Occasionally when designing small residential units the question comes up about what the minimum size is for a room and what the minimum ceiling height needs to be.
For single family homes and duplexes we go to the 2006 International Residential Code. It states that each dwelling unit shall have at least one room with a minimum of 120 square feet. Other habitable rooms need to have at least 70 sf excepting kitchens. (Bathrooms, closets, halls and utility spaces are not considered habitable rooms so they’re also exempt). In order to count the square footage towards the minimum it must have a flat ceiling over 7′-0″ or you can count the area under a sloped ceiling from the point it hits 5′-0″ and over.
There are also minimum ceiling height requirements under the 2006 IRC. Generally you need 7′-0″ clear minimum. Beams spaced 4′-0″ or more can be at 6′-6″ above the floor. Where you have basements with no habitable spaces (bedrooms, kitchen, living, etc) the ceiling can be at 6′-8″ and beams and ductwork can be at 6′-4″. Where you have the sloped ceilings at least half the room needs to have a ceiling height over 7′-0″. In bathrooms you need 6′-8″ over the fixtures and a minimum of 30″x30″ at that height where you have a showerhead.
The 2006 International Building Code kicks in for commercial buildings and buildings with more than 2 dwelling units. The room sizes are very similar to the IRC with a minimum of one habitable room over 120 sf and other habitable rooms needing to be 70 sf. Habitable rooms excepting the kitchen also have to be a minimum of 7′-0″ wide and there needs to be 36″ of clear space in front of counters and appliances.
On the minimum ceiling heights the IRC requires habitable spaces and corridors to be a minimum of 7′-6″ and bathrooms, kitchens, storage areas and laundry areas can have ceilings at 7′-0″ minimum. Where the ceiling slopes at least half the room needs to be over the minimum and you can’t count any area under 5′-0″ toward minimum room size. You can also furr down a ceiling to 7′-0″ for up to 2/3 of the room.
I haven’t utilized the rules for efficiency dwelling units but they exist in the 2006 IBC. For efficiency units you need a living room of not less than 220 sf and an additional 100 sf for occupants beyond 2. The unit has to have a closet. It also must have a kitchen sink, cooking appliance, refrigeration and a minimum of 30″ in front of each appliance. It also must have its own bathroom with a water closet, lavatory and a bathtub or a shower.