“The cost of living has gone up along with the cost of mortgages,” said Sydney Blumstein, a real estate broker with Corcoran. Ten years ago, a couple could make a little extra money by renting out a spare bedroom for brief stints on Airbnb, but new laws in many cities have imposed a 30-day minimum on short-term rentals. “That’s why the sublet community has grown, or the rotating roommate scenario,” said Ms. Blumstein.