Housebuilders are rushing to exploit guidance that will allow them to save millions by building homes to the present energy standards after the uprating of Part L next month

If housebuilders register their intention to start work on a site before Part L 2010 takes effect on 1 October, they gain an extra year before they have to start work under the old rules.

And once work starts, the entire scheme is allowed to comply with Part L 2006, regardless of how long it takes to complete.

Local authorities report that thousands of plots, including many without planning permission, have been registered in the run up to the deadline.

The move has been criticised by green bodies. Andrew Warren, director of the Association for the Conservation of Energy, said the change effectively meant Part L was being delayed by a year. He said: “This does undermine work on improving the energy standard of new homes in England. I do wonder if Andrew Stunell [the Lib Dem minister in charge of Building Regulations] knows what is going on.”

According to Davis Langdon, a typical mid-terrace home built to Part L 2006 will cost £2,000 less to build than one constructed to 2010 standards.

The haste to register exploits the “transitional arrangements” that straddle a change in regulations. These were less generous when Part L 2006 was introduced; then housebuilders had to comply with the new version if work had not started on site before it took effect.

The communities department this week denied transitional arrangements had changed. However, Steve Evans, building control manager at the NHBC, said it had used an interpretation that allows builders to delay starting work for a year, and this was approved by communities department officials.

Local Authority Building Control, which advises council inspectors, had stuck to the 2006 interpretation, but changed its mind after thousands of plots were registered with the NHBC.