I have got planning permission to convert a terrace house into a pair of flats. within the plans there is a small ground floor extension to the back of the house for a new bathroom. On starting the work I asked the building inspector if we could extend the extension to cover all of the back wall. He said he had no problems with this but to check with the planning department. They have told me that if it is a house I don't need planning permission but if it is a flat I do and recommend that I put in a pre planning application. As I have just started the work, is it classed as a house or a flat