KirstinMc wrote:@Papinian - it's a single storey extension and not in a conservation area. Thanks for your perspective.
If it's a single storey extension, you have a 40 foot long garden, you are not in a conservation area and you are adding on to the original house and not an already existing extension then what you are being told re only being allowed to extend out 3m is wrong. It is possible that whomever you are speaking with is not up-to-date - the permitted development rules were liberalised in 2012 and single storey rear extensions of up to 6m could quality for permitted development whereas only 3m extensions qualified previously. More details here:
http://webarchive.nationalarchives.gov. ... _may13.pdf
The only basis upon which there can be an issue with such an extension is that it damages the amenity of a neighbour (e.g. completely overshadows their garden) but as you have said your neighbours are fine with what is proposed that doesn't seem to apply in your case.