If the entire area is equally high density, then congratulations, it sounds like you live in a very desirable location in the middle of a lot of public and private infrastructure: if that's the case, you should be taxed, to encourage even higher density development and even more productive use of the land you're on.
How do we take the average density? Is it the average height weighted by land area, or floor area?
"average height of building on a piece of land" vs "average height of building a piece of floor is in"
Well, unless all buildings are the same height, the second average will always be higher, because taller buildings have more floor!
Most people would go with the former for the average. And it does make sense in general to average over the fixed thing. But that mean building will pay a greater portion of the total tax revenue than before.