I would say if you had an appraisal done at the time the property was built and assuming it represented cost not the replacement value for insurance purposes that's a starting point but then there is the cost of any improvements made since. I can't say about valuations used in tax bills. Most of the time they're supposed to be based on fair value which would be higher than your cost. another issue is of the total cost you used for depreciation did you pull out the value of the land since this is not depreciable. where did you get the basis you used for depreciation. you do have the option of getting an appraisal, not bulletproof from IRS attack, of what the construction cost would have been and even the costs of improvements.
