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Consultants have refused to use the government-backed software for calculating compliance with Part L on the grounds that it contradicts the written element of the new regulations.

Architects and building service engineers have rejected the Simplified ºÚ¶´ÉçÇø Energy Model software and have resorted to a long-hand methodology to ensure reliable results - even though this could cost clients double what they would pay for an SBEM calculation.

Professionals are relying on existing programmes to calculate the carbon emissions of every element of the building separately and then adding them together to produce a final result.

Trevor Butler, head of sustainability at architect ºÚ¶´ÉçÇø Design Partnership, said the software, which was written by BRE for the ODPM, was difficult to use and produced results that contradicted Part L documentation.

He said: "We are using the 2002 version of Part L and making it relevant by calculating the extra 28% ourselves. We are waiting for new software from other companies, such as IES or TAS, to come through, to incorporate this long-hand method and we will proceed with them."

He continued: "The results are not consistent with what the government are telling us to do. For example, you get lower carbon emissions with the SBEM software when you use single glazing in a building, but Part L documentation says you can't use single glazing."

Bulen Hourshid, a partner at Davis Langdon Mott Green Wall, said failings in the SBEM software were costing clients money.

He said: "One of our clients spent an extra £800,000 switching from a fan coil system to a chill beam system because the SBEM software said the building would not comply with the new Part L using a fan coil. We then inputted the exact same data using the long-hand system and the building passed the new regulations."

BRE defended the program this week. In response to criticisms that SBEM does not allow users to change the values for natural daylight - giving abnormal results - the firm said these values were factored in and were adjusted every month by the program.

BRE said it would continue to make improvements to the software.

The ODPM is considering legal action against The Stationary Office, a document management service. The office allegedly printed error-ridden Part L documents after it downloaded them from the ODPM website.