A retroactive appraisal is an appraisals that offers a valuation as of a certain date in the past. If the date is a few years ago there is more data available and the appraisal is relatively straight foward. However if the date is 20 years ago, there is little data available, and the appraisal becomes much more complex. Many times retroactive appraisals are done due to a death, and the valuation is done at the time of death. However these can be ordered for any reason.
Attorneys, accountants, executors and others rely on L Ellis Appraisals Inc. for retroactive valuations because such appraisals require special expertise and training. They require a firm that's been in the area for some time and can effectively research comparable contemporaneous sales.
Real property isn't like publicly traded stock or other items which don't fluctuate in value very much or for which historical public data is available. You need a professional real estate appraiser, bound by the Uniform Standards of Professional Appraisal Practice (USPAP) for a high degree of confidentiality and professionalism, and you need the kind of quality report and work product taxing authorities and courts need and expect.