I don't think this issue of air dams is nearly as cut and dried as a lot of people here think.
First, as Julian points out above: lots of vehicles, especially full-size trucks, have no underbody paneling and very deep air dams that extend well below the lowest-hanging components. The 2019- Ram even has an extra 2.5-inch air dam that folds down at high speeds, despite the fact that the fixed portion already extends below the lowest-hanging component under the truck:
Second, even production cars with engine undertrays, rear diffusers, and center undertrays still use air dams. I've spent years now looking under nearly every production vehicle at the Chicago Auto Show, and you all would be surprised how many cars have very smooth underbodies and an air dam, both with and without separate front wheel air dams.
Bottom line: I don't think it's prudent to rule out air dams with smooth underbodies, or to adhere to any blanket rules regarding their depth.
For reference, if the OP's car is a Fiesta--here's what the front underside looks like (on American market cars). No undertray, full-width air dam, no separate wheel strakes:
