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What is an ADU?
An Accessory Dwelling Unit (ADU) is a fully functional residential unit located on the same property as a primary home. ADUs typically include a kitchen, bathroom, living area, and a separate entrance. These units are one of the most valuable improvements homeowners can make to their property.
ADUs are also called granny flats, in-law units, or backyard homes. They can be rented out, used for family, or turned into a home office. In California, ADUs boost resale value fast. Most homeowners recover their build cost within a few years of rental income.
An ADU is legally separate from your main house. It has its own address in most cities. It can be sold with the property but not sold separately, unless local rules allow.
Types Of ADUs In California
Garage Conversion ADU
Convert an existing garage into a modern living unit including a kitchen, bathroom, and living space. This is the fastest and most affordable ADU type. You already have the foundation and walls, so construction time and cost drop. Most garage conversions finish in 3 to 5 months. Best for homeowners who don’t use their garage for parking and want quick rental income.
See our dedicated Garage Conversion services page.
Attached ADU
An extension to the existing home that includes a separate entrance and full living space. An attached ADU shares one wall with your main house. It’s a good fit when you want extra square footage without losing yard space. Utility hookups are cheaper here because they connect to the existing home lines. Build time runs 5 to 8 months.
Detached ADU
A completely independent structure built in the backyard providing privacy and long-term rental opportunities. A detached ADU stands alone in the backyard. It gives tenants full privacy and gives you the highest rental value. It’s the most flexible option because it doesn’t touch your existing home’s structure. Build time runs 6 to 10 months, depending on size and utility work.
Junior ADU (JADU)
A smaller ADU created within the existing home footprint, often utilizing unused interior space.
A JADU is capped at 500 sq. ft. by California law. It must include a kitchenette and can share a bathroom with the main house. JADUs cost the least to build because they need no new foundation or roofline. They’re ideal for aging parents or adult children who want privacy but stay close.
Not sure which type fits your lot? Our guide, Garage Conversion vs. ADU: Which Is Better?, breaks down the trade-offs side by side.
ADU Investment Ranges
ADU Type | Typical Cost | Typical Timeline | Best For |
JADU | $40,000 – $100,000 | 2–4 months | Family members, low budget |
Garage Conversion | $50,000 – $150,000 | 3–5 months | Fast rental income |
Attached ADU | $80,000 – $250,000 | 5–8 months | More space, shared utilities |
Detached ADU | $130,000 – $400,000+ | 6–10 months | Max privacy, max rental value |
What Changes the Price?
- Size: More square footage means more materials and labor.
- Finishes: Custom cabinets, tile, and fixtures raise cost.
- Utility hookups: New water, sewer, or electrical lines add expense.
- Site access: Tight lots or slopes cost more to build on.
- Permit fees: These vary by city. Ask us for your city’s current fee schedule.
Accessory Dwelling Unit (ADU) Guide For Southern California Homeowners
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FAM Promotions, Inc. DBA ACT Construction
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California Licensed General Contractor – License #883252
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Serving Southern California Since 1993
Fast-Track Permitting
california SB 543: Stop Waiting on City Hall: Our 2026 "Fast-Track" Process
Building an ADU or converting your garage in California just got faster. Thanks to Senate Bill 543, homeowners now have legal protection against bureaucratic delays. At ACT Construction, we leverage these new laws to get your project out of “permit limbo” and into construction.
The 15-Day “Completeness” GuaranteeUnder the new 2026 laws, the city no longer has months to “review” your paperwork.
15-Day Shot Clock:
Automatic Approval
If the city misses this 15-day window, your application is automatically deemed complete by law—starting the 60-day countdown to your permit approval.
No "Moving Goalposts
If the city asks for corrections, they must provide an exhaustive list. They cannot legally add new objections later in the process. Built-In Savings for Small ADUs.
Built-In Savings for Small ADUs
Are you looking to build a rental unit or a guest suite?
Zero School Fees
If your ADU is under 500 sq. ft. of interior livable space, you are now 100% exempt from school impact fees.More Livable Space: The 2026 laws now measure ADUs by interior space only. This means we can often build a larger footprint than before while still staying under the “fee-exempt” limits, potentially saving you $10,000+ in impact fees.
More Livable Space
The 2026 laws now measure ADUs by interior space only. This means we can often build a larger footprint than before while still staying under the “fee-exempt” limits, potentially saving you $10,000+ in impact fees.
California ADU Rules You Should Know
- Permits: Every ADU in California needs city or county building permits.
- Renting out your ADU: State law allows ADU rentals. Short-term rentals under 30 days may be restricted locally.
- Parking: In most cases, no extra parking is required. State law removed parking requirements for ADUs near transit and for many conversions.
- Property line setbacks: State law sets a minimum 4-foot setback from side and rear property lines for most detached ADUs.
- Owner-occupancy. Not required statewide for most ADUs, this rule is now permanent. Some JADU rules differ, since owner-occupancy still applies if the JADU shares a bathroom with the main house. City rules can vary, so confirm with us before you plan.
- Height limits: State law allows up to 16 feet for detached ADUs in most zones, with up to 18 feet allowed near major transit stops.
How Long Does an ADU Take to Build?
Most ADU projects move through four phases:
- Design & feasibility : 2 to 4 weeks
- Permit review : 15 to 60 days under the new fast-track law
- Construction : 3 to 10 months, depending on ADU type
- Final inspection : 1 to 2 weeks
Total time from first call to move-in ranges from 4 months for a garage conversion and 12 months for a full detached ADU.
Our ADU Development Process
Property evaluation and feasibility consultation:
We visit your property, check zoning, and confirm what you can legally build.
Architectural design and planning:
We design layouts that match your lot size, budget, and goals.
Permit preparation and city approvals:
We submit and track your permit using the Latest fast-track rules.
Full construction management:
We manage every trade under one team, so you don’t juggle contractors. See our full range of construction and remodeling services.
Final inspection and project completion:
We handle city sign-off and walk you through your finished unit.
Curious who’s behind the process? Learn more about ACT Construction, our license, and our 33-year track record.
Areas We Serve
- Chino Hills
- Diamond Bar
- Yorba Linda
- Irvine
- Anaheim Hills
- Walnut
- San Dimas
- Rancho Cucamonga
- Claremont
- Upland
Why Homeowners Choose ACT Construction
- 33+ years building in Southern California. Founded in 1993.
- Licensed general contractor. California License #883252.
- One point of contact. From design to final inspection.
- Fast-track permit experience. We know the 2026 SB 543 process extremely well.
- Transparent pricing. No hidden change orders.
Frequently Asked Questions
Q: What is an ADU?
An ADU (Accessory Dwelling Unit) is a self-contained home built on the same lot as your main house. It has its own kitchen, bathroom, and entrance.
Q: How much does an ADU cost to build in California?
Costs range from $50,000 for a garage conversion to $400,000+ for a large detached unit. Size, finishes, and utility work drive the final price.
Q: How long does ADU construction take?
Most projects take 4 to 12 months from design to move-in, depending on ADU type and permit speed.
Q: Do I need a permit to build an ADU?
Yes. All ADUs require city or county permits before construction starts.
Q: Can I rent out my ADU?
Yes. California law allows ADU rentals. Some cities restrict short-term rentals under 30 days.
Q: What’s the difference between an ADU and a JADU?
A JADU is capped at 500 sq. ft. and can share a bathroom with the main house. A full ADU can be larger and fully independent.
Q: Do I need extra parking for an ADU?
Usually not. State law removed parking requirements for most ADUs, especially near transit.
Q: Will building an ADU increase my property taxes?
Yes, but only on the added value of the new structure. Your existing home’s assessed value stays the same under Prop 13 rules.
Q: Does ACT Construction handle permits for me?
Yes. We prepare and submit all permit paperwork and manage the city approval process for you.
Q: What areas does ACT Construction serve?
We build in Chino Hills, Diamond Bar, Yorba Linda, Irvine, Anaheim Hills, Walnut, San Dimas, Rancho Cucamonga, Claremont, and Upland.
Q: Is a garage conversion cheaper than a new ADU?
Usually, yes. A garage conversion reuses the existing foundation and walls, which cuts cost and build time. See our garage conversion vs. ADU comparison for a full breakdown.
Q: How do I get started?
Book a free consultation. We’ll evaluate your property and give you a realistic cost and timeline.
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