We are adding a fairly large addition onto our house with a full basement. The builder is using 12 inch concrete block to build the foundation which is 22 feet wide and about 35 feet deep. We thought they would pour concerete into the holes of the blocks to make the foundation stronger, but they said it doesn't need to be filled and is strong enough. About 6 feet of the basement is under ground and we are adding 2 floors above. Should I worry about the foundation being strong enough with out the reinforcement of poured concrete and rebar in the block holes?? How would I know when pouring conrete into the blocks is neccessary?

Thanks,

Jason