We had to remodel our apartment in HK to the tune of $100,000 USD for the very reason you mentioned, efficient space use. We carved a study off excess space in the bedroom as my second child took my study. Apts in HK generally do not have closets so we had to build in wardrobes, etc. If we had only taken care of the space issue, it probably would have only cost $10-15,000. But we also had to re-do 30 year old pipes as well as our balcony and floors. I think a lot of space use is local contracting.