I don't know where you are located but most municipalities here require no less than 48" high knee walls in attic conversions in addition to other requirements based on calculations using height and area. You can utilize the space with deep drawer roll outs.

any lower than 48" and the side walls wouldn't be usable at all.

Hi guys! Current code (the IRC, as accepted by all 50 states) calls out 5' as the lowest wall allowed at a rood to wall juncture. 7' minimum average ceiling height, ergo, if you have 5' at the perimeter you need to have 9' in the center.

Ironwood, I beg to differ re 5' height. The 5' dimension is for calculating habitable floor area in a sloped ceiling room. It does not determine the height of knee walls.

IRC Section 304-all habitable rooms must have a floor area no less than 70 sq. ft. with no horizontal dimension less than 7'.
Section 305-habitable floor area is calculated from finished floor up to a finished ceiling height of 7' or higher and out to where sloped ceiling is 5' off floor.

Scott Design...I stand corrected...5' is the lowest allowable ceiling to measure the required floor area. But 305 does say that 7' is the minimum height for 50% of a sloped ceiling room...ergo average height of the room if the lowest point of the "room" is 5'.