converting window to square bay

[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

 

[noseall]

A bay widow is considered an 'extension' as far as building reg's are concerned so it will fall under controlled works.

The window joining at 90° will be no more vulnerable to cold bridging as those joining in line.

The walls will need to comply to current reg's. 100mm outer skin, 100mm full fill (dritherm) cavity, 100mm lightweight block work, is fairly typical.

For a warm deck you are looking at upwards of 100mm celotex/kingspan/quintherm insulation sitting above the roof joists.

All surfaces i.e. floor, walls, roof will need to satisfy latest thermal regulations.