The Rating System language (pg. xiv) says "include all directly conditioned square footage, whether finished or not, that meets building code requirements for living space (e.g. head room, egress, etc.)." Then there are CIRs and other rulings that relate:
When determining the square footage of the home for the HSA, include all directly conditioned space within the thermal boundary, calculated to the exterior wall.Include all normal living spaces and service areas (e.g., garages, utility rooms, closets, entries, crawl spaces, attics, basements) that are within the thermal boundary and meet ANSI Z765 requirements for ceiling height EXCEPT those that are separated from primary living spaces by insulated, weatherstripped doors AND are neither directly heated nor cooled. In the rare case of mild climates where normal living spaces in the home are neither heated nor cooled, include all spaces that are reasonably expected to be used as living areas or are normal adjuncts to living areas, such as closets, utility rooms, entries, etc.
Updated 10/1/13 for rating system applicability.