I am surprised that very few people are even considering the "legally permissible" issue. I thought that was taught in Appraisal 101, or maybe I am not remembering my original training too well, from it being a long time ago.
I am not saying that it has no value or you can't give it value but, IMO, you need to address the permit/legal issue in any case. Just because it is common for an area to have illegal additions and/or conversons, etc., doesn't make them all legal. If your subject happens to be the lucky one where code inforcement decides finds an illegal conversion, etc., that may lead to additional costs, to either get it permitted or destruct portions that are unpermitted, thereby, reducing it's value in the market. And that may not have anything to do with the quality of construction.
I have seen beautiful additions and conversions of very high quality, which were unpermitted, which had to have been converted back to original use, or actually torn apart or down, since they did not conform to code standards. And these additions and conversions were in areas where they were common. Even if you are a few inches off a boundary setback, you may be required to tear up an improvement to rectify the situation. I am not a code enforcement expert and would not want to be put up to those standards, so I would disclose, disclose, disclose, or in appraisalspeak, explain, explain, explain.
The market does not care if it was legally done, the tax assessor happily ignores legality in favor of revenue, and the local building authorities never even blink.
Typically, very few of us verify permits and most of us assume the main living area is legally permitted, if our actual measured square footage is reasonably close to the Assessors'. But, if this is square footage NOT reflected in public records, you should explain why you decided to count it as living area,etc.
And per your Assessor comment, you are right. I was an Appraiser at the County Assessor's office and we would just measure any new square footage we saw when we were physically at a property. We were not affiliated wth the building dept and typically did not check permits, unless it was brand new improvement, from ground up and we needed to see the "final" and "C of O" on it, but we still may not've remeasured the changes or additions they did prior to completion. In other words,the public records square footage does not necessarily reflect legal, permitted squrae footage. I had one house in high end neighborhood, that was on record as being a 1,200sf house, but was added onto by the contractor/owner, and was over 4,000sf when I measured it. That didn't change the fact it was illegally added onto, although, we did change the assessor sf to reflect what I measured. Those are 2 separate issues, that many appraisers blur into 1 (i.e. legal sf vs Assessor's sf, same with use codes-by the way, just because it is used as a duplex on public record, doesn't mean it is legally zoned for it-at least in my county).
Once again, IMO, just because that type of improvement may be common in the market, that is no excuse for not addressing the legal/permit issue.
We, as fee appraisers, & in particular as either willing or unwilling fannie form slaves, seem to be the only ones with a different opinion about this type of space. I find that interesting as we are supposed to measure and report market acceptance/reaction and the market says no issue.
(this of course excludes the really badly done stuff)
The maket also doesn't care about USPAP or appraisal theory, etc., but we still need to incorporate them within our work. It is only the FNMA form slaves, as you call them, that may not address these issues properly, since they are not addressed in the form.
We are not supposed to measure and report maket reaction in a vacuum. The market doesn't dictate HOW we do our work. Just because he market values something that goes against our training, doesn't mean we don't address that fact.