Major environmental benefit to using existing commercial and residential buildings, as opposed to construction of new "green" buildings, is the subject of "Old Buildings Combine Sustainability, Preservation", by John McKinney in June 6 Miller McCune.
Interesting links in the article document the decades of energy savings and lower carbon emissions a new "green" building would need to offset the energy, and carbon emissions, involved in that building's construction - energy already used and emmissions already created in the construction of the existing building.
My office went up in 1914, and my home in 1926. While restoring them, I added a bit of insulation, increased attic air circulation, added some sun-reflective window screens, and planted shade trees, all inexpensive and not degrading of the historical integrity; my energy usage is not too much higher than it would be in a new building, and I didn't have to build anew. They are marvelous old structures, fun to be in, and I've preserved a bit of local history.
"Most everyone, though, remains resistant to reusing and retrofitting buildings. Architects like to start from scratch, developers don’t want the hassles of rehabbing existing buildings, and new construction is a mainstay of the U.S. economy.
“The most unenlightened in this regard are the traditional environmental advocates and the U.S. Green Building Council and their LEED certification,” Rypkema jabs. “If it isn’t about a waterless toilet, solar panels or saving the rain forest, those groups don’t think it’s about the environment.”