There is currently no standard requirement on gaps between buildings, which is usually determined on a case by case basis using air ventilation studies and considering the size of the development. There is also no standard requirement for green features.

The source said care would be taken to prevent developers from profiting from the proposed green space by increasing the overall development density.

The move follows controversy over present arrangements that award bonus floor area in return for incorporating features such as sky gardens, utility platforms and bigger lobbies and corridors that are assumed to improve the quality of life in the developments.

Secretary for Development Carrie Lam Cheng Yuet-ngor said last month the government would consult the public this year on how green features should be incorporated into new developments without increasing the density of the project.

The consultation is expected to start as early as next month.

The source said building permeability and the green ratio - which refers to the minimum space between blocks and the amount of green space respectively - were major issues under consideration.

Based on air ventilation studies and overseas examples, a government consultant had advised that leaving 30 per cent of a site's length as gaps would ensure air flow and prevent 'walled buildings'.

'Walled buildings are formed when developers put buildings along the same line to maximise sea views,' the source said. 'To prevent walled buildings, developers would be required to leave at least 30 metres as non-building area if a site is 100 metres long.'

A standard requirement would be easier to implement than working out a figure for each site, the source said. 'As a start, developers could be asked to meet a minimum requirement since studying the wind environment of every single site would require much more effort.'

The source said a minimum of 30 per cent green area had been a requirement for developments on the mainland for many years.

'A site should have 30 per cent green area when we look at its aerial photo,' the source said. He acknowledged that the requirement could add management and development costs to government departments and developers in exchange for a better environment.

Patrick Lau Sau-shing, the legislator representing the architectural, surveying and planning sector, said increasing the permeability of developments would eliminate podium structures but encourage taller buildings, which might violate the government's new height restrictions.

He said imposing a site-specific amount of non-building area was preferable, and reducing the allowed development density would be the most effective way to minimise the wall effect.

Andrew Thomson, chief executive of the Business Environment Council, an independent non-profit group that promotes corporate social and environmental responsibility, welcomed the initiative but stressed that the business sector would prefer a more flexible approach as the wind factor varied at different locations. 'We would welcome a guiding principle. Prescriptive requirements could result in boring designs.'

A Development Bureau spokeswoman said the consultation would be held as soon as possible.