Hi Baldric,
I live in Victoria too, Melbourne. I too am wanting to build a hot house for my ever increasing collection of cacti. I already ahve a hot house (1.5 x 1 meters) which is built on a steel frame. This eables me to sit down at the table and do my propagation, maintenance etc.. This house has now become full and I am in the process of drawing out some plans. I must confess that I have'nt thought of dimension like 10x6 meters. I was htinking more on 4x6 meters, which should be sufficient. I keep most of my big cacti outside and they do not get affected by frosts for the past 3 years.(fingers crossed). I use a hot house only for cacti like lophophora, gymnocalycium, lithops, cereus, notocactus etc which need to be kept dry during winter.

Thank you,

AJAYKAMATH, I WISH YOU WELL . ANOTHER MEMBER EMAILED ME FOR WHICH I WAS MOST APPRECIATIVE . IN PASSING SHE MADE REFERENCE TO HEAT BANKS OF MILK BOTTLES FILLED WITH SEA WATER . THE YET ANOTHER EMAILED THE SUGGESTION I LOOK UP PASSIVE SOLAR GREEN HOUSES ON THE INTERNET. WHAT A GREAT SITE , THANK YOU TO THAT EMAILER TOO.