Side Extension: Usually the minimum size of a side extension is half of your main dwelling (50%). However, this size can be more dependent on the size of your garden width. Rear Extension: For a terraced and end-of-terrace house, the allowed rear extension size is between 3m-6m depending on the borough. For a detached house, you can extend your rear up to 9m. All these rules are contingent upon the extension not taking up more than 50% of your original garden space. Note: Anything outside local permitted development rules will require full planning permission.