Question
The new owner of the property next door to us is a builder/developer.  He demolished the original building and is putting up a very large house.  He excavated the entire property to a depth of 5 feet and in the process removed all the roots of our California Sycamore on his side of the property line.  Our sycamore tree is only 2 or 3 ft. away from the property line so he did quite a bit of damage.  Our tree is 60 ft. to 80 ft. tall and has a natural  lean over our house.  We are very concerned. The developer plans to put in a driveway along the property line above the area where the roots were extracted.  Can the tree survive the damage?  If so, will a driveway do further damage?

Answer
Sounds like you should be concerned fro a couple of reasons.
A trees root system extends about 1 1/2 time the width of the branches and usually in the first 2-3 feet of soil. You can cut up to about 25-30% of the root system without causing damage to the tree. The damage comes in die back of the foliage on that side of the tree and if extensive could cause mortality.
If as you say all the roots on one side of the tree were dig up up to about 2-3 feet from the tree trunk the tree will not be able to sustain the foliage with this amount of root loss. At the least the half of the tree on that side will die.

But more important is the lean toward your house--if the root are gone on the side away from the house and it is leaning there is nothing to hold the tree and possibly during a wind storm the tree will or could come down. I would classify this tree as a hazard and should be analyzed for possible replacement.

I would call the State Forestry agency and ask one of their Foresters to come take a look and advise you on the possibility of the tree falling and/or dying. Their number should be in your local phone book. I would approach the neighbor to pay for the removal and replacement costs. Sorry!