A Wohnberechtigungsschein (WBS) is a certificate required to live in apartments that have been built using state subsidies. A Wohnberechtigungsschein is only available to people below a certain income level. To qualify, your residence permit must have been issued for one year. You can obtain further information on the WBS permit from the local housing authority ( Amt für Wohnungswesen) listed in the phone book under Stadt.
People with limited means can also apply for a housing allowance ( Wohngeld) from their local town hall.
When looking for a flat it is often helpful to visit you local Wohnungsamt (Housing Office) to get information on Wohnungsgesellschaften (state-run housing associations with flats for rent).
Another form of subsidized housing are state subsidized student residencies.