There are no requirements for new homes funded by Government to be built to code level 5, only level 3. Currently, the only costs analysis we have completed of building to the code are those in “Cost Analysis of the Code for Sustainable Homes” published in July 2008, written by Cyrill Sweett. The report is available to download at
http://www.communities.gov.uk/publications/planningandbuilding/codecostanalysis.
We are currently in the process of updating the cost analysis for the consultation on the code, in light of changes to Building Regulations in 2010.