The building fabric refers to what the building is made out of. This will differ according to location and what is best to respond to the climate specifics of the location.
For example, sometimes it is better to have a dense material on the ground like concrete that can heat up and store warmth and heat to radiate back out during the day and stabilize the room temperature. Other situations it may be better to have a raised timber floor that air can circulate under, the best example of which can be seen in a Queenslander style house. It makes sense, where it is so hot that a more lightweight material should be used to prevent the building heating up like an oven and staying hot.
The single most important building material would have to be insulation. It has a range of R 1 to R6.5 for residential purposes where R equals resistance of heat transfer through mass. The effectiveness of even a little bit of insulation is large on how much you can stabiles the temperature within your building. It is your best friend for keeping out hot air in the warmer parts, and keeping it in throughout the cooler parts. Either way, you need it, in the walls floors and ceilings, even under concrete.
It is our job to educate the client on what is best in performance and how we might make it look good.