Hong Kong use an unique system where Gross floor area include a sharepart of common spaces, thus two blocks with identical apartment units but with different number of units per floor had different gross floor area stated for each apartment, because in the block with fewer apartments per floor the common areas were divided to fewer apartments. In other countries Gross floor area meaning is like Hong Kong’s Saleable area.
Net floor area is the internal room space between face of walls.
Saleable floor area include full thickness of external walls, half thickness of walls shared with other units, internal walls thickness, as well as balconies, but exclude bay windows.
Gross floor area also include a sharepart of common areas (lifts, staircases, corridors) so it should not been taken in serious.
Housing Authority shows internal floor area in case of PRH estates, while for TPS, HOS and PSPS estates it shows saleable and gross floor area.
The efficiency (Saleable floor area) is usually 75-85% of Gross floor area, higher value for the blocks with bigger apartments and for low-rise blocks, since the common area of a blocks with a given number of units per floor is approximately same for regardless of apartments size. More details about what is gross floor area and efficiency here insitu.com.hk and eaa.org.hk.
Home owners should be interested in internal floor area, the percentage of internal floor area from saleable area vary much, as newer and taller buildings require thicker walls, take a look how thick are the walls of The Masterpiece. Higher floors may have thinner walls so larger rooms.
BEWARE! Most, if not all private developers, as well as all real estate websites, publish gross floor area as primary value of apartment size, confusing people. We cannot make a law to force developers to quote the saleable floor area and put an END of this annoying confusion?
Starting from 2012 or 2013, websites like Centadata.com and GoHome.com.hk added saleable floor area beside gross floor area in every listing.