A drastic reduction in the city's projected population has led to calls for the government to scale back plans for more land reclamation and overhaul its strategy for building up a land bank.
The government launched a consultation exercise in November on six options for building up the land supply. But some, including building on farmland and reclamation outside Victoria Harbour, proved controversial.
The plans were based on a projected population in 30 years of 8.89 million, a figure put forward by the Census and Statistics Department two years ago.
But this was revised down to 8.47 million last month after Chief Executive Leung Chun-ying said that mainland women without local partners would not be allowed to give birth at the city's hospitals from next year.
The difference is 420,000 - a significant amount when, for example, 787 hectares of agricultural land at Kwu Tung North, Fanling North and Ping Che will be required for a new town providing homes for 150,000 people.
The Development Bureau will next month identify 10 sites for possible reclamation, from 25 initially put forward. But sceptics say the population projections indicate a flaw in the bureau's planning.
Green Sense president Roy Tam Hoi-pong, who fears marine life will be threatened if the government goes ahead with further reclamation, said the department's new figures should give pause for thought. "Reclamation should be avoided as far as possible," he said. "When demand falls, they should cut back the scale [of reclamation]."
Ng Mee-kam, a professor of urban planning at Chinese University, said the government should not just accept the population projections at face value.
He said: "More important questions would be: how do we want our city to be? Will there be policies proactively shaping the quantity and quality of our population?"
Without asking such questions, Ng said, the new land was likely to end up in the hands of developers intent on building expensive flats rather than meeting the needs of the city.
Paul Zimmerman, head of the urban planning group Designing Hong Kong, said the city needed a development masterplan. "You can't say, 'Let's keep [land] in my pocket and see how it will be used'. The response of residents could be very different if you tell them the reclaimed land is for sports development instead of building flats," said Zimmerman, who is fighting plans for reclamation at Sandy Bay in his capacity as a Southern District councillor.
The Development Bureau earlier said 4,500 hectares of extra land would be needed to cater for 1.8 million more citizens in 30 years, of which 1,500 hectares would need to be found now.
The figures were based on the population and land usage in 2010, when 7 million people used 7,600 hectares of residential land and 9,900 hectares of land for infrastructure, about 2,500 hectares for every million people.
A bureau spokesman said it would stick by its plans for building up the land supply, which would be made available "when appropriate". He said: "By doing so, the government will be able to supply sufficient land when demand rises."