This is not a clear yes or no answer. An extension will require planning permission if it is not ‘wholly to the back of the house’. If the extension is ‘wholly to the back’, then it can be built up to 40sqm without planning permission when calculated in conjunction with and other extensions carried out exempt of planning previously. The split between ground and first floor areas varies and depends on whether your house is detached, semi-detached or terraced so greater detail would be required to identify your specific situation. Other regulations / rules apply to exempted developments like separation distances of extensions ./ windows from the boundaries they are facing etc., so again, you should consult your Architect to identify these within your own design.