We got SE drawings and calcs for a single storey flat roof extension and ground floor reconfiguration on our detached house, involving removing the entire back wall of the house and adjusting /removing a couple of other internal load bearing walls. We paid around £1250, that was including a site visit at the outset to chat through our requirements (which was really useful as apparently there were different options). We had another quote at a similar level but without the site visit. We’re in Surrey. Yours does seem high. Is getting another quote an option?