Question
thanks for the service.  i live in Pittsburg, Ca.  Soil is heavy clay.  
i have a 12X20 ft section that i want to plant a lawn.  too small
to til, plus there's irrigation tubes under.  the soil is 4" below the
patio level.  can i just dig out 3" of clay.  put in 6" of some
additive.  put sod on top an have a great lawn for years?  do i
need to til if i have 6" of top soil - whatever is recommended for
the area?  should i dig deeper?  if this is ok, what type of mixture
do your recommend for a top soil?  thanks.

Answer
If I understand your question correctly, you are asking if you can dig up 3" of soil, mix with 6" of topsoil (on top) ? This would be ok. Grass can grow very well with 9" of amended soil. The soil should either be a sandy loam, or alternatively 75% sandy loam, 25% compost.

It is a good idea to dig the 3" and then mix it with the 6" layer of soil as above. You will need approximately 4-4.5 cubic feet of amended soil.

The only thing to test/watch out for will be drainage.
I am not sure how the drainage is below the 3" layer you plant to dig. If water settles and stays for a long time, you may need to dig a bit deeper and consider improving drainage before adding more topsoil. But otherwise you should be ok.