Hong Kong use an unique system where Gross floor area include a sharepart of common spaces, thus two blocks with different number of identical apartment per floor could had different gross floor area, because in one block the common areas would be divided to fewer apartments than in other block. 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 (public housing) shows internal floor area in case of PRH estates, while for TPS, HOS and PSPS estates it shows saleable and gross floor area.
Most private developers, as well as all property classifieds websites, published gross floor area as primary value of apartment size, confusing people. Only in 2012 or 2013, websites like Centadata.com and GoHome.com.hk added saleable floor area beside gross floor area in every listing.
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.
As home owner, you may be interested in internal floor area, but the percentage of internal floor area from saleable area vary much, as newer and taller buildings require thicker walls, take a look at 64-storey The Masterpiece, at lower floors structural walls are 100 – 120 cm thick, higher floors have thinner walls so larger rooms.