This is were things can get a little complicated to understand
It is true that under the building regulations you should only have the area of glazing to the extension area that is no more than 25% of the floor area of the extension - but!!!! you can also add the area of the any windows that were originally external windows but have now been removed/covered by the extension. So lets say you had original kitchen with a window that was 2msq, this window is removed to extend the kitchen. The extension has a floor area of only 4sqm. Thus you would be allowed (under the 25% rule) 1sqm of opening in the extension PLUS the original 2sqm giving you a total area of 3sqm.
Another way (that is accepted by most sensible Building Control) is to have full thermal calculations done for the whole of the dwelling -You need to have 3 set done. One for the original dwelling before the extension, one as if the extension had been built to the minimum requirements of the regs (ie with the worst allowable U-values for walls, floor, roof, etc and now more than 25% glazing) the final set is for the extension as you want to build it with the large amoutn of glazing - sometimes in this case it is neccessary to do other things to the original dwelling eg increase roof insulation, change you existing windows to better thermal ones, if no cavity insulation get them pumped.
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