Accessory Dwelling Units

ADUs in Fullerton, designed and built by one firm.

Garage conversions, detached ADUs, attached ADUs, and junior ADUs — from feasibility review through permits and construction, led by a licensed architect who is also a licensed general contractor.

We design, permit, and build accessory dwelling units in Fullerton and North Orange County — detached ADUs, attached ADUs, junior ADUs, and garage conversions. Every ADU starts with a feasibility review of your lot, so you know what fits and roughly what it costs before you commit to full design.

What can you build on a single-family lot?

California law requires cities to allow ADUs on most single-family lots, and in many cases a combination of units: an ADU created within existing space such as a garage, a junior ADU inside the house, and a new detached ADU (Government Code §66323). What actually fits on your lot depends on lot size and shape, setbacks, existing structures, utility capacity, and — in Fullerton's Residential Preservation Zones — the City's design guidelines.

Four types of ADU

Garage conversion

Turns an existing garage into a living unit. Usually the lowest-cost path when the structure is sound, but older garages often need foundation, framing, insulation, and utility upgrades to meet current code.

Detached ADU

A new, separate building in the backyard. The most privacy and the most design freedom; also the highest cost, and it must include solar under the Energy Code unless an exception applies.

Attached ADU

A new unit built onto the main house with its own entrance. Treated as an addition for energy-code purposes.

Junior ADU (JADU)

A smaller unit created within the walls of the existing house, such as a converted bedroom suite. Under state law, cities must require JADUs to be rented for terms longer than 30 days.

How long does an ADU permit take?

For a lot with an existing house, California law requires the city to approve or deny a complete ADU application within 60 days (Government Code §66317), and to tell you within 15 business days whether the application is complete. The clock starts only when the application is complete, and correction rounds add time — which is why clear, coordinated drawings matter as much as the design itself.

Why one firm for design and construction

ADUs are small buildings with tight budgets. When the architect and the builder are the same licensed firm, the design is priced as it's drawn, field questions are answered on site, and there is one party responsible from feasibility to final inspection. If you'd rather bid the permit set to other contractors, you can.

Deciding between an ADU, an addition, or a remodel? Read Remodel, add on, or build an ADU?

ADU questions

No, for an ADU. When a garage, carport, or covered parking structure is converted to an ADU or demolished to build one, the city cannot require the lost off-street parking to be replaced (Government Code §66314(d)(11)). Some cities interpret the rules differently for a junior ADU created from an attached garage, so we confirm with the City for your specific case.
A newly constructed detached ADU generally must include a solar PV system under Section 150.1(c)14 of the 2025 California Energy Code, unless an exception applies. The requirement can also be met by adding modules to an existing solar system on the same lot if the utility allows it. Attached ADUs and conversions of existing space, such as garage conversions, are treated as additions or alterations and are not subject to the new-construction solar requirement.
Only the ADU is reassessed. Under Proposition 13, the county assessor adds the market value of the new construction as its own base year value; the assessed value of your existing house and land does not change.
In 2026, ADUs we see in Orange County and Los Angeles typically range from $150,000 to $400,000+, including design, permits, and construction. Garage conversions typically run $150,000 to $250,000 because the shell already exists; new detached ADUs typically run $225,000 to $400,000+ depending on size, site conditions, and finish level. Utility connection distances, foundation requirements, and fire-rated assemblies are common cost drivers. We give you a written range for your specific lot after a site visit and feasibility review.
Typically 10 to 18 months total: about 4 to 8 weeks for design and engineering, 6 to 16 weeks for permitting, and 4 to 8 months for construction. State law requires a decision within 60 days of a complete application, but plan check corrections can extend permitting — which is why complete, coordinated drawings matter.
Often, yes. State ADU law limits when a city may deny a permit to legalize an existing unpermitted ADU (Government Code §66311.5), though the unit still has to be brought into compliance where the City requires it. We document the existing unit with as-built drawings, evaluate what needs to change, and handle both the permit and the corrective construction.
A detached ADU can be up to 1,200 square feet (Government Code §66314). A city cannot set a maximum size below 850 square feet for a studio or one-bedroom ADU, or below 1,000 square feet for an ADU with two or more bedrooms (Government Code §66321). An attached ADU may be limited to a percentage of the existing home's floor area, but never below those minimums. A junior ADU is limited to 500 square feet of interior livable space.
On a lot with a single-family home, a city must allow a detached ADU of at least 16 feet in height. That rises to 18 feet (plus up to 2 feet for roof pitch matching the main house) within one-half mile of major transit, and an attached ADU can go up to 25 feet or the local height limit, whichever is lower (Government Code §66321).
For a new ADU, a city cannot require side and rear setbacks of more than 4 feet (Government Code §66314). No setback is required when an ADU is created within an existing legal structure, such as a garage conversion. And an ADU of up to 800 square feet, 16 feet tall, with 4-foot setbacks cannot be blocked by lot coverage or floor-area limits.
Under 2025 changes to state law, cities cannot charge impact fees on an ADU with 750 square feet of interior livable space or less, or a junior ADU of 500 square feet or less. Larger ADUs can be charged impact fees. Permit, plan check, and utility fees are separate from impact fees.
A junior ADU is a unit of up to 500 square feet of interior livable space created within the existing single-family home. If it shares a bathroom with the main house, the owner must live on the property; if it has its own bathroom, owner occupancy is not required under 2025 changes to state law. Adding a JADU cannot by itself trigger a fire sprinkler requirement if the main house doesn't have sprinklers, and cities must require JADU rentals to be longer than 30 days.
Fullerton's local ADU standards are in Fullerton Municipal Code Chapter 15.17, adopted to implement state ADU law; state law has changed several times since, so we check both for your lot. For general questions, the City's Planning Division can be reached at (714) 738-6559. We confirm zoning, setbacks, and any preservation-zone guidelines for your specific parcel in the first consultation.
Not always. California Business and Professions Code Section 5537 allows anyone to prepare plans for a wood-frame single-family home of up to two stories plus basement. But any portion that departs from conventional wood-frame construction must be designed by a licensed architect or engineer, and the City still needs permit drawings that pass plan check, including structural and Title 24 energy compliance. Hiring an architect who is also your contractor means the person who draws the plans is responsible for building them.
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Written by Aaron Song — licensed architect and general contractor

CA Architect License #C-42133 · CSLB #1152969 (B - General Building Contractor) · EPA Lead-Safe Certified Firm · Master of Architecture, Harvard
Last updated October 2026. Code references are to California law as of this date; your City's review of your specific project governs.

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