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House builders are unwittingly designing homes which are too cold, according to a leading sustainable building organisation.

The standard assessment procedure (SAP) - which underlies the code for sustainable homes 鈥� overestimates the amount of heat in the average home, because it is based on calculations from a typical 80's house where incandescent light bulbs and other appliances were more prevalent.

The claims come from a report by the Association of Environment Conscious Builders (AECB) which compares the far more specific PassivHaus standard with SAP.

According the report, the UK measure also: 鈥淯nderestimates the amount of heat that is lost through the fabric of the building. It does not assume a realistic room temperature. The net result is a colder home, a higher heating bill than the designer expected, or both.鈥� SAP also drives developers into the arms of expensive micro-renewables at the expense of well-made buildings, alleges the AECB.

The group said it had showed an 鈥榠nterested鈥� BRE, DCLG and DEFRA the report but that they doubted whether revisions to Part L for 2010 would cut the mustard.

When asked if anomalies would be corrected in two year鈥檚 time, AECB spokesperson, Liz Reason replied: 鈥淗a ha.鈥�

鈥淭here will be revisions made by 2010,鈥� she continued. 鈥淏ut the question is 鈥榳ill they be enough?鈥�

Michael Sturdy of Hodkinson, an energy consultancy said the utility of SAP was being stretched: 鈥淪AP is now being used as a design tool, which is not its intended purpose.鈥�

However, he said: 鈥淭he issue of 鈥榝avouring鈥� renewables and 鈥榰nder selling鈥� fabric improvements is a real issue and an argument that is also akin to the current Part L requirements; promotes energy inefficient buildings.

鈥淚t is fair to say that these issues and problems with SAP are not new issues, and those of us that use the SAP tool in conjunction with trying to 鈥榮olve鈥� the Code soon realise that something is amiss; practical, and possibly the most sustainable solutions do not seen to be available.鈥�