Often you don't, but it depends on your house and where it is. Many single-storey rear extensions can be built under what's called permitted development (rights that let you do some work without a full planning application).
Those rights have limits on depth, height and materials, and they don't apply in the same way to flats, listed buildings or homes in conservation areas. Some extensions also need a 'prior approval' step with the council first.
The rules change from time to time, so check the current guidance on the Planning Portal or with your council before you start. If you're not sure, ask us. It's one of the first things we look at on a site visit.
Building regulations are separate from planning. Even when you don't need planning permission, the work still has to meet building regulations.
General information for a demo site, not advice for your project. Always check current rules with your council.