Tiny houses on tiny pieces of land in metropolitan areas

We need some sort of an apartment in a Metropolitan area Australia. Even in the inner suburbs there are tiny patches of unused land, like a parking lot, say 30m2 would be largely enough.
Building the house and planning is the fun and easy part, but the problem is getting that land and getting the building approved.
Maybe it is only a dream - usually lots need a certain size to be subdivided. Does anyone has any idea how to work around building laws (and still be legal)?
Being legal is important because even this tiny lot costs a fortune. I imagine building on the top of an existing garage for example.