Illegal construction in the context of "Rooftop slum"

Rooftop slum (Chinese: 天台屋) or penthouse slum generally refers to illegal housing on the rooftops of apartment buildings. In Hong Kong, some people are unable to afford traditional apartments and are forced to wait years for affordable public housing. They therefore live in squatted shacks on top of buildings. According to the Hong Kong population census, there were 47,091 rooftop dwellers in 2011 and this number is likely to have dropped as working class areas are redeveloped.