at the point of diminishing marginal returns

This picture of a restored steel kitchen is featured on the Retro Renovations website. It is in one of Frank Sinatra's houses.

There has been some discussion on the economics blogs I read about the relative decline in productivity from the 1970's onwards. An illustrative case is the progress of kitchen development in American houses. Some very good arguments have been made that the major improvements to kitchens were realized by the late 1940's. Progress since then has been mostly cosmetic.

This argument could apply to residential architecture as a whole. Modular kitchens, central heating and electricity were established as consumer "must-haves" by the 1930's and the building boom of the 50's through the 70's was a period of implementation rather than innovation. Another development was the modularization of construction methods and materials--4x8 panel products, power tools, dimensional lumber, and standardized millwork all came to fruition in this period.

Gains from the last several decades are more ambiguous. Enclosure quality improved, with regard to insulation, windows and weatherproofing, and there has been a steady advancement in mechanical systems. But, efficiency gains have been largely offset by an increase in overall dwelling size. I expect to see incremental improvements in enclosure assemblies, which will have the effect of reducing the size of mechanical systems. In general, we seem to be at some sort of asymptote.