Your homeowners policy assumes one household
A standard form contemplates the family that lives there. Introduce a paying tenant in a casita and the property no longer matches that description. Some carriers accommodate it with an endorsement, some require a different form entirely, and some will not write it. Carrying on without saying anything is the option that fails at a claim.
The tenant creates liability on your property
Their guests, their deliveries, their use of the shared yard. As owner-occupier you are exposed to all of it, and the liability limit on an ordinary homeowners policy was set for a household rather than a small landlord.
Permitted status matters to the insurer
A conversion done without permits can be a problem at a claim, because rebuilding it may not be permitted and the carrier may limit what it pays. Whether the unit is legal is a question worth settling before it is rented rather than after a fire.