A concept described in StewartBrand's "HowBuildingsLearn" (chapter 2), based on the work of FrankDuffy?. Brand describes six "layers" common to buildings:
Site - where the building is located
Structure - the foundation and load bearing elements
Skin - user interface
Services - wiring, plumbing, heating, cooling, etc.
Space Plan - interior layout
Stuff - furnishings
He describes how each of these layers changes more rapidly than the last, and how buildings that don't allow that fail. Buildings are often torn down because their antiquated services are embedded too deeply in their structure, for instance.
Avoid coupling things that change at different rates. See also AlternateHardAndSoftLayers.