This not clear enough to provide meaningful feedback. This just raises lots of questions. Needs to be definative. e.g.
Is it 25% extra of the existing dwelling (including any extensions from the original)? or,
25% extra of the original?.
What would be the situation if I had a certifcate of lawful development to extend that was not excercised but because the original building was constructed to old standards/materials I wanted to build to rebuild using new standards/materials (which is much greener)? Is it 25% extra on top of the certificate of lawful development?
With regards replacing a bungalow with a bungalow surely it is better in a rebuild scenario to build a house with the same floorspace as the bungalow as this would leave less footprint on the land.