Foundation strength for second floor

We are considering adding an above the garage room. I understand that the foundation may not support a second story as-is. The house is in California and was built in 1960. It has a slab floor in the garage and a curb around the edges which is about 6"x6" where the sill is. I'm not sure about the width of the base of the foundation. Is it possible to strengthen a foundation by adding to it? Or is it usually a requirement to tear it out and put in a new heavier foundation? Fremont

It is possible and practical to add to the foundation. In most cases you will need to dig next to the existing foundation, drill holes in the concrete and insert dowels that will extend into the new concrete. If you make sure that the new concrete is securely tied to the old (using these dowels) you can usually assume that they will work together to support the new loads. However, it is possible that the existing foundation is sufficient. Consult a local engineer for best results and safety.