Am I required to have a doorknob lock on my exterior door?
Some jurisdictions, such as New York City, require self-closing, self-locking doors to the entry of multiple family dwellings (,8 or more units), but do not seem to require them on smaller housing units (such as one or two family homes) or on the individual apartments within a larger unit.
Even when locks are required, they do not need to be in the knob of the door. Often there is no in knob lock but an auxiliary self locking mechanism higher up on the door.  Many exit door locksets do not even have a knob, but a thumb latch. A single key often controls the self locking latch and the deadbolt.

I do not know your particular locale's code, but I would be surprized if there is a regulation about locks at all (other than a prohibition on a door that can't be opened from the inside without a key). It would be very strange if they required an in-knob lock, especialyl if you have an auxiliary latch or deadbolt.

SUPPLEMENT

I since have found more code in NYC concerning mandated deadbolts and door chains or similar devices on individual apartments, but still nothing requiring an in-knob lock.