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Why could rental costs for apartments/houses rise while buying prices can go up and down?
Economically, you would say that purchased and rented real estate are not perfect substitutes--they are largely separate markets.  Only a few people are able to easily switch from one to the other and that choice is sticky--for example, once you buy a house, prices would have to rise a lot for it to be worth it to sell it and move into an apartment. In both markets there is a supply and demand curve, but the slope of the demand curve for houses to purchase is much steeper than the demand curve for rentals. The market for new housing fluctuates rapidly because it requires a large change in housing prices to change the number of people looking to buy a house.  Most decisions to buy a house are not driven by the state of the housing market.  This describes a supply/demand graph with a very steep demand curve.  Additionally, because of the leverage provided by mortgages, the demand for houses depends critically on relatively small changes in the interest rate and availability of loans.  Thus the steep demand curve shifts all over the place as borrowing conditions change. On the other hand, apartment prices are more stable because people easily move from one apartment to another and people living in their parent's basements easily move into apartments if prices change.  A small change in the price or quantity of rentals brings about reasonable response in quantity demanded.  This is the situation where the demand curve is shallow.  In addition, rentals are not tied to interest rates tightly, nor are they as strongly tied to economic conditions (in a recession, people avoid buying but renters continue to rent).