Are there foundations at the 15" away that are sufficient for that loading? That would be surprising to me.

No way to guarantee no differential movement. When I do stuff like this, I go out of my way to warn the homeowners that their walls will crack. Of course our soils aren't exactly great, and settlement is pretty common though rarely does it need to be corrected.

A few important things, though. If you have pressure treated lumber, spec it as KDAT (kiln dried after treatment) or force the contractor to dry it out before they use it. That stuff can show up sopping wet and shrinks as it dries. Not an issue if the whole house is being built at once and it dries evenly, but if they line everything up real nice with the existing while it's wet, you could end up dropping a bit. Use only synthetic shims like HDPE or other suitable plastics.

Aside from the very astute remarks by phamENG (concur with all of it except I would note that the likelihood you will get anything other than HD off the shelf PT wood is between slim and jim, and jim left town), I would also think even if you get your new wall in perfect contact and solve the shrinkage / settlement issues at the new wall location, joist deflection is inevitable at the old wall location. Are we not taking a joist system that was under negative moment, at the existing wall support, and now changing the moment to a positive one at the same location by repositioning the wall? I do not see how that doesn't cause deflection in the joists at the old wall location / into the span adjacent too it.

Now, it's only 15" so not a great mile by any stretch. But still something will occur.

Jayrod - No, we'll have to put a new footing or thickened slab edge at that location.

Pham - Thanks for the comments, I appreciate it.

Enable - The existing wall will be removed, so I'm not sure why it would matter that the deflection at that point will change? The span would just increase by 15" (which would then match the span of the joists on the other half of the house - the side that isn't a walkout).

LD- Enable is pointing out that by moving the wall, you're going to change the shape of the floor system. You have existing joists which have crept over time. You're now reversing the stresses in the joists at that point. As these changes work themselves out, any partition walls, floor finishes, etc. that sit on that floor are also going to start moving. It doesn't take much to start damaging things. Not catasrophic, of course, but the little irritating things that, while insignificant to us, mean the world to the homeowner and are complex enough to require hours of explaining.

It's unlikely that you could eliminate the possibility for differential movement depending on the new foundation type. There will always be some consolidation of the subgrade unless you're pouring directly on rock. Since you're only a thousand miles south of me (give or take), I'd expect you're not bearing it directly on sound rock.

And as for Enable's comment, you are increasing the span of those joists, therefore you will see increased deflection in those joists potentially causing damage to existing finishes in the area. It doesn't matter that their new span will match the span of other joists in the area, the modifications you are proposing change the loading/support conditions and the structure will react accordingly. Draw the deflected shape of the currently loaded joists, then draw the new deflected shape. It should become quite clear at that point.

I understand what you're saying about the new deflected shape. There are no partitions on the upper level to worry about and there will be new floor covering installed on that level after the structural modifications, so the floor would already have the dead load portion of the new deflection at that point. Very interesting point to consider, thanks! Whether this modification ends up in scope is still up in the air, I was just trying to fully understand the implications.