More likely the cost of building the structure needs to be adjusted for depreciation and/or cost of bringing it up to code. If a building costs more to fix up than starting from scratch, or effectively just needs to be demolished, clearly it's the building's value that's negative.
Although, there's no reason unimproved land value can't be negative, the land just needs to be burdensome.
The issue is that we're sorta back on the same issue as before where we need to estimate the value of the unimproved land except now it's (in my opinion) even more complicated. In this case, the structure doesn't need adjustment due to anything physically wrong it's just that no one is willing pay the theoretical value for it in that location. Think of an expensive anime wrap on a car, the wrap cost 2k or whatever but I don't care I really only want the car.
Good point on the value of unimproved land being negative, I was thinking of a different edge case. In the situation of "burdensome" land, maybe it does make sense for there to be negative value land with an associated tax credit if the owner is compelled to do something to remediate?
Although, there's no reason unimproved land value can't be negative, the land just needs to be burdensome.