Under the Permitted Development Rights system, a large number of home extensions can be built without the requirement of planning permission. You can extend up to 8m from the original rear wall of your property on a detached home, or up to 6m on a semi-detached or terraced home. In either case, your extension must not be higher than 4m. There are further restrictions if your project is to sit within 2m of a boundary.
Regardless of whether your new extension does or does not require planning permission, it will need Building Regulation Approval. Building regulations are rules approved by Parliament laid down to ensure the minimum design and construction standards are achieved. These cover all manner of subjects such as fire and other forms of safety, insulation, the drainage system, and access.