A lot of San Diego houses include a back unit, a converted garage, or a companion unit. In Bay Park, Linda Vista, Clairemont, and the College Area, you will see them often. Some are permitted accessory dwelling units. Some are not.
Permitted is the whole question
California allows ADUs on many single-family lots, and the City of San Diego has its own standards for size, setbacks, and parking. A listing that says “guest quarters” or “bonus room” is not the same thing as a permitted dwelling. Ask for the permit number and match it to the City of San Diego permit record. The unit on the ground should match the unit on the plans.
An unpermitted unit can complicate the loan, the appraisal, the insurance, and the next sale. A seller who says it has been rented for years has not settled the permit question. Put the permit review inside the inspection contingency so you can cancel or renegotiate if the file is empty.
What to look at on the property
Separate entrance, kitchen, and bathroom, if it is supposed to function as a dwelling.
How utilities are metered. Shared electric with no agreement becomes a dispute.
Parking. Older permits sometimes required a space. Confirm what was actually approved.
Access for a tenant without walking through the main house.
Condition of the foundation, roof, and HVAC, which may not match the main house.
Renting the unit
A legal ADU can be a long-term rental. It is not automatic short-term rental income. The City of San Diego licenses many short-term stays, and the rules depend on the property and the neighborhood. Confirm the current ordinance before you advertise a nightly rental. Also confirm the association rules if the house is in a planned community. Some CC&Rs restrict second units even when the city would allow one.
Do not underwrite the purchase with a rent number you have not verified. Ask what the unit rented for, then check whether that lease was legal and whether the condition supports the same rent after you own it.
If you are comparing houses with second units, start with the neighborhood guides and call Jon before you treat the extra kitchen as a dwelling.
