How do you value partly finished residential space?

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Thanks for any suggestions. I am interested in a property that has an upper and lower level living area. It was built in the 1980s, but only the upper level was ever finished. The owners (original) have lived there all this time without ever finishing the house. 

The lower level is basically the basement level, but since house is on a steep slope, the living area walks out into a really nice back yard, with big windows and lots of sun on 2 sides -- so that part of it does not feel like a basement. The rooms are framed out with painted sheetrock, but the floor is still concrete. There is also a fireplace, which has not been used since the house was built. Structurally it looks very good -- it was just never completed.

The house has about 1800 sf finished space upstairs, and another 1800 sf on the lower level. Some of that is traditional basement area, so I'd say the partly-finished lower living level space described above is about 900 s.f. The other half would be considered regular unfinished basement. (It's a nice basement though -- walls are painted and well-maintained -- space is totally useable and could definitely be finished at some point.)

How would you value this house? And what size of house should I compare it to? Right now the tax assessment lists 1800 sf finished on the upper level, 828 sf "finished" on the lower level (though as I said it isn't actually completely finished bc of the floors), and the rest unfinished basement. Thanks for any insight.