Designers are able to manipulate software so unsustainable all-glass buildings can hit carbon emissions targets.

Architects and environmental consultants are complaining about a government-commissioned computer program that calculates carbon emissions, claiming it is unreliable, unworkable and allows architects to design unsustainable all-glass buildings.

The software, designed for the ODPM by BRE, accompanies Part L of the Building Regulations. Its critics allege that designers are able to manipulate the software to allow them to hit carbon emission targets with buildings that Part L was supposed to discourage.

Computer software for modelling non-domestic buildings shows that all-glass designs will be accepted as compliant as long as their inherent energy inefficiency is offset using a low energy heating or cooling system.

Patrick Bellew, director at environmental engineers Atelier Ten, said architects had understood the new legislation would stamp out all-glass buildings. He said: "It's all very well offsetting the carbon emissions of a glass building with green heating and cooling systems in a computer programme but in reality we know it does not work like that. The fact that we can still design these buildings shows the legislation is not tough enough. There must be something wrong with the configuration of the software."

Bellew said the software, known as Simplified Building Energy Model, was based on assumptions that were not comparable to real life. For example, it only allows for one set of values for the amount of natural light a building receives, regardless of season. He said that no detail was given on the programme in order to adjust this.

He added: "The ODPM model is a black hole. We don't know what's going on on the inside of it so all we can do are the most basic calculations. The question is, when the ODPM releases an update of SBEM in a couple of weeks' time, will all-glass buildings still be possible? The government should have got its software up to scratch before releasing Part L."

Users are expected to have full training and sit an exam before they are authorised to use the software. BRE is still adjusting the software and no dates have yet been set for exams.

Part L is due to come into force next Thursday.