Published 20 October 2026 · 6 minute read
Four abbreviations drive almost every residential feasibility: FSR, GFA, GBA and NSA. Mixing them up is one of the most common and expensive early mistakes, because planning controls cap one number while revenue comes from another.
Floor space ratio (FSR)
FSR is the ratio of gross floor area to site area. An FSR of 2.5:1 on a 6,000 m² site allows 15,000 m² of GFA. It is the main density control in NSW local environmental plans; other states use similar plot ratio controls. FSR does not tell you how tall or how wide the buildings are: the same 2.5:1 could be an 8-storey courtyard block or two 15-storey towers, which is why height, setbacks and separation matter just as much.
Gross floor area (GFA)
GFA is the floor area counted against the FSR. Under the NSW Standard Instrument definition it is measured from the internal face of external walls, and it excludes, among other things:
- common vertical circulation such as lifts and stairs,
- basement storage and plant rooms,
- car parking that meets the consent authority's requirements, including access to it,
- terraces and balconies with outer walls lower than 1.4 m.
Corridors, lobbies and habitable rooms are included. Always check the exact definition in the planning instrument that applies to your site, because states and councils differ.
Gross building area (GBA)
GBA (also called gross building or construction area) is everything you build, measured to the outside of the external walls, floor by floor. It is the number cost planners price, because you pay to build cores, walls and plant whether or not they count as GFA. Basements are usually reported separately.
Net sellable area (NSA)
NSA is the internal area of the apartments you sell, typically measured to the inside face of walls and excluding common areas. Retail is usually measured as net lettable area (NLA). Measurement conventions vary by state, contract and valuer, so confirm which method your agent and valuer use before relying on per-square-metre prices.
How they connect: efficiency
The ratio that links them is efficiency, usually NSA divided by GBA (some people use NSA divided by GFA). For a double-loaded corridor building in the mid-rise range it typically sits around 70 to 80 percent. Single-loaded galleries, deep floor plates, many small lift cores and complex shapes all lower it.
| Measure | Measured to | Includes cores and corridors? | Used for |
|---|---|---|---|
| GBA | Outside of external walls | Yes | Construction cost |
| GFA | Inside of external walls | Corridors yes, lift and stair cores no (NSW) | FSR compliance |
| NSA / NLA | Inside of apartment or tenancy walls | No | Revenue |
A worked example
The designer's sample site is 6,082 m² with an FSR of 2.5:1, so the GFA cap is about 15,200 m². One generated courtyard option reaches about 14,600 m² of GFA (FSR 2.40:1) from 16,500 m² of GBA over 7 storeys, and delivers about 12,100 m² of net sellable area: an efficiency of around 73 percent of GBA. At $11,500 per m² that NSA is worth roughly $139 million before GST and selling costs. Change any input and every one of these numbers updates.
How AI Will Do That CAD calculates them
- GBA is each floor plate (outside of walls) times the number of storeys.
- GFA is each floor plate less the external wall thickness and the lift and stair cores.
- NSA is the internal area of every apartment (frontage less party walls, times depth less facade and corridor walls). Retail ground floors are reported as retail area.
- Common area is GFA less NSA: corridors, lobbies and service spaces.
These are concept-stage measurements for comparing options. Before lodging a development application, have the areas measured by your architect under the definitions that apply to your site.
Next: how basement parking drives cost, or a 10-minute feasibility workflow.