Building area types: GBA, Usable, Rentable, and Net Assignable Area

One-line orientation

These area terms draw different boundaries around the same floor. Learn where each boundary is measured and which shared spaces it includes.

Key points

• Gross Building Area (GBA): Measured to the outside face of exterior walls. Includes everything inside: occupied spaces, circulation (primary + secondary), mechanical rooms, vertical penetrations (stairs, elevators, shafts), and service areas. The largest and most inclusive figure.
• Usable Area: Measured to the inside face of exterior walls and the finish face of walls enclosing vertical penetrations. Includes net (assignable) area and secondary circulation. Excludes core areas shared by multiple tenants (building service core).
• Rentable Area: Includes Usable Area plus service areas and primary circulation. Excludes vertical penetrations. Used for lease calculations; the exact extent may vary by lease terms.
• Net Assignable Area (NAA): Measured to the inside faces of the walls enclosing each assignable space — wall thicknesses are not counted. In BRM’s programming model, Net Assignable Area (NAA) excludes general circulation and mechanical rooms. Other standards may treat service spaces differently. This is the space actually dedicated to programmatic functions — what the program document lists.
• Where the terms come from: Usable and Rentable are office-leasing measurements standardized by ANSI/BOMA Z65.1; NAA (net assignable square feet) is the programming / institutional measure (FICM). BOMA measurements support leasing, while institutional programming standards define assignable program area. Do not combine their boundaries in one calculation.

Building area types: a conceptual boundary comparison

The nesting is a study model only; confirm one compatible measurement standard before calculating.

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Conceptual study model only — NAA is measured inside the walls enclosing each assignable space, and BOMA and FICM measures must not be combined in one calculation without confirming compatible definitions.

Confusions / comparison

Area type	Measured to	Includes	Excludes
GBA	Outside face of exterior walls	All areas — occupied, circulation, mechanical, vertical penetrations	Nothing (most inclusive)
Usable	Inside face of exterior walls; finish face of walls enclosing vertical penetrations	Net area + secondary circulation	Shared building core (tenant-shared service areas)
Rentable	(Derived from Usable)	Usable area + service areas + primary circulation	Vertical penetrations (stairs, elevators, shafts)
NAA	Per the institutional programming standard in use	Programmatic / assigned space	In BRM’s model: general circulation and mechanical rooms; confirm other exclusions by standard

Related

→ prog-efficiency-net-to-gross (this module): how the three efficiency factors use these four area types · prog-adjacency-diagrams (this module): the diagramming tools used once NAA is established · prog-criteria-to-block-diagram (this module): how the program’s NAA requirements are sized and arranged.