> One thing I don't understand is the "their real estate values will tank 80-90%" part. I live in a transforming rural area and one-by-one all the farmers have been selling their land to developers to make planned neighborhoods. They control the supervisors, zoning, and planning boards, and they have to be dragged kicking and screaming to allow these developments. However, their land values have gone up a ton in the last decade: basically a whole order of magnitude from on the order of 10's of thousands per acre to 100's of thousands per acre.
> I think the same applies to your single family home next to a 15 story condo building. Instead of selling to a another family, now they're selling to a big developer and can price it waaaaay higher, because the developer will be getting significantly more value out of the land than they were.
I think the way to think about property values is this: what is the highest value economic activity that can be accomplished on the property?
In your first example, it's pretty clear that building lots of houses (in the presence of sufficient demand) is a much higher value economic activity than farming.
The home to condo example is a bit more complicated. For the very first person to sell to a developer, I think your point is sound.
However, each additional bedroom that's created in the city decreases the value of all other bedrooms by a small amount. This not only directly decreases the value of a single family home, but it also decreases the value of a condo building, which means that developers will be willing to play less money for land.
Eventually, builders will stop building - the cost of construction will be more than they can sell new condos for. At that point, I believe the remaining home owners will have suffered a dramatic decrease in the value of their property.
Where does the equilibrium land? I don't think anyone knows for sure. But new construction goes up in some pretty low value real estate markets. 80-90% is just a guess on my part, but I don't think it's too far off based on current prices.