It turns out - for instance looking at Mel Bartholomew's work1 that most crops and garden flowers don't need more than six inches deep of rich soil to do well. He claims that a four-foot (120cm) square box will produce enough salads to keep a person or two going for a full growing season and that three boxes should produce most of the veg and salads a person needs. If the soil isn't loam based, so it's light and water absorbent it's not going to be all that heavy (good for roof gardens). He's very insistent about his soil mix, but I've taken to using growing bag mixture which I'll feed with compost when I change crops. It's working well for me so far.
The main value of his stuff is that he splits the beds into 30cm x 30cm (1ft x 1ft) square, marked with (in my case) bamboos laid on the soil. A different crop generally goes into each, moving around all the time. The extra constraint of planting crops into those small spaces is really helpful in making planting decisions as you go along. The boxes are big enough to hold moisture and provide a living soil for the plants and watering is much easier than multiple containers, which I can never manage to keep properly watered.
Shrubs and trees obviously need different treatment, but they have to live in bigger, deeper containers that are big enough to hold enough moisture anyway.
He fails to reference anyone else, pretty much, but you can see similar stuff in other places. Gardening books tend to be really bad at attributing ideas to other people unless there's a guru involved - the permaculture crowd are better at references for some reason.↑
As you can see the boxes aren't very big as you note; however, there are only 24 boxes for 184 condos. So, I expect this particular garden space will be pricey, but well utilized.
