Measuring a curved top window for double glazing

[besonders1]

hi, I have just bought an old victorian terraced house and want to replace the rotton windows myself, (I know I need to apply for planning permission BTW)

The only prbolem is that the top of the windows have a decrative brickwork above the litel that makes the window top go into a semi "curve" design almost like an arch rising to about 1 inch approx. would it be within building regualtions to just measure the windows as if the top was straight and fill the gap with that expanding foam that companies use and paint it or does the whole window need to be designed to have a curved top in line with the opening brickwork?. please see diagram below

http://s705.photobucket.com/albums/ww54/besonders1/?action=view&current=untitled.jpg

[bd3cc]

Usually there is an insert in ply that fits up into the cavity, which is rectangular, and can be painted

[bd3cc]

Fit ply behind the outer brickwork and in front of the window and paint it

[youasked]

There are plenty of window makers that will make you a window with a curved top. Are you fitting wood or PVCu. Make a template with hardboard or plasterboard and shop around.