If your turnover exceeds, or is expected to exceed, £85,000 in any twelve month period you will need to register for VAT from the following month.
Please note that this is a rolling 12 month period, NOT your financial year. It is a common mistake so, each month, you need to keep an eye on your sales in the previous 12 months.
We’ve had a few clients go over the VAT limit in the middle of their financial year and only find out when we do their year end accounts some time later. This means that they may end up having to pay the VAT themselves so it is worth keeping an eye on your invoicing.