I find it encouraging that the mayor of London is being so ambitious when it comes to energy targets (2 June, page 12). However, when you start comparing what is desirable to what is realistic, you realise that the thinking is a little less than fully joined-up.

This proposal would make more sense if the desired 20% actually related to something real, say the calculated energy use per square metre for a typical building, which commercial real estate advisers would consider saleable to an investor.

The truth is it becomes increasingly difficult to save 20% on an already highly energy-efficient building, such as the ones we are designing in our practice, while staying within the constrictions that the market imposes.

A much better approach for the mayor would be to move investors, and their advisers to embrace a specification that is not derived from the wasteful 20th century, thus encouraging developers to develop with energy efficiency as a desire rather than an imposition.

David Walker, partner, Walker and Martin