Final answer:
The non-rentable space accessible by tenants in a building is called the common area, which includes lobbies, hallways, and other shared spaces, with associated costs covered by Common Area Maintenance (CAM) fees.
Step-by-step explanation:
The non-rentable space that is accessible by tenants in a building is referred to as the building's common area or common space. These areas are generally maintained by the landlord or property management and can include lobbies, hallways, restrooms, and sometimes amenities like fitness centers or conference rooms. The cost of maintaining these areas is often included in the operating expenses of the building and shared among the tenants through their lease agreements, typically under the term Common Area Maintenance (CAM) fees.