Even FHA handbook talks about what is typical for neighborhood adds value...I would not call zoning and report an address. That is way outside your scope as an appraiser.
I always ask the owner, did you pull a permit? If they say no, or they bought the house that way ( with illegal addition) I simply disclose it in my report. I will give the addition value, if the market seems to indicate it has value ( most of the times it does) then I will state though the addition was done in a workmanlike manner, appraiser cannot verify if it was permitted, and is making the extraodinary assumption that the addition was not permited. Let the lender decide whether or not they want to lend on it. That is not our responsibility.Our repsonsiblity is to come up with a value for the subject. If the market /paired sales shows value for additions/garage conversions etc, most of which were not permitted, then they have value. But we also need to disclose, so lender can't come back later re if a permit was pulled. Many counties simply increase the owner's property taxes once they discover additions to a home.
A statement like this can be made " family room does not appear to be permitted/appraisers cannot verify if a permit was pulled. Appraiser is giving the family room value on the report, as these types of additions on older homes are typical for the area, and paired sales show buyers are paying more for homes with extension/additonal living area added to home."
Again it is what is typical for the area. If your neighborhood is older homes, not high income demographic with lots of cars in driveways and you observe many additions/garage conversions, and some of the conversions look home made/not to code, you might make the assumption they were not permited) sometimes you can tell from property tax records if the taxes are too low for the amount of sqaure feet) But if you are in an area of newer or more expensive homes where it would be typical to get a permit pulled and your subject does not, that can influence your decision whether to add value for it .