Assessor's land value too high?

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The county assessor's apportionment of land v. improvements seems a bit high to me. On the 4 houses I purchased this summer, the land values are 42%, 42%, 33%, and 22% of the total assessed value. For depreciation purposes, I'd obviously prefer to shift value from land to the improvements. The total assessed value for each of these houses ranges from 24K to 40K, so I'm not going to go broke playing it safe, but hate to give up thousands in depreciation if there is a defensible way around it. Any recommendations?