BP - depends on the way the original house and the addition were backfilled. Normally, you want your new foundations to go on "undisturbed" soil...so that it won't settle. When you dig for a foundation, however, you end up making the excavation wider than the footing and foundation wall, and have to "backfill" on the outside of the new foundation.
That means the backfill material won't be "undisturbed" soil. If the original and the bath addition were backfilled correctly (meaning compacted soil, or gravel) then you probably have a good base to build on.
If not however, then anything you put on the backfill will likely settle. So if you attach the new to old with rebar, and the new settles, you'll be affecting the existing wall. Not a good thing!
But it the existing bath addition's been around for a while, and hasn't settled, it should be fine. Your job is to make sure you do it right with the closet addition. Just be sure the new footings are on undisturbed soil and/or compacted fill, and you'll be fine. I would consider attaching the new footings to the bath footings, since they'll be at the same level, but I wouldn't attach the new footings to the concrete block of the house. Hollow block isn't meant for that kind of bearing.
If you have any questions about this once you dig, you need to find a qualified pro in your area to work with. It's a cliche, but the foundation really is the most important part - everything above depends on it!
Hope this helps,
Richard Taylor, AIA
www.rtastudio.com
www.rtaplans.com
