I'm assuming HLA was a typo for GLA. I would probably not call it GLA, but it might be a close call. I might call it GLA if it had ducted heat and air; but, you indicated a window unit. The fact that they use it year round is not all that significant, but if it has heat ducts that could sway me the other way.
I believe that this is an ANSI grey area. I've seen a lot of houses that have varying quality from one area to another. Would you knock out a bedroom for having paneling when the rest of the house has drywall? Still, I'd probably go with the Duck on this one.
Nevertheless:
Whatever you call it, it's worth what it's worth. Just because you call it living area doesn't make it worth a dime more than if you call it an enclosed porch.
So, if you want to make people happy, you might consider calling it GLA but adjusting based on an overall rate that is lower than if you did not include it.
But, if you want to be sure not to run afowl of ANSI: The preferred way to handle it would be to not include it as GLA and then compare it to other homes that have enclosed porches, also not counted as GLA, and adjusting based on their size and quality. Probably, if you could show the owner that reasoning, it would smooth some ruffled feathers.
This is one of those cases, where I don't really think it matters what you do, as long as what you do is consistent with the local market and is well-explained. In other words, if the other similar enclosed porches you can find and verify were counted as GLA by the MLS, assessor, or whatever source you use, then I would count it. If not, then I would not.