Here's a simple, non-technical answer:
Take your house in Canada and measure the length and width, multiply and you have gross space. Measure room-by-room and you have net space.
Not such a big difference in a house, big difference in an apartment complex where measuring the outside dimensions leaves you with the elevators, hallways and stairwells in your end answer. Sometimes in HK a 1,000 square foot apartment can be smaller than a 900 square foot apartment, because of the common space.
Thanks for all the clarification. I will now look at the net size. I already know it's going to be a shock going from a 2400sq ft house here in Canada to a 700 sq ft apartment in HK and now it's not really even 700 sq ft. Or would that be close if I was looking at a flat in a village house? They don't seem to have elevators or hallways.
Yep the useable area is higher with village houses but the floor plans may not suit you. Some older properties have a two-foot square area in the toilet for you to shower in without a shower tray or curtain.
My first apt was 1196 and the second is 1230 but the actual difference is huge. It totally depends on how they lay it out so whilst the measurements are useful you can't really tell till you get there. Plus you have to think of storage, not only did my last place have a tiny kitchen but every cupboard had a pipe running through it so you could only really make use of 2 cupboards. Neither apartment offered any storage outside of the bathroom and kitchen and in the first place you could only just fit the bed in the room.
It is not that bad - actually - it is quite a relief. When my husband and I moved here from Kitchener 2 years ago we moved out of a 3-storey 2200sq ft place into a 2-bedroom 774 sq ft apartment (usable size 581 sq ft). We sold pretty well everything back in Canada and soon realized how much "junk" we actually had. You can't have junk here b/c there is just no room for it. And I sure don't miss cleaning that house back in Canada!