A heritage study of Government Hill calls for a legislative change to create a 'special protection area' to safeguard the historic zone from inappropriate development.
The study also warns that the views from Government House, the chief executive's residence, would be blocked and the low-rise character of the hill harmed if a bulky office block replaced the west wing of the Central Government Offices complex after it is demolished.
Critics said the 'conservation package' for Central announced by Donald Tsang Yam-kuen in his policy address on Wednesday was only an ad-hoc measure for individual buildings rather than a genuine advancement of heritage policy.
The study was released the day after Tsang said the east and central wings of the CGO complex - the centre of administration since the earliest colonial times - would be used by the Department of Justice after the government headquarters moved to the Tamar site in 2011. The 13-storey west wing, less historic, will be razed to make way for an office block.
Tsang did not mention any legislative changes for heritage.
The study consultant, Michael Morrison from British-based architectural firm Purcell Miller Tritton, recommended legislative changes for a special protection area if the government wanted to give guidance to potential developers as to what could be done with the site in future.
The special area could cover the well-wooded spaces and low-rise buildings in the area.
The three CGO wings, in the functionalist architectural style, were deliberately designed in the 1950s as low rise to preserve the view of the harbour from Government House.
In the report, which was commissioned by the government, Morrison wrote: 'This view may have long since disappeared but the low-rise buildings and the trees combine with the other well-wooded areas to give a large green space in an otherwise heavily developed part of the city ... This is something to be maintained in any future development of the site.
'Protection of this sort will require an amendment of the relevant legislation in relation to preservation in Hong Kong.'
The architect suggested the new building on the west wing site should cover a smaller area than the current ground plan to avoid blocking out the light and the views of Government House. The government should set up a management company to have close control over the proposed development, Morrison said.
Vincent Ng Wing-shun, a member of the Urban Design Alliance, agreed a designation of a special protection area was necessary. Government Hill was part of Hong Kong's origins and the core of power, he said.
'The integrity of the site should not be harmed. The design of the future office block must be compatible with the surroundings.'
The chairman of the Institute of Architects' heritage and conservation committee, Eric Lee Chung-ming, said the measures unveiled in the policy address would not secure permanent preservation of the site.
The Antiquities and Monuments Ordinance only covered individual buildings, never an area, he said.
'The next administration might not share the same feelings for heritage. Even if there is a category of land zoned for historic buildings, you can always rezone. A stronger heritage law is necessary.'
Bernard Lim Wan-fung, a member of the Antiquities Advisory Board, agreed about the designation and said any tall building, say over 50 storeys, would not be acceptable.
Katty Law Ngar-ning, who leads a concern group that keeps an eye on development in Central and Western, said it was not only Government Hill but the wider historic area of Central that needed protection from unco-ordinated redevelopment.
Secretary for Development Carrie Lam Cheng Yuet-ngor said the design of the office block would go through the Town Planning Board and public consultation. Its height, density and impact on the hill would be dealt with carefully, she said.
Lam also said the government would offer two sites on the new Central harbourfront - in front of Two IFC - to private developers through open tender. The areas will feature low-rise structures for exhibition, shopping, entertainment, civic and community uses with public open space.
Vincent Ng said as long as the tender spelled out clear requirements about public access and use of the buildings, it was acceptable to allow private developers to manage public open space.
Other conservation measures for Central include revitalising Central Market, the former police married quarters in Hollywood Road and the Sheng Kung Hui church complex.