When we built sunroom the builder discovered soakaway in middle of garden under where sunroom was going. Had to move it to end of garden but regulations meant he had to keep it a certain distance from house and from wall and we got caught on distance. Not sure of the distances involved but after an architectural consult he got it sorted. Also pipes at back had to bemoved, was a reasonable but not difficult job and involved moving manhole too. A good builder with experience of extensions like this should have come across this before and be able to guide. Try and see other work builder has done of same type, we did and helped a lot.
On planning, generally once to rear of house and under 40sq mtrs with single storey only, is planning exempt but most co councils have the leaflets on this on their websites. THeres also some rule that a minimum amount of garden space be left afterwards. Check to be sure first. Builder will assume that you have planning for what you're putting up or are exempt from planning, its not his problem.
Either way, once done it'll be a major boost to life at home.