If it is in your garden and more than 5 metres from your house, you can build it up to 30 sq metre floor area and 4 mts to the highest point. If built from mainly non combustible materials, such as brick/block and tiled roof, then it can up against the boundary. If timber built, then a metre from boundaries is the rule.
There are other things to consider, like proximity to highways, which you should check and also need to consider what your local authority allow, but i've built 2 block workshops following the above guidelines and both times the council have turned up. Both were fine after being scrutinised. I'm moving house next friday and about to build another. Confident that there ill be no issues.