Hi got planning permission for a small single storey extension due to be built soon. The extension is 18.75m2 internal surface (4.075x4.600m) , 22m2 if the depth of the walls are taken into account in measuring it (4.600x4.600m). The planning permission states the extension is 18.75m2 but the builder is quoting for an extension that is 22m2. At 2k a m2 this is an expensive difference so I am wondering is there a consensus as to how the size of an extension is calculated?