Adding Above Grade Living Square Footage is always a plus, barring the rare exception where you might ruin your own view which I have seen.oh yea additions, my last house had a (i'm guessing) deck converted into an enclosed sunroom, and the bottom was just gravel and tarp (basically the deck) it was always freezing in the winter and when i went underneath it was just bats loosely sealed with plywood, that coulda used a partial crawlspace to regulate temps more.
but the sunroom looked really nice and i'd consider it a good addition for value and usable sq/ft
would a crawl space have been better? as far as i know, it's the only house on the block w/ an addition (you can see the house further down has the same sized deck)
Regarding, the addition in your photo....
In my opinion an actual concrete footing that created an actual crawl space would have been better for livability/climate control, however, without more data I cannot speak to the cost effectiveness of it. What I mean by that is what if that concrete footing install cost is $10,000, but would only make a difference of $400 annually in heating/cooling costs. That is a 25 year break even, not exactly cost effective. I mean maybe you could recoup that in a resale, but the typical market participant isn't going to think of that angle. And the vast majority of so called Real Estate experts (Realtors) will never even mention it, they are sales people they will just highlight the positives of the added sq.ft..
EDIT: Even if the footing only costs $5,000 that would be a 12.5 year breakeven, not super cost effective either. Made up numbers are just that, I am just using them to illustrate my point.