Shorterm the vacancy rate will determine if rent goes up or not but in the longterm rent can't rise much faster then income growth. That is net income growth after inflation/tax and welfare cuts. Looking how fast rents have risen the last few years, one can safely assume that the max treshold has been reached or will be very soon. If real estate investors bank on ever rising rents above inflation they are dreaming. Every bubble throughout history has popped and so will this one. You can argue the timing not the outcome.
