The Passivhaus approach can be implemented when retrofitting existing buildings as well as when building new ones. Heating energy use in an existing building can be reduced to 10% of what it was prior to renovation. The "chainsaw retrofit"--so called because this approach involves removing the overhangs of the existing house with a chainsaw!--is one way to add the additional insulation. Matt Wasse in our office has created an animation to illustrate how this approach works. Have a look.