Reebok Ronnie

Hi All im new to this site, wonder if any kind persons could advice on building cost before I engage surveyor , I don't want to waste money if I can't afford the extension. im looking for indicitive cost for double storey garage , the garage size external is 25 x23 feet basic electric sockets and lighting , the upper floor is one room used for recreational gym etc so again basic , because it's attached to the house only three sides are required many thanks