Accessory Dwelling Unit Builder You Can Trust in California
From design to permits to move-in day — Tree Builders Group builds custom ADUs that increase your property value, generate rental income, and create lasting space for your family. Licensed California Contractor | CSLB License 1091953 | Fully Insured
What Is an Accessory Dwelling Unit (ADU)?
An accessory dwelling unit (ADU) — also called a granny flat, in-law suite, backyard cottage, secondary dwelling unit, or secondary suite — is a self-contained residential unit built on the same lot as a single-family or multi-family home. ADUs have a private entrance, a full kitchen or kitchenette, a bathroom, and sleeping space, making them legally habitable as an independent dwelling.
California has made it significantly easier to build ADUs since 2017, with the state continuously relaxing zoning restrictions, reducing permit fees, and eliminating owner-occupancy requirements. Today, most homeowners with a qualifying lot can add an ADU — and many can add a Junior ADU (JADU) as well, for a total of two or three units on one property.
ADUs are the most practical tool homeowners have for increasing property value, generating passive rental income of $2,000–$4,000/month, housing aging parents or adult children, or creating a dedicated home office or studio.
Types of ADUs We Build
Tree Builders Group designs and constructs every category of accessory dwelling unit recognized by California law. We assess your lot, goals, and budget — then recommend the best path forward.
Detached ADU
A fully independent backyard structure with its own foundation, utilities, and private entrance. Maximum privacy — ideal for rental income or multigenerational living.
Attached ADU
An addition that shares a wall with the primary home. Shares structural systems and is ideal when backyard space is limited. Lower cost per square foot than detached.
Garage Conversion ADU
Transform your existing attached or detached garage into a fully permitted ADU. The most cost-effective option since the structure already exists — requires electrical, plumbing, insulation, and finish upgrades.
Junior ADU (JADU)
Up to 500 sq ft of habitable space carved from within your existing primary residence. Requires an efficiency kitchen and separate entrance. The fastest and most affordable ADU option.
Above-Garage ADU
Build a livable unit above an existing detached garage. Maximizes use of vertical space while preserving yard area. Popular in many Areas across California lots with limited footprint.
Interior Conversion ADU
Convert a finished basement, bonus room, or unused floor of your home into a legal ADU. Adds a private entrance and kitchenette while keeping construction costs lower than a new structure.
What Can You Use an ADU For?
Accessory dwelling units are one of the most flexible real estate assets a homeowner can build. Common uses include:
Rental Income
Multigenerational Living
Guest House
Home Office / Studio
Property Value Growth
Au Pair / Caregiver Suite
15+ Years of Local Experience
We've navigated San Jose, Santa Clara, Sunnyvale, Palo Alto, and every Bay Area city's building department. We know what gets approved — and what gets flagged.
Why Homeowners Choose Tree Builders Group for Their ADU
Full-Service Design-Build
One contract. One team. Architecture, engineering, permits, and construction — all under our roof. No finger-pointing between contractors when problems arise.
Fixed-Price Proposals
Our detailed written proposals lock in costs before work begins. No surprise change orders. No hidden fees. Transparent pricing is non-negotiable at Tree Builders Group.
AI-Powered 3D Design
Visualize your ADU before a single permit is filed. Our 3D design tool lets you walk through your future unit, change finishes, and adjust layouts in real time.
5-Star Rated Track Record
300+ completed projects across California. We maintain 5-star ratings on Google and Yelp because we treat every project like it's our own home.
Our ADU Construction Process — Start to Finish
Building an accessory dwelling unit in California involves design, permits, and construction — each with its own timeline and requirements. We manage all of it under one roof so you never have to juggle multiple contractors.
We visit your property, assess lot conditions, zoning, setbacks, and utility access, and identify the best ADU option for your goals and budget.
Our designers create detailed architectural plans and a 3D walkthrough so you can visualize every room, finish, and layout before we break ground.
We submit all applications, manage plan check revisions, and coordinate with your city's building department. In San Jose, we leverage the city's streamlined ADU portal.
Our licensed crews execute the build with weekly progress updates and on-site supervision. We manage all sub-trades — framing, MEP, finishes — on a fixed schedule.
We schedule all final city inspections, secure your Certificate of Occupancy, and complete a detailed walkthrough before handing you the keys.
Get Inspired by Projects Near You
Whether you own an architectural castle in the hills, a beachfront property along the Pacific coastline or a modest home close to the mountains, our residential home remodeling and construction professionals will exceed your highest expectations.
Accessory Dwelling Unit FAQs
What does it cost to build an accessory dwelling unit in the California?
Accessory dwelling unit costs in the California range from $80,000–$120,000 for a Junior ADU (JADU), $120,000–$150,000 for a garage conversion, $150,000–$350,000 for an attached ADU, and $250,000–$550,000+ for a new detached unit. San Jose permit fees typically run $8,000–$12,000. Construction costs in the California average $400–$650 per square foot in 2026, running 20–30% above the state average due to labor rates, seismic requirements, and local amendments.
How long does it take to build an ADU in California?
In San Jose and across the Bay Area, design and permitting typically takes 2–4 months, and construction takes 4–9 months depending on ADU type and complexity. Total timeline from first consultation to Certificate of Occupancy averages 8–14 months. Garage conversions and JADUs tend to be faster; new detached structures take longer due to foundation work and utility connections.
What is the difference between a detached ADU, attached ADU, JADU, and garage conversion?
A detached ADU is a fully separate structure in your backyard — most private and most expensive. An attached ADU shares a wall with the main home. A Junior ADU (JADU) is up to 500 sq ft carved from existing interior space, with an efficiency kitchen and separate entrance. A garage conversion transforms your existing garage into a legal ADU — the most cost-effective option.
Do I need a permit to build an ADU ?
Yes — all ADUs in California require a building permit. However, California law requires cities to ministerially (without discretionary review) approve ADU applications that meet objective zoning standards, which has dramatically simplified the process since 2020. Tree Builders Group handles all permit applications, plan check submissions, responses to city comments, and final inspections on your behalf.
Can I rent out my ADU? What rental income can I expect?
Yes. Bay Area ADUs commonly generate $2,000–$4,000 per month in rental income, depending on size, location, and finish level. California law allows ADU rentals with a minimum 30-day tenancy (short-term vacation rentals are subject to local rules). Your ADU must receive a Certificate of Occupancy before renting. San Jose does not require owner-occupancy for ADU rentals, making them viable investment properties.
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How much value does an ADU add to my home?
A professionally built ADU typically increases Bay Area property values by $150,000–$300,000+, representing a 20–35% increase for most properties. The exact uplift depends on ADU size, finish level, location, and whether it’s income-generating. ADUs also improve a home’s marketability by offering flexible use — rental income, multigenerational housing, or a home office.
What is a Junior ADU (JADU) and how does it differ from a regular ADU?
A Junior ADU (JADU) is a unit of up to 500 square feet created within the existing walls of a single-family home. It requires an efficiency kitchen (not a full kitchen), a separate exterior entrance, and in some cases may share a bathroom with the main home. JADUs are faster to permit and build than conventional ADUs and are often the most affordable path to adding a legal dwelling unit. California allows one JADU plus one standard ADU on most single-family lots.
Are there financial assistance programs for building an ADU?
Yes. The ADU Accelerator Program — a $2.5M state-funded initiative — offers rebates up to $15,000 for homeowners in 15 East Bay cities, with higher amounts for units deed-restricted to low-income households. Marin County extended ADU building fee waivers through 2026, with savings up to $10,000. Some cities also offer pre-approved ADU plan sets to reduce design costs and accelerate permitting. Ask our team which programs apply to your city.
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