You could make a better case - but as FNMA defines it and ANSI confirms it, the earth contact portion is not the same as the above ground. I don't care what the room count is or materials, but in the OP's case, it clearly is a concrete wall below and frame above. I've analyzed several such properties (they are not common here) and when you find a SFR very similar to the upper level, adjust out the size and the land value you are left a chunk for the basement and that typically is much less per SF except on the lakeside properties on steep slopes and it tends to be the "same" more or less since land is such a premium on the lake. And I adjust accordingly.