what do i do if i failed to add the additional closing cost allowances to the cost basis in prior tax returns?
What do you mean "return**S**? You state that in the plural. Your closing costs can only be claimed on one tax return, and only in the tax year you actually paid them. If you use the program the way it is designed and intended to be used, it will ask you for specific closing costs and the program will handle them accordingly.
Basically, any costs associated with the acquisition of the mortgage are deductible. For example, origination fees. Whereas any cost associated with acquisition of the property are added to the cost basis of the property. For example, title transfer fees.
