The above is correct regarding indemnity insurance.
But if this is England / Wales, the council cannot force removal of a new structure that has been constructed without PP if more than FOUR years have passed. The way this can be regularised (but it is not essential) is through a Certificate of Lawful Usage or CLU. Google for this and you will find more details. It effectively involves submitting drawing plans to the Council of the same std as would have been required for PP, with a fee very similar to PP. The Council cannot refuse to grant the CLU.
What's probably more of an issue to you is that without PP, the structure has almost certainly been erected without Building Regs approval. This is different - it assures the quality of the structure (foundations get checked, that sort of thing). If this is so, your seller WILL need to provide you with an indemnity policy in case of structural or other buildings problems in future.