Casualty losses are deductible in the year you sustain the loss, which is generally in the year the casualty occurred. You have not sustained a loss if you have a reasonable prospect of recovery through a claim for reimbursement.
If you have a casualty loss from a federally declared disaster that occurred in an area warranting public or individual assistance (or both), you can choose to treat the casualty loss as having occurred in the year immediately preceding the tax year in which you sustained the disaster loss, and you can deduct the loss on your return or amended return for that preceding tax year. See Revenue Procedure 2016-53 for guidance on the time and manner of making and revoking an election under Code Section 165(i). Review Disaster Assistance and Emergency Relief for Individuals and Businesses for information regarding timeframes and additional information to your specific qualifying event.
You would need to amend your 2016 to add the fire. Remember that the deduction is only available when you itemize, so make sure it will help you to do it. You can amend in TurboTax 2016 and see if you would get anything back or enough to make it worth doing.
Here is how to amend:
https://turbotax.intuit.com/tax-tips/amend-return/how-to-file-an-amended-return-with-the-irs/L6kO691...
