What is the cheapest way to efficiently heat individual rooms in a multi-tenant house?
There is no large common room other than the kitchen. The house is basically an L-shaped hallway with 3 bedrooms branching off it to one side and the kitchen branching off to the other. On the other side of the kitchen is a master bedroom. Doors are often closed because of the house being multi-tenant.
I would recommend you look into a mini-split system.
This will allow each of the tenants to control the temperature in their individual rooms. It is a very efficient system and will also alleviate any unnecessary arguments between the tenants.
For more information:
Read Randy Potter's Q&A "I can't afford geothermal. Is an air-to-air heat pump my best option for heating?"