I dug 6 inches below the sidewalk grade, trucked out about 30 1-yard trucks of soil, packed in 4 inches of concrete base and 2 to 3 inches of gold-colored rocks.

Hi BP,I'm looking at a HUD property with BURIED OIL TANK and may also need MOLD Remediation in the garage and another room.My bigger concern is potential Oil tank leaks and soil remediation that can run thousands of $s.

The sewer bills vary by location and they even factor in the "ERU" which measures the roof and paved space on the property which creates water run off into the sewer system vs space where storm water would be absorbed into the soil.  

Howeverall plumbing is laid and the slap is poured over it thus it can be very expensive to repairThere are areas where the soil is said to be "massively expansive" and it expands or contracts with water. 

I would plan to take the ceiling down so you can get a good look at what is happening underneath.

Take in consideration that this settlement usually happens on the first 20-30 years from the date the house was built, depending on the type of soil... some exceptions applies (flooding or oversaturated soil, earthquakes, to name few).Cost will largely varies depending on what actually the issue is and how severe is.  

i.e  pulled out the storage section underneath the stove and in the back corner there was a rust spot that was also blown up in the picture.  

Insulating the floor space (underneath the joists, with rigid or batt insulation- not shown in this drawing) will keep your living spaces warm during the winter, which given your climate is not so important probably.

It was important for me to move to a place with entrepreneurial soil.

Like a couple years ago when we were building  an addition and found contaminated soil and an abandoned oil tank .