Private developers have been criticised for abusing government regulations over the car parks in residential buildings, making money from floor area for which they did not pay a land premium.
In some cases, developers are building up to nine floors of car parks that they can then sell or lease without paying land premium, a green group says.
Some car park spaces are being rented out as car-sales lots.
Research by the group, Green Sense, found that projects built near MTR stations also have a lot of parking space, despite government policy encouraging people to make use of public transport.
Existing planning guidelines requires developers to provide a certain number of parking spaces depending on the size of the development.
These areas will not be counted towards the property's gross floor area - the maximum area they can build on a piece of land.