The short answer is that it is determined by square footage.
According to the City and County of Denver Residential Housing Regulations Chapter 27 DRMC Adopted by the Board of Environmental Health on October 11, 2007:
Section 2-201 Minimum Space and Location Requirements
A.
No person shall occupy or let to another for occupancy any dwelling or dwelling unit which does not contain at least one hundred and fifty (150) square feet of
floor space for the first occupant thereof and at least one hundred (100) additional square feet of floor space for every additional occupant thereof. The floor space shall be calculated on the basis of total habitable room area, provided that uninhabitable area equal to ten (10) percent of the total habitable area may be counted in determining the maximum permissible occupancy.
