so could have quite different implications for building energy use. Systems that have equal load-bearing capacities might have very different weights, and the heavier system might require substantially larger footings and foundation. The extra materials would increase the embodied energy of the structure, and thus total environmental impacts (Trusty & Meil, 1999). As Wayne Trusty points out (Malin, 2002, p.15), The real functional unit is a piece of space to fill a certain need. Thats the level which we should ultimately compare. One important challenge for green building assessment is the need to move beyond the commercial and multi-residential sectors, and especially beyond new construction. The focus of most building assessment systems has been new commercial and high-rise construction. But most of the building stock that will exist in 30-50 years exists right now, and the greening of the built environment depends very much on upgrading the existing buildings. A few systems, like the Green Globes adaptation of BREEAM, backed by Canada Mortgage and Housing (CMHC) here in Canada, have been primarily oriented to existing buildings and building operation. But most systems are now just beginning to adapt their systems for single-family housing and for older buildings. LEED-EB for existing buildings was launched in 2004 after a 2 year pilot phase, with about 70 buildings representing a wide variety of building types. LEEDHomes is a residential programme also in development. Working with existing buildings presents different kinds of challenges, since they cannot benefit from the kind of integrated design that save money from the outsetfor example by using the building structure or envelope as part of the buildings heating or cooling system.