Here is the short answer: you must register for VAT when your VAT-taxable turnover goes over the registration threshold, which is currently £90,000. But that one sentence hides a few details that trip people up constantly, so it is worth understanding exactly how the test works before you either register too early or, more painfully, far too late.
Thresholds are reviewed periodically, so always check the current threshold on GOV.UK before you make a decision. The £90,000 figure applies at the time of writing, but the mechanics below matter far more than the exact number.
The £90,000 threshold is a rolling 12-month test
This is the single biggest misunderstanding we see. The threshold is not based on your tax year, your accounting year, or the calendar year. It works on a rolling 12-month basis.
At the end of every single month, you look back over the previous 12 months and add up your VAT-taxable turnover. If that running total has gone over £90,000, you have crossed the threshold — even if your official accounting year has barely started.
So a business that turns over £8,000 in a good month, month after month, will quietly sail past £90,000 partway through the year. The trap is that people only check their turnover once a year at accounts time, by which point they may have been trading over the threshold for months without realising.
A few things count towards "VAT-taxable turnover" that people forget:
- Standard-rated, reduced-rated and zero-rated sales all count. Zero-rated is still taxable — it just happens to be taxed at 0%.
- Sales you make to customers who are themselves VAT registered count too.
- Genuinely VAT-exempt income (such as some financial services or certain rents) does not count, and neither does anything outside the scope of VAT.
The 30-day forward-looking test
There is a second, separate test that runs alongside the rolling one, and it looks forwards rather than backwards.
If at any point you expect your VAT-taxable turnover to go over £90,000 in the next 30 days alone, you must register immediately — and the registration takes effect from the date you realised, not from the end of the 30 days.
This one bites when a big contract lands. If you win a single order that will, on its own, take you over the threshold within a month, you cannot wait for the rolling total to catch up. The classic example is a tradesperson who signs a large fit-out job: the moment they know that job will breach £90,000 in 30 days, they are on the hook to register.
What are the deadlines?
The deadlines are tight, so it pays to know them:
- Under the rolling test, you must register within 30 days of the end of the month in which you went over the threshold. Your registration then takes effect from the first day of the second month after you went over.
- Under the 30-day forward test, you must register by the end of that 30-day period, and you are registered from the date you first expected the breach.
Miss these and HMRC can register you retrospectively — which leads to the most expensive trap of all, covered below.
Voluntary registration: when it helps, when it hurts
You are allowed to register for VAT before you hit the threshold. Sometimes this is a genuinely smart move; sometimes it just adds admin for no gain.
It can help when:
- Most of your customers are themselves VAT registered (other businesses). They reclaim the VAT you charge, so your prices do not really go up for them — and you get to reclaim VAT on your own costs.
- You buy a lot of standard-rated stock, equipment or services, so reclaiming input VAT puts real money back in your pocket.
- Being VAT registered makes you look more established to the customers and suppliers you want to work with.
It can hurt when:
- Your customers are mainly members of the public or small non-registered businesses. They cannot reclaim VAT, so adding it either makes you 20% more expensive or eats 20% out of your margin.
- You have few costs with VAT on them, so there is little input VAT to reclaim in the first place.
- You simply do not want the extra quarterly admin yet.
What registration actually means day to day
Once you are registered, three things change:
- You must charge VAT on your standard and reduced-rated sales, show it on your invoices, and hand that money to HMRC. It was never really yours — you are collecting it on their behalf.
- You can reclaim VAT on most of your business purchases.
- You must file VAT returns, usually quarterly, and pay any difference across to HMRC.
Crucially, VAT returns now have to be kept and filed digitally under Making Tax Digital. That means keeping your records in compatible software and submitting straight from it — no more typing figures into a HMRC web form. We explain how that works in our guide to Making Tax Digital, and it is worth getting your bookkeeping onto proper software before you register rather than after.
The Flat Rate Scheme in brief
If your turnover is modest, the Flat Rate Scheme can simplify things. Instead of working out VAT on every sale and every purchase, you pay HMRC a fixed percentage of your VAT-inclusive turnover, with the percentage set by your trade sector. You still charge your customers the normal 20%, but you keep the difference between what you charge and the flat rate you pay over.
It reduces admin and can occasionally leave you slightly better off, but it is not automatically cheaper — businesses with lots of VAT on their costs often lose out, because under the flat rate you generally cannot reclaim input VAT on day-to-day purchases. It is worth doing the sums both ways before you opt in.
Common traps to avoid
A handful of mistakes account for most of the VAT pain we see:
- Leaving it too late. If you breach the threshold and do not register on time, HMRC backdates your registration. You then owe VAT on all the sales you made after you should have registered — even though you never charged your customers a penny of it. That 20% comes straight out of your own pocket, plus possible penalties. This is the big one.
- Only checking turnover once a year. Because the test is rolling, you need to watch the 12-month running total every month, not annually.
- Forgetting zero-rated sales count. Zero-rated turnover still pushes you towards the threshold.
- Assuming a quiet month resets the clock. It does not — the rolling window always looks back a full 12 months.
Deregistration
VAT is not necessarily forever. You can deregister if your VAT-taxable turnover falls below the deregistration threshold (this sits a little under the registration threshold — again, check the current figure) and you expect it to stay there. You must deregister if you stop trading or stop making taxable supplies altogether. Deregistering means one final return and, sometimes, accounting for VAT on stock and assets you still hold, so it is worth planning rather than rushing.
The honest summary
Register when the rolling 12-month total tips over £90,000, or the moment you know you will breach in the next 30 days — whichever comes first. Consider registering voluntarily if you sell mainly to other VAT-registered businesses. And whatever you do, watch that rolling total monthly, because the costliest mistake in VAT is finding out you crossed the line months ago.
If you are not sure where your running total sits, or whether voluntary registration would help or hurt in your situation, it is a five-minute conversation rather than a guessing game. Get in touch and we'll check it with you before it becomes a problem.
