[ckissick]

The answers all depend on the house. As for 2X4s in walls, most 2-story houses have 2X4 walls, same as 1-story. Around here, people add a second floor all the time, and they don't generally have to beef up existing walls, or at least not very much. What they do, is they transfer the additional loads to existing foundations and/or add new square footings or drilled piers for point loads. The existing spread footings may need to be underpinned.

Here in California, we don't deal with unreinforced block walls. If a wall is old and unreinforced, it gets replaced.

Each house has to be evaluated individually. You would need to consult with a structural engineer who can inspect the house. You could also have a contractor inspect the houses for less detailed, but pretty good evaluations.

The biggest problem may be getting a construction loan. Most of my clients can't get them anymore.