This is an interesting article from the Telegraph about a property developer who demolished a pub in Battersea without planning consent. He applied for consent after the demolition had been completed and was refused that consent.

He now has the considerable costs of trying to reinstate the original pub with original materials.

Planning permission is not always required for demolition works but you may need that consent for demolition in a conservation area. This property was located in a conservation area. You may also need listed building consent to demolish a listed building. The easiest way to deal with such questions is probably to speak with the local planning authority before you carry out the works.