VAT is a complicated area and the answer to this question comes in two separate parts, namely when can you register for VAT, and when must you register for VAT. You must register for VAT once your turnover exceeds the threshold which is currently set at £85,000 for the 2017/18 tax year, however, you may decide to voluntarily register before this time.
There are two main reasons you may wish to voluntarily register your company for VAT. Firstly, being VAT registered gives the impression that you are a large company, and therefore for some business this is a tactical decision taken to appear larger than they actually are. Secondly, once your company is registered for VAT you must charge VAT on the goods and services you provide, however, this also means you are able to reclaim the VAT other businesses you are charged from other businesses. Depending on your company, the amount of VAT you are able to reclaim may be in excess of the VAT you will be liable to pay.
You should think extremely carefully before registering for VAT and take the professional advice of an accountant before going any further.