Builders have been told to stop work after they knocked down the sties at Nil Solitaire, which has been designated a building of local interest.
The property was purchased by MAP Developments Ltd on 6 October for £1.2 million.
Assistant planning director Peter Le Gresley said that they had written to those believed to be responsible for the demolition, and were now investigating the matter.
Mr Le Gresley said: ‘The old granite pigsties which were part of the listed building have been demolished, and we are investigating.
They have been told to stop work and not remove any material from the site.’