The assessor would have included it in the sq footage if it was properly permitted when it was constructed. My guess is that it was not permitted and so it shouldn’t be used in the calculation of the square footage.
You can’t do much today about 1957 construction with a possible (“grandfathered”) circa 1965 screen porch. I seriously doubt anyone down at the city would bother you about it.
In any case, spdrun is right. If you think the price is fair for the whole package, then make an offer. Otherwise, let it go.
You can’t get all wrapped up in “price per sf” in older, established areas, even if they were built on tract. After 50+ years, the majority of them have been remodeled and/or vastly improved so it would be difficult to next-to-impossible to find true “model matches.” For instance, you can’t really put a price on a curved-glass window or an original American Standard pastel bathroom.
