Hello again,

I just wanted to add...

Here is a good blog post on the topic of searching for footprints for various cities:

http://www.undertheraedar.com/2015/07/urban-footprints-some-building-outline.html

It details more strategies for accessing OSM data, including a good video tutorial on how to use a different tool within Quick OSM. 

It also points out that the most authoritative footprint data often comes from the relevant municipality itself. It might be worth finding a Cantonese speaker to contact the HK Planning Dept to see if they will share the Building layer that apparently exists in their possession, according to this link:

http://www.pland.gov.hk/pland_en/p_study/prog_s/landscape/landscape_final/app6-6.htm (scroll down)

Good luck!

eb