The WWF is widening its eco-league table of housebuilders to embrace the top non-listed developers.

The conservation charity has announced that it is including the 20 biggest private housebuilding companies in its sustainability benchmark of the sector, which will cover 70% of UK housing output.

The tables will enable the environmental performance of non-listed companies such as Miller and Cala Homes to be compared with quoted giants such as Crest Nicholson and Berkeley.

As well as a six monthly report based on companies’ published returns, the WWF and Insight Investments, the HBOS’ ethical investment division, are proposing to conduct a separate audit based on information which is not in the public domain.

As well as regular audits of companies’ overall performance, there will be more specialised reports on aspects of environmental performance, such as climate change and water. Individual projects will also be vetted.

To cover the costs of information gathering, the WWF and Insight are proposing to charge companies £5000 to belong to its benchmarking club. Companies that join will have the opportunity to determine how the eco-vetting is carried out.

The Housing Corporation and English Partnerships have used the benchmarking reports to supplement their own ratings when deciding which companies to award development cash to. A number of non-listed companies were already in negotiatons about joining the scheme.

Paul Pedley, the chairman of listed company Redrow, welcomed the inclusion of the non-listed companies in the benchmarking scheme. He said: “The quoted housebuilders deliver something like 50% of housing output, which leaves 50% that isn’t measured. Extending the benchmark to the next 20 will make sure that everybody’s playing on a level playing field.”

He added that non-listed companies should not be scared about being benchmarked because it provided firms with a good way of displaying their environmental credentials to planners and public bodies, such as EP.

In the most recent league table, which was published last September, Crest Nicholson came top and Westbury bottom of the 12 biggest companies.