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45 degree rule is used for planning permission. Only relevant to neighbouring habitable rooms tho - not bathrooms kitchens etc.
Permitted development will let you do a 3m rear ex on a terrace - with a max flat roof eaves of 3m I think. But check the booklet for all the rules.
IMO In many ways the PD rights are fucking local character but that ship has sailed and every *persons house is their castle I guess
Is the 45º rule of extension size actually enforced? Looking at lots of the ones out my back window I can't see how they pass.