I am certainly no authority on this subject, however . . .! I live in a house in the Bucktown area of Chicago. It was built in 1882 as far as I can discover. Our house is considered a "below grade house." The city added new sewer pipes in the early 1900's -- (I was told but haven't verified the dates) -- and to accommodate the new sewers, raised the level of the street. All the houses that were built on those streets prior to the raising of the street level became "below grade." Many home owners raised their houses to bring their front doors to street level but their yards remained "below grade." We raised our house in 1997! Lifted the house up, dug a basement, built a whole new foundation and walls to set the house back down on. Quite an experience -- as we lived in the house while all this was being done. Our tiny front yard and our back yard remain below the street level. The mystery to me is why 5% of the living area of the house you mention would be considered above grade. Sometimes people raise part of the grade of the lot for a porch or a garage. Come on over to Bucktown and take a look. There are several houses in a row on Wabansia between Honore and Wolcott that are below grade. There are a lot of others, too in Wicker Park as well as Bucktown. The other very interesting bit of Chicago history that is related to this is -- after the street raisings, toilets were installed UNDER the sidewalks to connect to the new sewer pipes. There were doors in the cement retaining walls (between the lower front yards and the sidewalks and streets) that opened into the toilet rooms. Sometimes a couple of houses or more would share a toilet. I was told that it was a colloquial expression in several languages to say that someone was "under the sidewalk" to mean, of course, that they were using the toilet! Hope that this is of help to you. I'm sure that the Chicago History Museum's Architecture Department could give you information as well.