There are other options, sure. The tax assessors could simply guess, or outright dictate whatever "unimproved value" they choose (i.e. the highest tax they think they can get away with). If the the point is to establish the "unimproved" value of the land through an auction, however, you can't do that when the land and its improvements are inseparable. These are not two distinct economic goods with independent values. The improved land is one good, with one combined value.