If you can show that the existing farmhouse was last used as a house and the walls and roof are substantially intact then applying for either a replacement dwelling or a rennovation of whats there would depend on the state of the existing structure and whether it could be considered to important to the character of the area.
Depending on which county its in there will be varying policies on replacement dwellings, you should either give the local planning department a ring or have a look at the development plan.
As far as an anurement clause goes that would again depend on the county council and their development plan