The social mix principle tries to promote a rational distribution of urban public resources and provides adequate housing for different revenue groups through city planning regulations - ensuring social equity and promoting economic efficiency. An affordable and accommodating city is a core feature of a sustainable city. For example, Vietnamese cities are struggling for the provision of open space in dense inner-city centres. In Hanoi, between 2000 and 2010, the area devoted to parks and public gardens per capita went from 2.17m2 to 1.58m2 (HealthBridge, 2015).