I would think that generally, if none of Prop's other situations are true

, then it should be positive. It could signify developers moving into an area. The new townhouses will probably sell about the same price as a full block. Theoretically, as this happens it makes the remaining development blocks worth more as the price of the land increases.
The thing is that as this continues, this land is probably more valuable to developers than OOs, as a streetscape full of units may not be as attractive a propostion as a street full of houses to live in. Which is why i have just bought a PPOR in an area under a single-dwelling-covenant. So i guess the answer is "up to a point." It will depend partly on the area.