Decks, Patios, Porches and Docks - What kind of foundation do we need?

In NJ,

Planning to add a sun room on top of the existing car port (deck).
The car port also serves as deck, about 6.5' high, with a span of 20 feet, using steel I beams. The floor has 8 inch depth concrete.

The architecture plan shows the new Floor load will be 40 PSF and new Roof load, 26 PSF.

The existing deck has 8 iron columns sitting on round concrete foundations (11" in diameter). When we check the bottom of one foundation, it has only 2 feet deep. The question is do we have to dig deeper to make a hole underneath the foundation and pour more concrete to make it 3 feet in depth? (we will support the column before doing that.)

Any other better plan or ideas? Thank you in advance.

Check with your local building code office for foundation requirements. Codes tend to vary from area to area. These tend to cover design, embedment, height, materials and workmanship.

Thank you. We are making an appointment with them. If too complicated, we will give up.