I understand that the area of glass of an extension can be no more than 25% of the new floor area, however you can offset the area off window removed from the original building by the creation of the extension.

Our extension (internally)will be 28.14 sqm
the proposed amount of glazing is 12.9sqm
The amount of glazing removed from old house is 7.4sqm
Hence the additional glazing is 12.9-7.4 = 5.5sqm

This as a percentage is 19% of the new floor are so i am ok. Is this right

28.14m2 x 25% =7.03m2 allowable
proposed glazing = 12.9m2
7.03m2 + existing of 7.4m2 =14.43m2 allowable
Your proposed amount is less therefore you are ok.

Thanks.
Just wanted to check I hadnt misunderstood it