You can look at the real estate tax assessment as some will provide an allocation for the structure and the land value and use that percentage (not the actual values, however).
For example, if the assessment allocated $100,000 to the land and $200,000 to the structure, then you could use that percentage (e.g., if the appraisal came in at $600,000 combined you could use $200,000 for the land and $400,000 for the structure).
It would, of course, be optimum if you could get the appraiser to provide a break down for the land and structure value.
