Builders next door brought house at auction with no planning permission,?:
The whole thing sounds pretty dodgy to me. An extension without the proper footings is not legal and if it has been built without planning permission that to is not legal.
The letter that you write should not be to help them make a sale on a property that has parts that could fall down in a gale and perhaps cause injury. It should be to state that it had only been built for the time that it has and that the building should be inspected as there are no proper foundations. If you knowingly write an incorrect statement and the truth comes out then you could be caught for aiding and abetting an illegal sale. The whole thing is a mine field so either stay out of it altogether or put the local authority in the picture. The estate agent should also be made aware of the problem as he could be in real trouble.