Do You Need a Permit to Build an ADU (Accessory Dwelling Unit)?

Permitting authority

Local building & planning department

Typical fee

$1,000–$15,000

In the US, ADU construction requires both planning/zoning approval and a building permit in virtually every jurisdiction. Since California's landmark ADU legislation (AB 68, SB 13, AB 881 in 2020, and subsequent updates through 2024), many states have followed with laws that pre-empt local zoning restrictions — but the building permit is still mandatory. Typical costs: plan check fees ($1,000–$5,000), building permit ($2,000–$10,000 based on valuation), school district fees ($3–$5/sq ft in CA), utility connection fees ($5,000–$15,000 for sewer/water tap). The permit process includes plan review (4–12 weeks), foundation inspection, rough framing inspection, rough plumbing/electrical/mechanical inspections, insulation inspection, and final inspection for certificate of occupancy. Impact fees vary dramatically — some cities waive them for ADUs under 750 sq ft per state mandate. Owner-occupancy requirements have been rolled back in California but still apply in many other states. Setback requirements (typically 4 feet from rear/side lot lines for detached ADUs) and height limits (typically 16–25 feet depending on jurisdiction and roof type) remain. Some jurisdictions now offer pre-approved ADU plans that fast-track the permitting process to 2–4 weeks.

Permitting authority

Starostwo Powiatowe / Urząd Gminy (Wydział Architektury)

Typical fee

0–2 000 zł + koszty projektu

In Poland, building a separate dwelling unit (known as budynek mieszkalny jednorodzinny or lokal mieszkalny within an existing building) requires a pozwolenie na budowę in most cases. Since the 2022 amendments to the Prawo budowlane, wolno stojące budynki mieszkalne jednorodzinne o powierzchni zabudowy do 70 m² (freestanding single-family buildings up to 70 m² footprint, single-story or with attic) can be built under an uproszczona procedura zgłoszenia (simplified notification), provided: the building is for the investor's own residential needs, the plot is covered by a miejscowy plan zagospodarowania przestrzennego (MPZP) or a warunki zabudowy decision exists, and the structure is max 2 stories. This 'dom do 70 m²' procedure eliminates the need for a kierownik budowy (construction manager) and dziennik budowy (construction log), though a projekt budowlany (architectural design) by an architect with uprawnienia is still required. For buildings exceeding 70 m², a full pozwolenie na budowę with complete projekt budowlany is mandatory.

Permitting authority

Gemeente (Omgevingsloket)

Typical fee

€500–€5,000

In the Netherlands, building a separate zelfstandige woonruimte (independent dwelling space) nearly always requires an omgevingsvergunning under the Omgevingswet. The key distinction is between a mantelzorgwoning (care home for relatives) and a reguliere zelfstandige woning (independent residential unit). A mantelzorgwoning for a family member requiring verzorging (care) may be vergunningsvrij under certain conditions in the achtererfgebied — but the moment the care relationship ends, the building must lose its woonbestemming (residential designation). A self-contained unit with its own keuken (kitchen), badkamer (bathroom), and separate toegang (entrance) that could be rented out independently is a woning splitsing (dwelling split) and requires both an omgevingsvergunning and a splitsingsvergunning from the gemeente. Many gemeenten have additional rules under the Huisvestingsverordening that restrict kamerverhuur (room rental) and woningsplitsing to maintain housing stock balance.

Permitting authority

Ayuntamiento (Urbanismo) / Comunidad Autónoma

Typical fee

€500–€5,000

In Spain, building an additional dwelling unit — whether as an ampliación (addition) to an existing vivienda, a conversión of a local (commercial space) or garaje into housing, or new construcción of a vivienda auxiliar — requires a licencia de obra mayor from the Ayuntamiento. The project must comply with the CTE (Código Técnico de la Edificación) requirements for habitabilidad (habitability), including minimum superficie útil (usable area, typically 25–40 m² depending on the CCAA), ventilación (ventilation), iluminación natural (natural light), and aislamiento térmico y acústico (thermal and acoustic insulation). A cédula de habitabilidad (habitability certificate) must be obtained upon completion. The proyecto básico and proyecto de ejecución must be signed by an arquitecto colegiado, and the obra must be supervised by an arquitecto técnico (aparejador) as director de ejecución. In comunidades autónomas like Cataluña, specific regulations on viviendas de uso turístico may further restrict ADU usage.