How big is this extension

[thomp1983]

I'd put in a planning application for the design you outlined in your first post.

The planning officer should be sensible enough to see your only extending the original property size by 4m and that the roof is being altered for practical purposes, contrary to the ideas you'll get on here in my experience planning officers are generally helpful once your application is in and will try and get your plans to something they are willing to pass

 

[AGD]

I've submitted a couple of similar applications, my LA treat them as permitted development.