I’ve taken the liberty of expanding Duffy’s “four S’s”—which are oriented toward interior work in commercial buildings—into a slightly revised, general-purpose “six S’s”: • SITE - This is the geographical setting, the urban location, and the legally defined lot, whose boundaries and context outlast generations of ephemeral buildings. “Site is eternal,” Duffy agrees. • STRUCTURE - The foundation and load-bearing elements are perilous and expensive to change, so people don’t. These are the building. Structural life ranges from 30 to 300 years (but few buildings make it past 60, for other reasons). • SKIN - Exterior surfaces now change every 20 years or so, to keep up with fashion or technology, or for wholesale repair. Recent focus on energy costs has led to re-engineered Skins that are air-tight and better-insulated. • SERVICES - These are the working guts of a building: communications wiring, electrical wiring, plumbing, sprinkler system, HVAC (heating, ventilating, and air conditioning), and moving parts like elevators and escalators. They wear out or obsolesce every 7 to 15 years. Many buildings are demolished early if their outdated systems are too deeply embedded to replace easily. • SPACE PLAN - The interior layout—where walls, ceilings, floors, and doors go. Turbulent commercial space can change every 3 years or so; exceptionally quiet homes might wait 30 years. • STUFF - Chairs, desks, phones, pictures; kitchen appliances, lamps, hair brushes; all the things that twitch around daily to monthly. Furniture is called mobilia in Italian for good reason.Brand, Stewart. How Buildings Learn (pp. 24-25). Penguin Publishing Group. Kindle Edition.