Question
QUESTION: hi

i live in california
i will be planning on rebuilding on a current home site with a basement
there is a coast live oak that is approximately 17 ft from the new building perimeter. the oak has a 28 in diameter, and it crown span on my side of property is at most 23 ft
the current house is only 8 feet from the tree, has a 2.5 ft perimeter foundation
the neighbors are contesting
i can see in no way how this planned construction can hurt tree to the extent that it will perish
1- less than 10 % of TRZ will be affected
2- less than 10% of root mass will be effected
the roots in question have been covered by a home for over 60 yrs
the water table as determined by boring is greater than 20 ft down

thanks for your help

ANSWER: A couple of things come into play here--the loss of root from the foundation digging and the protection of the Critical Root Zone around the tree. IF I am reading this correct the current house 8 feet from the tree trunk and has a 2.5 foot deep foundation??  IF so the tree has more than likely grown roots under the foundation (assume this is a slab foundation). It is true that the amount of roots that will be cut will be well within the allowable 5 BUT removing the present foundation could cause major damage to the root system.  I would make sure that the critical root zone is protected during the construction. To prevent construction damage, fence off the critical root zone. Estimate the critical root zone by measuring the diameter of the trunk in inches, multiplying that number by 1.5, and using that number in feet as the radius of the protected area. A 28 inch diameter tree needs protection of 42 feet around the tree. Need to put up some sort of barrier around the tree and not let ANY machinery, Storage of construction equipment, soil stockpiles, masonry stockpiles and other heavy items inside this barrier. These can compact soil permanently even if they are only on the site for a short time. The roots of the tree on the side of the current building will not be as wide spread as the other side. I am unclear where the new building will be but it sounds like it maybe within the critical root zone. The problem will be keeping machinery out when you dig the new footing.

IF much care is taken you should be ok--may get some branch dieback on that side of the tree crown but not death. I would however fertilize the tree with 10-10-10 fertilize at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. I would go ahead and do this now and remember NO machinery etc. . in the critical root zone. I hope this is what you asked and I did not complicate it too much.

---------- FOLLOW-UP ----------

QUESTION: hi

to clarify
current house without basement 8 feet from tree- 2.5 ft perimeter foundation
new house 17 ft away with BASEMENT
if these roots have been covered by old house for 60 yrs, there is also a driveway on my side so more than 54 feet of tree root have been covered on my side, what kind of value would these roots have. the soil is clay, impervious, and the water table is more than  22 ft deep

Answer
The roots under the house would grow under the perimeter foundation but would still be present. There would  also be roots under the drive. IF the tree is on your property It is your tree and unless local laws prevent it you can do what you want ---cut it down if you want. I would try to save it by protecting the roots left around the tree and keep as much traffic off the root zone as you can. IF the tree is on the adjacent property you may have a problem--I would call the local office of the State Forestry agency and ask one of their Urban Forester to come out and advice you. This is a free service and they can see things that can not be written in a question.
In summary I would go for it and try and keep the tree.