Under the current regulations, no land premium is required for building an air-conditioning plant room, which used to be designed for detached houses to store a centralised air-conditioner. But some flat owners have in fact paid for the room, as some developers include it as part of the saleable area.

The new definition announced by the institute yesterday includes the 'utility platform' - a balcony-like platform - but not the air-conditioning plant room as saleable area.

'The air-conditioning plant room is commonly found in multi-storey residential developments in recent years,' the institute's vice-president, Stephen Yip Moon-wah, said. 'It should not be counted as saleable area because people living in the flat do not practically use it.'

A number of surveyors said that some developers were abusing the right to build an air-conditioning plant room. They were building the rooms larger and describing them as storerooms.

Air-conditioning plant rooms are commonly found in floor maps of newly built developments. In the floor map of one luxury apartment in Kowloon Tong, the air-conditioning plant room is roughly the size of the kitchen or small bedroom.

The size of a plant room can vary from 10 sq ft to over 30 sq ft, said Charles Chan Chiu-kwok, managing director of valuation and professional services at property consultant Savills.

There is no maximum area set for the plant room under current building regulations, 'but its size has to be justifiable and approved by the Buildings Department', he added.

'Some developers include the room's area as saleable area because, they say, the room is exclusively used by flat owners,' Mr Chan said.

Surveyor Pang Siu-kei said buyers might mistake the plant room for a storeroom because developers' floor plans do not explain what they are. 'Some buyers eventually turn the plant room into a storeroom and hang the air-conditioners outside the buildings,' he said.

However, Mr Pang said the utility platform should be counted as saleable area since it can be used for drying clothes or storing a washing machine.

'The function of a utility platform is similar to a balcony. They should be counted, as they are useable,' said Lawrence Poon Wing-cheung, the chairman of the institute's saleable area working group.

Dr Poon said yesterday that the institute had not changed the definition of saleable area but clarified it to avoid different interpretations by developers. He said the clarification was made after consulting the views of the government and developers.

Early this year, the institute suggested excluding the area of bay windows from the saleable area. Yesterday's announcement is the institute's second attempt to clarify the definition of the saleable area.

A spokeswoman for the Transport and Housing Bureau said the government would study the institute's suggestions.

A spokesman for the Real Estate Developers Association declined to comment on the new definition, saying the association is having discussions on the matter with the government.

Cut down to size

How the saleable area of one flat would be reduced by the new guidelines

A/C room: -37 sq ft

Utility platform: -17 sq ft