Changing single garden door with double one

Hi, does anyone know if I would need planning permissions to change an old small door giving access to our garden with a set of double french doors (in wood)? Also, would I need to get a builder to widen the opening before getting the window company to install the doors or is it something they normally do themselves? I guess there would be some minor structural work needed to widen the door arch. Any tips from someone who has done this recently would be great!