Allowable amount of glass in an extension

[donmaico]

Now i know that 25% of the floor area of any proposed extension plus existing window and door areas are allowed without having to install extra insulations as well to the main building , ie cavity wall ,but i am wondering if its based on the size of the total size of the openings or total amount of glass involved minus the frames etc? i am looking to have a roof lantern fitted which obviously has a greater amount of glass than a Velux fiting the same sized opening as well as a 4 mtrs wide pair of bifold doors in a 8 x 4 mtr extension My existing kitchen doors and window plus the wall will be removed and a suitable RSJ put in place . I also have a pair of double glazed french windows in a 1650 x 2100 mm opening in an adjacent lounge which will remain in situ, so in effect the kitchen opening will cover half the width of the rear wall where the extension will be built.Can anyone advise me

 

[noseall]

i am wondering if its based on the size of the total size of the openings or total amount of glass involved minus the frames etc?

It is based upon the total size of the openings.

 

[donmaico]

Ok thats a relief.Makes things much easier

 

[ajrobb]

I think if it has enough glass, it can be called a conservatory instead of an extension - different planning and different regs.