If I wanted to build a self driving car, and had the money, equipment, and knowledge to do do, would it be legal? Could I take my hands off the wheel/feet off the pedals while driving, as long as I still paid attention to the road and could override the computer if necessary?
If you buy a large piece of land, closed to the public, then yes.
You would need a license for your car to allow drivers to use it without hands on the wheel, or the driver will get arrested.
But first you need a license that allows using the car on public roads at all. That’s what all the car manufacturers have to do. Requires for example crash tests, tests how polluting the car is, and so on.