A business with turnover in the previous 12-month period exceeding £90,000, or turnover is expected to exceed £90,000 in the next 30 days is required to register for VAT.
If a trader is already registered for VAT, starting another business means registration has to be immediate.
It is essential you monitor the turnover closely. There are some real horror stories around where there have been delays in registering. There are some hefty penalties for late VAT registration as well as cash flow implications when the arrears also need to be paid!