The rear of the house is north facing, so currently quite dark, but with the large skylights it will be very light (as my next door neighbour's is). The building firm I last spoke say it would be no problem putting a gable roof on the extension (even with the first floor extension) bit like these images.
Regarding the toilet I sketched this layout quickly, so no issues with just checking the toilet where it is off the hallway but making slightly larger.
No comments on my first floor layout?