The OP mentioned 10 x 10, which is a good size but as in all things construction, twice the size doesnt mean twice the cost. If you want reasonable use all year round ( especially in irish climate) a solid roofed structure is best, insulated, wired, heated etc as an integral part of the main structure. When we built we wanted the "conservatory" experience but also needed lounging and eating and "party" space and thanks to a brilliant builder got just that, usaable 24/7x365. ( Xmas dinner for 16 held out there ) Consider seriously your desired use over time from the space and see as many examples as you can.