Edit a saved drawing

I use SketchUp for very basic 3D drawing of internal measurements of houses basically square boxes most of the time, occasionally I need to go back to a saved drawing and add maybe a conservatory or a garage or an extension but am unable to do so, any help would be greatly appreciated.

Sounds like you are using SU for some kind of business. If so, you should be using SU Pro. Any drawings created would then be stored locally on your own system and you would have no difficulty retrieving them.