Accessory Structure

An accessory structure is a building on a residential lot that is secondary, or subordinate, to the primary residence — think detached garages, sheds, workshops, pool houses, carports, and studios. Zoning codes treat accessory structures as a category with their own rules: how close to the property line they can sit, how tall they can be, and how much of the lot they can cover.

The key distinction for housing is whether an accessory structure is also a dwelling unit. A storage shed or detached garage is an accessory structure but not a home — it has no provisions for cooking, sleeping, and sanitation. A detached ADU is both an accessory structure and a dwelling unit, because it adds an independent living space, and a casita used as a true second home falls in the same category. This is why a detached ADU is sometimes described as converting or replacing an accessory structure.

The difference matters for permitting. A non-habitable accessory structure may face lighter requirements, while creating a dwelling unit triggers the full path — a building permit, habitability and egress standards, utility connections, and ultimately a certificate of occupancy. Converting an existing accessory structure such as a garage into an ADU is a common, often lower-cost path precisely because the shell already exists.

Accessory structures are also governed by dimensional limits such as setbacks, height limits, and lot coverage, which together decide whether a planned structure actually fits. Because definitions, size thresholds, and which structures need a permit vary by jurisdiction and change, treat this as a general overview and confirm how your city classifies and regulates accessory structures with the local planning department.