I agree with all who suggest connecting to existing central heating. Also look to ensure the concrete floor incorporates insulation. As well as thinking about warmth in winter, don't overlook how hot they get in spring and summer, even if not fully south facing. Ours got like a furnace, so look to incorperateas many opening windows as poss(top opening fanlights for better security). Finally, is it a lean to type, or Victorian/Georgian style. If Lean To, rather than go for regular twin or tripple polycarbonate, ask your builer about the much better stuff (about 4 inches thick> I can't recall the brand name, but it is semi clear and contains strentheners made of aluminium, which means you can even walk on it (useful when cleaning the roof), it also means that the weight of snow is less of a problem. Hope this helps