4'-6" x 21'-3" Bump out-on piers?

I want to create a bump out on the first floor of my house in order to create enough space for two bedrooms where there is currently a large living room (that will be moved elsewhere). The bump out would need to project 4'-6" from the foundation wall, so while cantilevering might be a possibility, it would be too problematic in my opinion for a couple of reasons. First, because as I understand it, 2' of cantilever is usually considered the maximum for a bump out anyway. Second, even if it could be cantilevered, I'd need 9' of joist inside the foundation which would be a project in and of itself due to the fact that the basement underneath is fully finished with sheetrock ceilings and recessed lighting, not to mention other wiring and ductwork which is in the joist bays. Basically, what I would propose would be to lag bolt a ledger to the rim joist and frame out the platform for the floor which would be supported by a pressure treated girder and whatever number of concrete columns was determined to be appropriate. The top of the foundation wall is about 36" above grade and I figure a ganged 2x12 would be used for a girder, so the piers would not need to extend far above grade. Frost depth for my area (NJ) is 48". There would be nothing in the bumped out portion of the walls, floor, or ceiling such as wiring, ductwork, plumbing, etc...and the bump out would be a single story with a simple gable roof on top that would connect with the existing gable roof (running perpendicular). Would this be a common solution to an "addition" such as this? 2x12 floor joists would allow for R-30 fiberglass batts in the joist bays, and rigid foam could be used on the underside as well, so I'm not worried about the bedrooms being cold. The piers could be easily hid with landscaping. Anything else I'm not thinking of?