Just 1 other question: Is it normally only necessary to lay reinforced concrete foundation under main walls (3 times the width of the wall?), or should the reinforced concrete foundation cover *all the floor area? Thanks.

Poured slab on grade concrete foundations, as a minimum, consist of: 1) A reinforced concrete footing which extends below your freeze line in the ground; 2) A reinforced slab on grade poured over an under slab moisture barrier and insulation. The use of moisture barriers, insulation, fill and reinforcing materials will vary with the conditions of your site and design.

The size of your footings will depend on the load they are designed to carry. For example, a one-story structure will have smaller footings than a two- or three-story structure.

Your local building official can help you with specifics based on your location and the type of construction you envision.

A blocked sewer line under a slab on grade can be repaired by removing the slab in the area of the blockage, excavating and replacing the piping where needed. It is not an unusual situation in remodeling of older houses, but it is messy and time-consuming.

Good luck!