It appears it was installed without the 18 inch minimum straight run of piping.

@Jonathan Feliciano this is a commonly used strategy in a lot of places, and it can definitely produce good returns and enhance the usability of a house when done correctly. ...though, there are also plenty of examples of it turning into a poorly-planned hack job that ruins the usability of a house...A few things to consider:-Does the garage connect to any wet walls, and is the sewer main easily accessed?

Size is 5 ft X 6ft 8 inches.

Concrete is the easy part.  2 x 2 spacing with 1/2 inch rod; 6 inches thick walls/sides; vapor barrier as needed.   

This adds I believe 1.5 inches from ceiling.Primary issue: Footfalls (just walking around would be hard for someone to focus downstairs)- 5/8" gyp rock affixed to the subfloor for footfall dampening w/green glue- Rockwool for high noise/voice attenuation- Sound channel clips for air separation- And to go further: double sheetrock with green glue.- Custom boxes around each of the light cans ($175 each) OR switching to wall sconcesThe primary cost is the sheetrocking labor itself, as It's not just the ceiling replacement, it's aligning the new ceiling texture with the walls.Given there is a stairwell from main floor downstairs, he recommended two solidcore doors, which parts + labor would be $2k each.What will we end up doing?

This usually occurs when a political argument can be made that a given solution will enhance the neighborhood but doesn't comply with the zone.