It depends what the original floorplan was, as well as on the size of your plot of land.
In theory there are extended permitted development rights for a terraced house up to 6m BUT these require an application. Without an application you can only extend 3m and there are other restrictions on heights, materials, etc.
Any planning application needs more than just floorplans - you'll also need elevations, site plans and location plans. Plus other studies may be necessary depending on the specifics of your project and site.
This link takes you to the official government guidance on what is allowed:
Permitted Development rights for householers (LINK)