Very simple example. Say I own a single family home in mountain view. Tomorrow, the zoning changes such that somebody can build a 10 story building on my plot. Obviously the value of the land goes up tremendously, and the incentive exists to sell to a developer to rebuild it.
So zoning for higher density should only improve value of homes.
Or put another way, say there were a SFH with a yard on a plot in the middle of Manhattan. How much is that home likely worth?