Without an appraisal at the time of purchase, you will need to estimate the value of the land/house when you bought it.
Assuming you purchased the property for use as a rental and immediately started using it for business (preparing it for rental and/or attempting to rent it), and assuming it was assessed in the condition you bought it (you made no major changes to increase/decrease home or land value prior to the assessment), you can use the assessor's home and land values to determine a ratio of the home to land values (e.g. 34,650/xx,xxx = yy% where xx,xxx is the sum total of home and land values) and then multiply that percentage by your original purchase price to determine the land value at time of purchase for entry in TT.
Assuming the sum of land + home value in your property tax statement is 50,000 it would work out as follows:
34,650 / 50,000 = 69.3%
Multiply the calculated percentage by the purchase price to determine land value at the time of the sale.
69.3% * 25,001 = 17,326 (land value)
25,001 - 17326 = 7,675 (home value) that must be depreciated
Alternately, you can use your 2010 appraisal in the same fashion (it should break down home/land value for you, which can be used to calculate a percentage).
Note that if you didn't immediately start using the property for business purposes (lived in it, etc.) before renting, you would want to use the value of the property on the day you started using it for rental purposes to establish the land/home values for depreciation/basis purposes in TT. If your 2010 appraisal is close to that date, use the land value specified in the appraisal.
Just remember it's all about the value of the home & land when you started using it for business purposes. Hope that helps.