It's kind of a silly thing that some systems have to reference a home that is something between a 1 story and a 2 story home.
Generally, it's referring to a home that has a partial upper floor, but not a full 2nd story footprint to match the 1st floor footprint. An example would be a home with an interior staircase to an attic that has been converted to living space with 1 or more bedrooms. The ceiling in the bedrooms may follow the downward slope of the roofline. This would be a 1.5 story home. Any extension beyond 1.5 up to 1.9 might be an effort to explain dormers or other modifications that added a bit more usable space. The unusable areas are either behind knee walls around the perimeter of the living space or in areas where the sloped ceiling is so low you can't walk easily. Generally, building codes require the average ceiling height in the room meet a certain height - often 7' - but this can vary by community.
Hope that makes sense and helps.
Where did you see the term "1.5 to 1.9 story house?" A 1.5 story house is often used to refer to a split level house--a partial second floor with rooms. The 1.9 term may refer to some sort of attic space. Where was this house located? Was it listed for sale on a website? If you have a little more information, I may be able to define this term for you.
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