I had all my clients retain a real estate agent (someone reputable and not family related) and provide a formal appraisal, nothing like "well, I think it is worth so and so". It was all provided on official letterhead, FWIW. Sometimes nothing is good enough but this was better than nothing.
When clients provided their own estimates, I did not accept it. They grumbled about the fees for an appraisal but at the end of the day, they understood that upon an audit, a formal appraisal was much more acceptable so they did it.
The moral of the story, do not have RE in the plan (together with life insurance 😀)