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I say it is what it says it is: a guest room. No provisions are shown for cooking. We would ask for more information and changed plans addressing cooking appliances, heating, ventilation, utilities, parking, zoning restrictions and addressing before we could call it an ADU or other any other type of dwelling. If the applicant insists on calling it a guest room, then we would clearly identify it as such on the permit and plans.
The image visually implies a sink and refrigerator, and does not show a graphic that looks like a cooking device (burners or oven). If this were on a single family lot, I would call it an accessory structure (such as JPohling's "pool house" recommendation), but not an (accessory) dwelling unit.
That said, it is easy for them to add a plug-in microwave or a burner coil afterwards... but bootlegging is a different issue.
where is it located on the lot in conjunction with what other dwelling or structures, you cannot have an accessory anything until you have a principle use