Not German so I can only talk about how it is going in another country:

- Massive change in the average household size: way fewer people live together now (delayed couple & family formation, divorce, etc.). If you go from 4 people per household to 2 people per household, now you need twice as many homes.

- Massive internal migration: declining population in a lot of rural areas and increasing in cities & their suburbs. So lot of empty houses and super cheap houses in Dumbfuck, Nowhere but scarce & expensive homes where people want to live.