It depends on the local planning regulations. It is worth a phone call to the Planning Department at your local Council to find out. There are usually regulations covering the size and height of the new building and its use. If you go ahead and rebuild without permission it is likely that somebody in the neighbourhood will call the Planning Department and, without the right permission, you could find that you have to pull the whole thing down! It is not just planning either, even if you don’t need planning permission, it is likely that you will have to meet building regulations.