They said some owners, who had planned a collective sale of their properties to developers, had put their plans on hold.

The Planning Department last Friday amended the Outline Zoning Plan of Wong Nai Chung, which covers the sites in Happy Valley.

Under the proposed new restrictions, buildings in 11 sites in and around Ventris Road and Shan Kwong Road will be imposed with a development plot ratio of five. The building height is limited to 115 metres above principal datum - the level to which land surveys in Hong Kong are referenced.

The buildings on five sites in Mui Hing Street and Hawthorn Road have been imposed a height restriction of 14 storeys including car parks. Blue Pool Court on Sing Woo Road has been imposed a height of six storeys in addition to one storey of car parks.

Happy Valley is the second district in which the Planning Department has imposed restrictions to prevent over-development. The first district was Kowloon Tong where the department in February imposed height limits for residential and commercial sites around Broadcast Drive, Beacon Hill Road, Grampian Road and La Salle Road.

Charles Chan Chiu-kwok, the managing director of Savills Valuation and Professional Services, said most owners of properties of more than 20 years old would consider a collective sale. But the new development controls would discourage developers from the acquisition of old buildings.

'The new development restrictions will dampen the value of the property,' Mr Chan said. 'But the individual owners do not notice the impact on their property prices. It takes time for them to have a clear picture on the market value of their properties and to adjust the price.'

Gabriel Cheng Hon-wah, a director of the investment department of Savills, said: 'The individual owners of Blue Pool Court planned to conduct collective sales a few months ago.

'The plan has been put on hold as the redevelopment potential of the site is limited while developers cannot gain additional gross floor area through the redevelopment.'

Property agents said individual flat owners of Winfield Gardens and Shan Kwong Court on Shan Kwong Road had planned to conduct collective sales. But the sites have been imposed development plot ratio and building height restrictions.

Colliers International residential sales department director Ricky Poon said developers would lose interest in old buildings as they could not gain additional development plot ratio with the new controls.

'The individual owners have been forced to give up their collective sales plan,' he said. 'But the new controls can prevent over-development in the district, which is suffering from the problems of high density and traffic jams.'

Kerry Properties' South Court redevelopment project on Shan Kwong Road and its neighbouring site, however, is not expected to be affected by the new controls.

Mr Chan said the project would not be affected because the building plans were approved before the restriction was announced.

Kerry could redevelop South Court with a development plot ratio of eight, he said.

Kerry is controlled by the Kuok Group, the controlling shareholder of the SCMP Group, publisher of the South China Morning Post.