Consequently it is always likely that one might want at least one more room than is needed by the family. It additionally provides buffer overflow should the pitter-patter of tiny drains on one's resources appear.
In such circumstances renting out a spare room on a longish licence should necessarily reduce rents since now there is an option which is likely to be cheaper. Renting out a spare room for other people visiting ones city should similarly have had a depressive impact on the price of hotels or an improvement in their offering.
Given the inconvenience and reduced amenity one gains from living in a house under multiple occupation there is further no specific reason why it should be the case that this does not more than offset the cash remuneration meaning there is no in principle reason why the capital cost of such a property should increase.