Plan to double the amount of on-site renewable energy to 20% criticised by both clients and green lobby

London mayor Ken Livingstone's tough energy rules, announced this week, have been greeted with scepticism by both clients and the green lobby.

In a revised version of the London Plan, Livingstone is proposing that every new development generate 20% of its energy needs from renewable sources. The previous rate of on site generation was 10%.

If all goes according to schedule, the requirement will go to public consultation in the autumn and be implemented early in 2008

Critics have argued that the hike in the renewable energy percentage is not achievable and therefore unlikely to be implemented.

Roger Madelin, chief executive of developer Argent, said current technology was not sophisticated enough to meet Livingstone's target. He said: "You can set whatever level you want but unless you create new technologies, that I currently don't know about, then it is a tall order. I'm not against targets but they have to be achievable. The Olympics are coming up, so we need to learn from someone who has got the answers and we need to learn together."

The British Property Federation also criticised the move, saying the mayor should focus on energy consumption in existing buildings.

Faraz Baber, director for regeneration and development at the BPF, said: "If no attention is paid to energy consumption by the occupiers then efforts to improve the energy efficiency of buildings will be redundant. An energy efficient building is useless if occupiers leave the lights on and keep the air-conditioning on full."

Argent's £2bn King's Cross development is one scheme that failed to meet 10% on-site renewable energy targets, as Madelin claimed it would take 523 wind turbines to generate enough power.

A spokesperson for the Mayor of London said: "Since the introduction of the London Plan, particularly in the past 18 months, we've seen applications with percentages of renewables that meet or exceed 20%. This proves that the technology does exist to meet the proposed 20% target.

"For King's Cross, Argent's own application shows that it is possible to meet the 10% target and are entering into a legal agreement to deliver this."

According to a report by the Green party in the London assembly, the mayor has approved many schemes that do not comply with the Greater London Authority's environmental policies (see box below).

Darren Johnson, a Green party London Assembly representative, said: "Doubling the requirement for on-site renewable energy is a welcome step and will be a real boost for small-scale technologies.

"But if there are too many get-out clauses developers will try to get away with not fulfilling the targets. It depends on how willing the mayor is to insist systematically on his own policies being implemented, instead of bartering and doing trade-offs."

Johnson added that the example of councils such as Kirklees in West Yorkshire proved that the 20% target was achievable.

"Councils such as Kirklees are looking at going beyond 20% so it shows it is achievable as long as there is the political will to apply it."