[neutronicus]

Not sure whether the original contention is untrue in the Northeast or just much more rapidly-varying in space.

Where I live (Baltimore) your second sentence is certainly true. There are boundaries across which the vacancy rate is basically discontinuous. In connected regions surrounded by such boundaries I think the original contention pretty much holds, though. There are a couple variables (nbeds, nbaths, sqft, parking space) that pretty much determine the value of the home and going HAM on any improvements they don't capture is probably negative ROI.