An empty building where people start living without the owner's permission.
squat
A very poor, dirty house or flat in a bad condition.
hovel /ˈhɒvl/
A small flat or house in a city owned or rented by people in addition to their main home and used when they are visiting the city.
pied á terre /ˌpjeɪd ɑː ˈteə(r)/