Structure

I'm imagining an east-west beam approximately where the old north wall of the garage is. This will be on new footings and will resist east-west lateral forces (such as wind on the west wall). I'm thinking that the new 2nd floor apartment will be on I-Joists that hang or rest on this beam.

The I-Joists will cantilever 2' or so over the BR3 south wall. I doubt that the existing footings on that wall can take the added load, so I (sigh) imagine we'll need a new beam spanning above the south wall, resting on new footings on either side.

This has the advantage of allowing us to replace the damaged BR3 window and the very cool Dutch door with any kind of door/window we want. We probably don't want a Nanawall, but we could do it structurally.

The new wall at the south end of the garage is load bearing, as well. It is 10' south of the east-west beam in the garage and 16' 6" north of the beam at the existing south wall.

The beams that span 20-24' are likely LVL 5 1/4" x 11 7/8" (Fb=3100), which are $60/lf or about $2,880 for two.

Chief supports I-Joists, but it ignores the fact that there are nominal sizes: 9 1/2", 11 7/8", 14" and 16" and the flange widths are 1 1/2" to 3 1/2".

The span charts are easy. 40 PSF live/10 dead or 40/20 for L/480 deflection says:

I'm assuming we want 40 live/20 dead, 16" o.c. and 9 1/2" or 11 7/8" depth. That means TJI-230 (9 1/2", 2 5/16" flange) or TJI-110 (11 7/8", 1 3/4" flange). These would be 28' 6" long, but we could lap them on the intermediate wall, meaning we would need 10' and 18' 6" lengths

Dunn Lumber sells Weyco Trus-Joist (r) in nearest foot lengths, up to 60'. The 9 1/2" TJI-230s are $14.78/lf, but the 11 7/8" TJI-110s are just $9.88/lf. 16, 29' joists is $6,858 or $4,584. Just for a reality check--actual prices depend on the total lumber order and the floor will require rim boards and other pieces. We need 18 sheets of subfloor, which is $100-$150/ea, or $2,700. We're pushing $10,000 just to reach the 2nd floor.

The apartment roof  is trusses, I imagine. They would go the short way (east-west), or 20' span. At 24" o.c., we need 15, incl. two gable ends. I'm also assuming a 40' crane.

I'm assuming 2x6 studs so that fiberglass batts get us a proper R-value, but we'll need to investigate that further.

Just as a reminder, the deck materials came from Marine Lumber Service, 525 S. Chicago Street, 800-317-4730. Larry Johnson. MarineLumberService. They can do ripping, trimming, planing and angle cutting.