So where did the surveyor get this idea from?
Most probably because the surveyor's speciality is residential property and, if there is a mortgage involved and he is working on behalf of the lender, there's good chance he's now referred it for valuation based upon the value as an 'agricultural property' as against solely the bricks and mortar of a 'house'.
The lender will have a vested interest in ensuring there are no agricultural ties or other obscure issues with regard to access or the boundary, etc.
If there is no lender involved, I'd be asking "Why?", as you're solicitor should be making the relevant checks on your behalf, with the surveyor concentrating on whatever type of survey you've requested!