Will definitely want it done right - certified etc and designed properly. I don't envisage it requiring PP as it will be pretty small in real terms, and should be below the dimensions requiring PP. Main purpose will be to elongate the kitchen slightly, nothing major as the garden is pretty small and don't want to lose too much of it.

Have had a few rough quotes which all seem to be in and around the same ballpark, so know what I'm facing into anyway. Won't be pulling the trigger on anything for a few months, but will update when things get moving! Thanks to everyone who has replied and sent PM's!