business use

[question ned]

i have a yard that alreay has B1 & B2 use , i want to open a builders & garden supplies ( not garden centre) do i need change of use.

[buildersmate]

Based purely upon what you say here, then yes, assuming you want to operate a retail unit selling stuff to the general public. Retail are the A class uses, which I imagine you already know. B1 and B2 are more to do with running business to business services.
The following downloadable file from Richmond Borough Council in London (picked out because it seems particularly heplful) may help you further but you really need to talk to your local planning department about what you are wanting to do.
http://www.richmond.gov.uk/ru/class_orders.pdf