The main problem is much greater than that builders are discouraged from improving because of a tax on buildings. The main reason we see slums, vacant building

The main problem is much greater than that builders are discouraged from improving because of a tax on buildings. The main reason we see slums, vacant buildings, fenced lots, parking lots, and single story buildings from 1900 next to new skyscrapers is that not taxing the annual rental value actually encourages hoarding land, since the value of land increases faster than anything else and the less an owner invests in improvements, the higher rate of return from "capital" (land) appreciation will be.