We don't have a lot of what the re
Agree with Glenn and by inference, JGrant.
In my experience, the vast majority of the scenarios that I see in the markets where I practice, what would be considered "below grade/basement area" in other parts of the country are simply not considered to be that in our markets. And, the GSEs don't have a problem with that. That isn't to say we don't have "basements"; we do, but true basements are very rare. What we do have are a lot of partially below-grade areas (especially on hillside slopes); they are marketed, valued, and identified as GLA all day long, every day of the week.
Having said that, every now and then we get a situation where the area in question is significantly below grade and it is best identified as a "basement" area. I get them from time to time (last one was about 2 years ago) and I find them to be a PIA because they are not that common and if I am going to value it on a GSE form to GSE expectations, then I'm going to segregate it out and do it that way.
Based on the photo, the area you identify would be a "basement" in my identification-scheme. As Glenn pointed out, the value may be the same as living area. Your challenge (IMO) is going to find similar homes with similar configurations that can be matched to the subject. If you cannot find them, then the next step would be to find dissimilar properties with a similar basement configuration and (a) show that these properties are marketable (not that difficult) and (b) use them as data to analyze the contributory value of the basement space (does that space trade at the same price as GLA or not?). I'm loath to put a non-comparable into the sales grid to demonstrate that one element (the basement) is acceptable in the market and to use as a basis to offset an adjustment, but that's me. I'd much prefer to do that analysis off-grid (dissimilar sales to support an adjustment conclusion) and then reference that analysis as the basis for my adjustment in the grid.
I always smile to myself when I read threads in other parts of the country/markets where they are arguing about basements because I considered myself lucky that I don't run into that situation. Until I do. Then the smile wipes clean pretty quickly.
Good luck!