Extension on a dormer

bs93

We are starting renovations to a property we bought, and we want to add a small extension to enable us to have 3 bedrooms and a shower room upstairs (currently 2 bedrooms and 1 shower room upstairs, with a bathroom downstairs which we plan to keep). We’ve had a structural engineer out who thinks it will be fine to get planning. Also got a father in law who is a builder so the cost won’t be extortionate. But I can’t get my head round how you extend onto a dormer type of construction? 
The extension will be on top of the kitchen extension (the smaller extension in the photo, on the left) 

DOH_a

So you’re proposing a first floor rear extension, which will attach onto a part of the original dormer? If so, you’d just build the extension similarly to how the dormer is constructed... tile hanging on an insulated timber frame with a continuation of the flat roof over.