How to establish values for casualty and theft deduction?

We had two storms that caused damage to our property in summer 2024. They were separate incidents. One led to a bad basement flood and the other to another basement flood and part of our fence and retaining wall washing out. What a mess. Both of these ended up being Major Disaster Declarations from FEMA.

I'm trying to use the workbook from publication 584 to find numbers to help claim the casualty and thefts deduction. From what I can understand I have to use the smaller of a) cost or other basis or b) fair market before minus fair market after. But, what I'm wondering is how I establish "cost or other basis" or "fair market" on a basement or a portion of a retaining wall and yard, when these two things have been at our house since we bought it 14 years ago. Is there a calculator or formula out there that I can use? No insurance claim. Hope this all makes sense. Thanks in advance.

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