The separatizer has the burden of proof to show that the asset is their Separate Property. Therefore, you need to provide proof (ie, the deed) that the property is your Separate Property.
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I'm sorry to read that you are going through that process. I know if it very stressful and more than a little bewildering.
If you owned a property prior to marriage, your ex's attorney is doing due diligence to ensure that the property title wasn't changed to add the ex at some point during the marriage. It isn't likely that the title was changed, but the ex's attorney has to make sure.
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It is a very normal request to have documentary verification of an oral assertion. Even if the property was mentioned in the prenuptual agreement, there is nothing particularly burdensome about the request.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.