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Short of an appraisal, or a separate contract for the land (I have seen that done before) or a clause in the sales contract (I have seen this as well) you need to have some reasonable basis for allocating the sales price to the land. One possible way is to look at your current property tax bill. Often the tax assessor lists the value of the land separately form the value of the improvements. This is one method that I have been successful with on tax returns. There is no such presumption about land vs improvements.
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