How to Plan an ADU in Altadena: Permits, Costs & Timeline
Accessory Dwelling Units (ADUs) are becoming one of the most popular home additions in California, especially in areas like Altadena and the San Gabriel Valley, where space, flexibility, and long-term investment value matter.
Whether you're thinking about building a detached guest house, converting a garage, or adding a rental unit to boost your income, planning your ADU project properly is essential.
Here’s your step-by-step guide to building an ADU in Altadena, including what permits you’ll need, what to budget for, and how long the process typically takes.
What is an ADU?
An Accessory Dwelling Unit (ADU) is a fully functional, secondary housing unit built on the same lot as your primary residence. In Altadena, these can include:
Detached ADUs (backyard cottages)
Attached ADUs (built as an addition to your home)
Garage conversions
Basement or attic conversions
Junior ADUs (JADUs) with partial kitchen and living space
ADUs are highly sought-after for multigenerational living, long-term rentals, Airbnb income, or simply adding resale value.
Step 1: Understand Altadena ADU Zoning Laws & Permits
Altadena falls under Los Angeles County jurisdiction, so homeowners must comply with L.A. County’s ADU regulations, which were updated in response to California's pro-housing legislation (SB 9, AB 68, etc.).
Key regulations:
Minimum lot size: No minimum for one ADU.
Maximum size: Up to 1,200 sq. ft. (detached), 50% of existing home (attached).
Setbacks: At least 4 feet from side and rear property lines.
Parking: Often not required if you're near public transit or converting a garage.
Height limits: Generally up to 16 feet; higher if attached or above a garage.
💡 Tip: Mareci & Steele Construction works directly with L.A. County inspectors to ensure all plans are compliant from Day 1.
Step 2: Know the Costs
ADU construction costs vary depending on size, finishes, utilities, and site conditions. In Altadena, typical starting costs are:
ADU Type
Estimated Cost Range
Garage Conversion
$90,000 – $140,000
Detached ADU (400–800 sq. ft.)
$150,000 – $250,000+
Larger Custom ADU
$250,000 – $400,000+
What influences the price?
Sewer, water, and electrical hookups
Foundation or slab work
Upgraded finishes or custom features
Structural challenges (slopes, soil, setbacks)
Permit and plan fees
We offer transparent budgeting from the start, so there are no surprises.
Step 3: Timeline, How Long Does It Take?
An ADU build can take 6 to 12 months from start to finish.
Phase
Time Estimate
Initial consultation & design 2–4 weeks
Permits & plan submission 1–3 months
Construction 4–6 months (depending on complexity)
Final inspections & occupancy 2–4 weeks
At Mareci & Steele Construction, we streamline every phase, from site planning and engineering to permitting and construction, so you’re never left guessing.
Step 4: Why Build with Mareci & Steele?
Local knowledge of Altadena and San Gabriel zoning rules
Design-build team: One point of contact from idea to execution
Permit handling: We deal with L.A. County so you don’t have to
Fully licensed and insured general contractors
Hands-on project management and transparent timelines
From garage conversions to new detached units, we’ve helped homeowners across all Los Angeles County, Altadena, Pasadena, Sierra Madre, and the San Gabriel Valley turn their ADU vision into a reality.
Ready to Build Your ADU in Altadena?
Contact Mareci & Steele Construction today for a free consultation on your ADU project.
📞 Call: (626) 724-4093
📍 Based in Altadena, serving all of San Gabriel Valley
🔗 www.mareciandsteeleconstruction.com