Here's my take: Even though housing is nominally "durable," it does, in fact, represent a diminishing capacity. So, extending the usable life-span of "existing" housing through renovations is, looked at this way, an actual addition to housing stock.
Good point regarding renovation adding to the longevity of a house. Indeed, at what point does it become "new construction" disguised? There's a story of the curator demonstrating the ax used by George Washington to chop down the cherry tree. It's totally original, except restorationists have...
Most informative discussion so far.
I would add emphasis to materials of construction. As global commodity prices rise, builders will have to charge more for their product. The cost of new homes will go up directly, existing homes indirectly. This could be the largest factor underlying...