Building Control and planning are completely separate departments with completely different functions. Building Control won't care about the outbuildings, all they are interested in is that the buildings have been constructed to comply with energy efficiency and structural stability and a few other technical areas. They aren't even interested how big the things are, as long as they're properly constructed.
Planning are interested in population density, look and feel and other aesthetics- they rarely emerge to see if what has been built is what was permitted, usually only if someone objects.
So.
Check (you and/or your legal advisor) all the conditions that were attached to the planning permission and make sure you fully understand all the implications.
Check the Land Registry entries for the house you're planning to buy
Your outbuildings- if in the future (10 years is probably the window, good spot
@blub) the council comes along and says 'oi, these shouldn't be here' then YOU will be on the hook for demolishing them, which may not be cheap if there's asbestos roofing involved or any other environmental issues (pig farm- is there a slurry bund or pit involved?). If your solicitor can get the builder to indemnify you against any costs involved then fine but if anything kicks off you'll probably have to chase him through the courts which will be messy and expensive. Don't forget- he isn't doing you a favour by leaving the buildings up, he's saving himself a few grand (depending on how big they are & how they're built).
Of course the other way would be to get a quote for demolition and knock that off your offer- shame that'll cost the old lady rather than the builder!
(I'm assuming here that we're talking about substantial agricultural buildings that wouldn't be permitted development in a million years (10m x 50m x 6m high, that sort of thing). )