Bren, there are ywo ways. The safest is to look at your property tax bill. It has your property broken down into land and improvement values. By dividing either one by the whol you can get the percentage each is valued by the county. This method is seldom challenged by the IRS. The second way is by getting a written appraisal from a certified appraiser and using the ratio he or she has used. This is usually very defensible if challenged by the IRS and may or may not result in a higher improvement value.