridging software or full accounting software.

What to do if you''re near the threshold

• Set a monthly reminder to add up your rolling 12-month taxable turnover. Most accounting software has a built-in report.
• If you''re approaching the threshold deliberately (e.g. delaying a launch), keep a written note of why — this is what HMRC asks for if they investigate.
• If you''ve crossed it accidentally, register immediately and notify HMRC. Voluntary disclosure mitigates penalties; being caught in an audit doesn''t.