According to Bloomberg news (see the article here) builders across the country are buying older, smaller single-family residences on large lots, demolishing them, and building a newer larger house on the property.
Closer to home I am hearing anecdotal reports of this practice beginning to happen in some of Portland’s closer in neighborhoods. This trend appears to be driven by the desire to be as close as possible to the urban core yet still live in a nice single-family residence. I see this practice becoming more common as traffic congestion increases in the Portland area and commute times to the suburbs continue to grow.