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  • Writer's pictureMaria Luisa Castellanos

Would you like to build an accessory dwelling unit or guesthouse?

Example of detached accessory dwelling unit
Main house and accessory dwelling unit by Miami residential architect, Maria Luisa Castellanos

Miami-Dade County has implemented a new ordinance that allows guesthouses and accessory dwelling units (ADUs) on single-family properties with a minimum lot size of 7,500 square feet. This additional unit allows for a separate kitchen, sleeping area, and bathroom for a “separate family of occupants”. There is no physical difference between an ADU and a guesthouse. The only difference is that a guesthouse is not for rent and an ADU can be rented out, but never for less than one month. Under no circumstances can either of them be used for vacation rentals such as Airbnb's.

The ordinance reads, “Subject to the conditions, requirements, and procedures set forth in this section, one accessory unit per lot shall be permitted in the following zoning districts located inside of the Urban Development Boundary: AU, EU, and RU, as well as GU for properties that are trended to AU, EU, or RU.”

Accessory dwelling units and guesthouses can range from 400 to 800 square feet of habitable area and must be located inside the Urban Development Boundary. The ordinance requires that the primary and accessory unit maintain an appearance consistent with the character of the neighborhood, and at the same time the ADU must be in character with the primary residence. The ADU or guesthouse can have a separate entrance.

An ADU may have electrical, water, gas, and other types of utility meters, a house address or unit number, and a mailbox that are separate from the principal dwelling, but a guesthouse cannot have utility meters, or an address, unit number, or mailbox separate from the principal house.

These units may be attached or detached, and they must be formally permitted by the Miami-Dade Building Department. This ordinance does not apply to houses within municipalities.

A Certificate of Use (CU) is required for each accessory unit and this certificate must be renewed every year. At the time of renewal, the unit will be inspected. Guesthouses are only required a one-timeADUs CU that does not need to be renewed.

There are penalties for not following the law and the rules regarding guesthouses or ADUs.

This ordinance only applies to residences in unincorporated Miami-Dade County. However, the City of Miami also allows ancillary dwelling units, but they can only be 450 square feet. The city regulations state, “Unit Structure shall be architecturally harmonious with the Principal Building. Any Facade abutting another property shall provide only clerestory windows along that corresponding Facade.”

If you want additional information on construction in Miami or want to know if your property qualifies for an ADU, contact me, Maria Luisa Castellanos, R.A., residential Miami architect, principal of one of the best architecture firms in Miami, at 305-439-7898 or email me at



Can ADUs be built in Transect T3-R of the City of Miami? Any push to do so if not?

Maria Luisa Castellanos
Maria Luisa Castellanos

No, ADU's can only be built in T3-L. You can do an addition to the back of the house which can be an almost independent unit. City of Miami zoning allows pretty big houses, but you cannot do an ADU in T3-R. If you want to discuss further, call me, Maria Luisa Castellanos at 305-439-7898.

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