When negotiating a deal on my new house (complete except for the
> interior stuff), the sales agent agreed to a number incentives
> including the addition of a velux window in the top (3rd floor) shower
> room. These were listed on our reservation agreement as special
> conditions with a note saying my deposit would be refunded if the
> conditions could not be met. You can guess what is coming but of
> course bakcing out now isn't practical.
Most Velux windows are installed from the inside. Basically, clear plaster board around the area, lift tiles from inside, cut and reinforce the roof timbers, then lift the frame into position from the inside and fix. You then make good the tiling and lift the glazed frame into position again from the inside. Finally making good the inside plaster work.
Not sure on the planning aspect, but if he needed planning permission, so would you.
Personally, I would suspect the job got forgotten during the build and it will cost much more to rectify the error at this stage - hence the attempt to get away with it. Stick to your guns!