I have received planning permission for my 2-storey extension. Why do I have to appoint a structural engineer?

You will need to appoint a structural engineer when you need loading calculations for the preparation of Building Control drawings.

In their deliberations on the acceptability of planning applications, the planning officers do not consider whether the proposed building will be structurally sound. This is considered by the Building Control officers, who will examine Building Control drawings supplied by your designer, together with loading calculations supplied by your structural engineer. The calculations might refer to new steels, an upgraded roof etc.