We can see what happened in the past with the Lily Pond etc. And if you read back through this thread you will see how the discussion developed. Basically, owners of agricultural land are allowed to fill it in to a maximum depth of 1.2 metres. It seemed that there was no requirement that the infill be composed of soil suitable for cultivation. The impression is that would-be developers cover the soil with rubble and in time apply for the zoning of the land to be changed arguing its unsuitability for farming and then go ahead to build.
Another consideration, that I still don't clearly understand, is that this area somehow falls through the cracks of normal land administration and enforcement due to some sort of planning zone setup (ask Living Lamma to explain) such that no government department seems able or willing to actually follow through with legal enforcement of Hong Kong's development regulations.
I have found one thing, though: there does seem to be some legal expectation that infill of agricultural land be composed of cultivable soil. See the following '
Town Planning Board' report Agenda Item 3.3.C.iii
Director of Agriculture, Fisheries and Conservation (DAFC)
advised that the materials used for filling of land for agricultural purpose as
claimed by the applicant had to be suitable for crop cultivation.
It seems fairly likely to me that unless some group with deep pockets and a lot of time and dedication rises to fight it - the whole valley will slowly fill up with village housing. It has already begun.