According to Reg 21(3) of the Building (Planning) Regulations (Cap 123F), the plot ratio (地積比率) of a building is obtained by dividing the gross floor area (GFA) (總樓面面積) of the building by the area of the site on which the building is erected.
For example, if a building has 12 storeys and the area of each floor being 1,500 square metres, the GFA of the building is 18,000 square metres. If the site area is 2,000 square metres, the plot ratio is 9.
Regulating Plot Ratio
The Government imposes restrictions upon plot ratio as a means of controlling the density of population. Provisions regulating plot ratios may be enforced in the following ways:
- The Government Lease/Conditions may prescribe the maximum permitted plot ratio, breach of which would lead to the refusal of the issue of a certificate of compliance or re-entry by the Government.
- The notes accompanying the Outline Zoning Plan may specify the permitted plot ratio. Consequently no development would be permitted as the building plans would not conform to the restrictions in the Zoning Plan.
- Where the developer does not abide by the permitted maximum plot ratio prescribed in the First Schedule to the Building (Planning) Regulations (Cap 123F), permission to develop may be refused.
Amalgamated Sites having Different Plot Ratios
In International Trader Limited v Appeal Tribunal (Buildings)  HCAL 161/2005 in the High Court of the Hong Kong Special Administrative Region, the Applicant acquired two sites and intended to construct a multi-storey building there. Part of the building was on land zoned R(C)7 and the remainder on land zoned R(A).
In respect of the land zoned R(C)7, the Outline Zoning Plan prescribed a maximum plot ratio of 5. In respect of the land zoned R(A), the maximum plot ratio was prescribed by the Building (Planning) Regulations applied; this was 10 and 9 for a building on a class C site and Class B site respectively.
The Applicant submitted plans for the multi-storey building to be constructed on the amalgamated site. The first plan was for a plot ratio of 10 which was rejected by the Building Authority. The second plan was for a plot ratio of 9 which was also rejected. The ground for both rejections was that the development comprised one building part of which would stand on R(C)7 land and this contravened the permitted plot ratio of such land.
The Applicant appealed to the Building (Appeal) Tribunal which upheld the rejection. The Applicant then applied for judicial review of the rejection by the Appeal Tribunal.
The Applicant argued that the plot ratio in respect of the less restrictive zone should apply to the straddling site.
The Court held that the developer might properly construct two buildings on the differently zoned portions, each complying with its prescribed plot ratio. Alternatively, the developer might position the building anywhere on the amalgamated site even where it straddled two different zones. However, when assessing whether in such a case the plot ratio was permissible, the zoning application to the individual portions of the amalgamated site could not be ignored. There was clearly a contravention of the permitted plot ratio for the R(C)7 portion of the site, and the Building Authority was correct in rejecting the plans. The application for judicial review was dismissed.