Foundation modification question
 

My house is on a slanted hill in SF (an earthquake area). I'd estimate
the hill to be between 30-40 degrees. The house is a 2-story SFH built
in the 1920s. The slope is higher on the left, and lower on the right.
As a result, the pavement in the garage is not level. The foundation
on the left side is higher than the right side. It is a 2-car tandem
garage, the area where we want to build the room is inside to the left
of 2nd car.. (so afterwards, one car, and rooms in front of it)

The garage looks kinda raw, with no dry-wall. We want to build some
storage room in the garage, but as it is, there isn't enough height
clearance. I'd estimate about 6-ft to 7-ft, depending on where you are
in the garage.

To make the floor level within each proposed room, some contractors
said they need to break the existing concrete floor, dug out the sand
(we live in a sandy area), and as a result, the base of the foundation
on the left side (higher) will likely be exposed, so they will
"expand" the foundation down-ward (put something to support the
current foundation while purring another one below). I estimate they
will need to dig down about a foot on the left side to make it match
the middle of the garage (not the lowest point, just the middle).

Now the contractor said it's not a problem, they've done foundation
expansion this way. But I am just a little worried. Does this sound
safe? Especially on a slanted hill?

The current garage, although doesn't look its best, feels very solid.
The house appears to have survived the 1989 SF quake without much
problem. I have 2 worries:

If we dig down, and assume the contractor do a good job, the existing
foundation will be sitting on a new foundation. Not the entire left
side of the house, but perhaps a 10 foot section. (how tall will the
new foundation be, I don't know, probably half a foot?). But during an
earthquake, won't the upper foundation move freely since it's not
bolted down with the foundation below? This seems risky to me. right
now the foundation can't move because the garage floor next to it is
concrete and pushes against it.

My 2nd worry is, my next door neighbor to the left (the houses are
next to each other), his garage floor is obviously higher than mine.
So it seems to me currently there is more pressure (soil) on the other
side of the left foundation than on this side (empty). If we dug down,
there will be more pressure, during an earth-quake, will the
foundation just tip-over to the right?

And to finish it off, the contractor we will be hiring is not
licensed. Hiring licensed foundation contractor to do the foundation
work is out of our budget. The current contractor is kind of a general
contractor that will be doing the dry-wall, etc.. He definitely have
experience, just not licensed probably due to language problems.

Should I be worried about the foundation update? Without breaking the
existing garage floor, it's hard to make the floor level, and breaking
the floor means the higher side's foundation will be exposed and needs
to be expanded down.

Apologize for this long email, my parents wants to divide the garage
into some rooms to make better use of it, but it's just the options
seems limited due to the slope in the garage and the low clearance.

Personally I am opting for just doing whatever we can without touching
the foundation or the floor, which seems to be in good shape. But my
parents wants to build some rooms so they can potentially rent it out
when they retire soon..

And one last question, for foundations, are there different grades of
concrete that needs to be used?

Thanks a lot!

Raymond