The huge stock of standalone buildings in the city, which are three, four or five-storey tall, has yet to warm up to green building norms.

A green building's design reduces the AC use, and its sanitary fittings are designed to stop water wastage, experts said.

In the long term, the residents of such houses can save up to 40 per cent of their energy bills, a senior official of the Indian Green Building Council said in Calcutta on Friday.

The state government offers benefits, including a bigger floor area for those adopting green building norms, as incentives.

Architects, engineers and officials from the sector said that while large projects, 10-storey or taller, follow green building norms, the smaller ones coming up on plots that are around 5 cottahs are still not interested.

There are a large number of standalone buildings that are yet to be brought into the ambit of green buildings. Bringing them into the fold is vital, the state's chief environment officer, Dharmdeo Rai, told a conclave on green buildings organised by the Confederation of Indian Industry (CII).

The Green Building Council has guidelines for houses for middle-income and low-income groups, too, where the plot size and the flat size are relatively smaller.

"One can design the building in a way that air-conditioner usage is reduced. There are types of concrete that have porosity and allow air to come in. What kind of glass is used, where, and up to what quantity — all of these can be calculated and help to create a green building. The main idea is to conserve natural resources and reduce the use of energy," said Anand.

An architect who has designed buildings big and small said what attracted builders to adopting green building measures was the opportunity to create more floor area that can be sold.

"In small projects, it is often not possible to construct what is allowed under usual municipal building rules because the road in front is not adequately wide, or the necessary open area requirements will be infringed upon to do so. It makes no sense to go adopt green building norms in such projects," said the architect.

He said the cost of a project shoots up by nearly 10 per cent to adopt green building measures, and selling such flats at higher rates can prove to be difficult.

Across New Town and Calcutta, several smaller buildings are still under construction. The overwhelming majority of them do not go for green building ratings.

"The state government offers more floor area to projects that adhere to green building norms," said Chandrima Bhattacharya, the minister of state for environment.

Sushil Mohta, the chairman of IGBC Calcutta, said the right choices will ensure a liveable earth in the future.

"The choices we make today in urban infrastructure, in real estate development, in transport, and in energy use will determine the liveability for future generations," Mohta said.