is surveyor local? If not, they won't know anything about what was built in Walkley. When I sold mine I was warned by the estate agent about this and told to make sure the estate agent pushes back on any buyer who claims the attic had been converted.
One thing you can check is whether on the same road all the terraced houses have an attic room. If they do, that's a sign they are all likely original.
That's not true. The house I sold was a 2 bedroom house - bedroom 2 was in the attic and was original. On the middle floor there 3 rooms - a bedroom, a bathroom and a room that was too small to call it a bedroom legally.