Two advocacy groups on Monday said brownfield sites could offer a quick solution to Hong Kong's housing problems as they called on the government to set a roadmap for their development in next week's Policy Address.

Greenpeace and the Liber Research Community said satellite images and site inspections had identified 433 hectares of large and accessible brownfield plots within the area designated for the administration's massive Northern Metropolis scheme but that are so far excluded from the development plans.

Hong Kong defines brownfield sites as land designated for agriculture but used for industrial purposes such as storage, workshops and recycling.

Karman Cheung of the Liber Research Community urged the administration to set a short-term target for making use of the plots the groups had identified, which she said could be ready for development in between four and six years, and yield about 108,000 homes.

She said sites covering 100 hectares, including areas around Kam Tin, were vacant already, and could boost land supply even more quickly.

Cheung brushed aside concerns that the brownfield sites were isolated pieces of land that would be unsuitable for housing development, saying the plots they had identified were on the edge of new development areas.

"The 50-hectare brownfield located in the Lau Fau Shan district is one of the brownfield sites [at which] we think the government can quickly provide housing. That brownfield site is quite large, and it's also very easy to handle because it's next to the Hung Shui Kiu [New Development Area]," she said.

Chan Hall-sion, a senior campaigner at Greenpeace, urged the government to make good use of brownfield sites and rethink its proposals for a huge reclamation project off Lantau.

"The Lantau Tomorrow [project] can only be completed at least 12 or 13 years later. So in terms of timing, Lantau Tomorrow isn't the answer to the problem in front of us," she said.

"In terms of hectares of land Lantau Tomorrow can provide, it's being said there will be at least 200 hectares for housing. But alternatively... brownfields can provide more housing land because just in Northern Metropolis there are 433 hectares of 'instant brownfields'."

She called on the government to also come up with a comprehensive policy that covers brownfield sites in other districts in the Policy Address. Chief Executive John Lee will give his maiden policy blueprint on October 19.