Are there any online services where you can draw/create your own drawings for building regs on a small single storey extension?

Yes, if you use the Chrome browser, install WebxCad or AutoCad WS from the webstore, and use them from the browser as an app

They won't do the design or specification though

Also TinkerCad for sketchup-like 3d modelling

Hi Paul,

As woody suggests - do you know what you want to build but just need a nice way of drawing it up, or are you trying to convert your requirement for a '4m x 3m single storey pitched roof rear extension' into a generic extension design with detail like "100mm cavity, 600mm wide strip foundation, C16 joists blah blah"?

The latter could be tricky but there's enough knowledge on this forum to get you there.

Gary