I live in a 35 year old split level home.
Currently in the process of having the kitchen totally redone and an
old addition torndown, rebuilt & expanded. Basement is under the
kitchen; attic is over the kitchen. Old addition butts up to the
kitchen. Old addition has it's own cinder block foundation wall.

When the kitchen floor was stripped down the to subfloor, you could see
a slight dip in the floor over the last 4 feet as you proceeded to what
was the dividing wall to the old addition. If you looked at the wood
beam in the floor at the end of the kitchen (where the old addition
started), you could the the kitchen floor sag down towards the middle
part. I didn't measure the sag but it was on the order of
three-fourths of an inch.

In my basement the foundation wall doesn't extend to the back wall of
the house, but ends 4 feet earlier. The basement beams extend past the
wall. If you looked under the old addition you'd see a plywood
enclosure. I imagine if you removed a piece of that you could look
into my basement.

The new addition was already done at this point. My contractor said it
was the weight of the house must be pushing the end of the beams down.
He said today they'd either have to be double beams or spaced closer
together. They are spaced 16 or 18" apart believe.

So I was wondering what to do (or if anything should be done). The new
addition was already built by this point. The old kitchen was there
when we bought it 15 years ago. We never saw a problem with the old
countertop (i.e water never ran to the back of it) It seemed to have
stayed level over the years.

So I don't know if I need to reinforce the support beams. But if I
wanted to what would be options? I do have a crawl space under the new
addition. I wouldn't be looking to jack anything up; just support it
from sagging further. thanks.