converting window to bay window

[gags37]

Hi
I have an 8ft window which i'm thinking of converting to a square bay (8ft *3ft) window. The existing house walls are cavity brick outer block inner. I'm not sure what to do about the walls for the new bay either cavity brick and block or outer brick and dry line internal. I'm also concerned about cold bridging as the new window would be joining the existing exterior wall at 90 degrees. I've done loads of DIY over the years but this is more advance to what I've done before so any advice/ideas would be appreciated. I'm looking at doing a warm roof with epdm, can anyone recomend insulation to use and thinkness required.
Cheers

[welsh brickie]

First you will need planning permission,And then building control will have to inspect the work.And the finished internal size will be hardly worth it
Why not go for a bow window conversion you will save time and money
You can use a preformed GRP fiberglass roof fully insulated.