ADU Builder & Home Additions in Topanga, CA
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Last modified: 2026-09-24T00:00:00.000Z
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Image: Detached backyard ADU with wood siding and black-framed glass doors at dusk
- Topanga
Design · Permit · Build
ADUs & Additions in Topanga
Detached ADUs, garage conversions and additions, from plans to move-in.
Book a Consultation Call 818-300-2393
Built for Topanga Homes
Topanga is a canyon rather than a town. Its homes gather in a handful of rural villages strung along Topanga Canyon Boulevard and Old Topanga Canyon Road, both mapped as scenic routes, and thin out across the hillsides in between. Most of the canyon sits inside the Santa Monica Mountains Coastal Zone, and nearly all of it is rated a Very High Fire Hazard Severity Zone.
A pool, terrace or guest house here is shaped by habitat mapping, septic layout and the rainy-season grading calendar before it is shaped by style, and settling those three early is what keeps a Topanga project moving.
Two patterns: the rural villages of Fernwood, Old Topanga, Topanga Oaks, Topanga Woods and the Old Post Office Tract, which the coastal program defines as antiquated mountain subdivisions, many created in the 1920s and often lacking infrastructure built to current standards, and scattered hillside parcels on the slopes between them.
In the villages, new development must be sited to avoid oak woodland, and where removal cannot be avoided it may not exceed 10 percent of the oak woodland on the property. Where new residential development is approved in H2 or H3 habitat, the building site, counting graded slopes, structures, decks, patios and other hard surfaces, is capped at 10,000 square feet or 25 percent of the parcel, whichever is less.
We are a design-build team, which means the people who draw your project are the people who build it. Design, engineering, permits and construction run through one project manager and our own crews, so nothing gets lost in a handoff between a salesperson and a subcontractor. We are licensed, bonded and insured, CSLB license #1119054 .
Permits in Topanga: Topanga is unincorporated, so building permits come from the County Public Works Building and Safety Division. Most of the canyon lies in the Santa Monica Mountains Coastal Zone, where Regional Planning issues coastal development permits under the certified Local Implementation Program; parcels at the northern end fall outside it, in the Santa Monica Mountains North Area.
Improvements to an existing, lawfully established home, pools included, can be exempt from a coastal permit, but not in H1 or H2 habitat, in an area designated as highly scenic, or where a septic system is added or expanded. Grading of 50 to 5,000 cubic yards needs a minor coastal permit, and grading is prohibited from October 15 through April 15 except in limited cases.
The program allows only one accessory dwelling unit or guest house per property, at no more than 750 square feet, on a septic system separate from the main house and outside H1 habitat, its buffer and the Quiet Zone, and each one must retire a transfer of development credit.
Permit details checked September 2026 against LA County Planning and LA County . Rules change, so we confirm them again for your parcel.
What We Build
- Coastal development permit and habitat review
- Grading scheduled outside the October 15 to April 15 rainy season
- Septic siting with stream and native tree setbacks
- Transfer of development credit for ADUs and guest houses
- Scenic route frontage and rooftop equipment limits
- Detached ADUs, ground-up
- Garage conversions to living space
- Junior ADUs within the main house
- Room additions and bump-outs
- Second-story additions
- Full kitchens and bathrooms in the ADU
- Separate utility metering where required
- Private entries, patios and landscaping
- Plans, engineering and city submittals
- Title 24 energy compliance
Local Conditions
What to Know in Topanga
These local conditions change the scope, the schedule and the budget.
01
CAL FIRE's State Responsibility Area map places nearly all of Topanga in a Very High Fire Hazard Severity Zone, and the coastal program asks new construction there for fire-resistant exterior materials, windows and roofing, with eaves and vents that resist flame and embers; County Fire fuel modification can also reach beyond the approved building site.
02
State seismic hazard maps, carried into the County's coastal program maps, place much of the canyon in earthquake-induced landslide zones, and in these mapped hazard areas a site-specific geologic, soils or geotechnical report is required before a coastal permit application is filed, so investigation comes ahead of pool, retaining wall and foundation design.
03
New septic leachfields must sit at least 100 feet from any stream and seepage pits at least 150 feet, and both must stay at least 50 feet outside the dripline of native oaks, sycamores, walnuts and bays, which constrains where a pool, terrace or guest house can fit on a lot.
04
Pools reviewed under the coastal program must use alternative sanitization, such as low- or no-chlorine systems, may not drain chlorinated water to a street, storm drain, creek or canyon, and within the program's hazard areas, which include Very High Fire Hazard Severity Zones, need double-wall construction with drains and leak detection.
Areas we serve in Topanga:FernwoodOld TopangaTopanga OaksTopanga WoodsOld Post Office Tract
Investment
What It Costs in Topanga
Real Southern California price ranges, not teaser numbers.
Detached ADU
$150,000 to $400,000+
What moves it: square footage, finish level, utility runs, and site access; garage conversions typically run well below a ground-up detached build
Garage conversion ADU
$80,000 to $180,000
What moves it: whether the existing slab and framing meet current code, how far utilities have to run, and the finish level of the kitchen and bathroom
Plans, drawings and permit management
$4,500 to $25,000+
What moves it: project type and size, whether structural engineering, Title 24 or a soils report is required, and how many jurisdictions or review bodies are involved
Your lot, access and finishes set the price. We walk the site, then show you where every dollar goes.
Get a line-item price for your Topanga property
The Process
How We Work in Topanga
01
Feasibility
We check lot size, setbacks, utilities and access to confirm what your property will actually support.
02
Design & 3D
Layout, elevations and finishes, modeled in 3D so you approve the unit before submittal.
03
Plans & Permits
We prepare the full drawing set, handle engineering and Title 24, and manage the city submittal and corrections. In Topanga that submittal goes to Los Angeles County Department of Public Works, Building and Safety Division.
04
Foundation & Framing
Site prep, foundation, framing and dry-in by our own crews.
05
Systems & Finishes
Electrical, plumbing, HVAC, insulation, then kitchen, bath and interior finishes.
06
Final Inspection
We pass final inspection, walk the unit with you and hand over the keys.
Questions
What Topanga Homeowners Ask
Who issues the permits for this work in Topanga?
Los Angeles County Department of Public Works, Building and Safety Division. Topanga is unincorporated, so building permits come from the County Public Works Building and Safety Division. Most of the canyon lies in the Santa Monica Mountains Coastal Zone, where Regional Planning issues coastal development permits under the certified Local Implementation Program; parcels at the northern end fall outside it, in the Santa Monica Mountains North Area.
Improvements to an existing, lawfully established home, pools included, can be exempt from a coastal permit, but not in H1 or H2 habitat, in an area designated as highly scenic, or where a septic system is added or expanded. Grading of 50 to 5,000 cubic yards needs a minor coastal permit, and grading is prohibited from October 15 through April 15 except in limited cases.
The program allows only one accessory dwelling unit or guest house per property, at no more than 750 square feet, on a septic system separate from the main house and outside H1 habitat, its buffer and the Quiet Zone, and each one must retire a transfer of development credit.
We prepare the drawings, make the submittal and handle plan-check corrections ourselves, so you are not the one chasing plan check.
What makes building in Topanga different?
Two patterns: the rural villages of Fernwood, Old Topanga, Topanga Oaks, Topanga Woods and the Old Post Office Tract, which the coastal program defines as antiquated mountain subdivisions, many created in the 1920s and often lacking infrastructure built to current standards, and scattered hillside parcels on the slopes between them.
In the villages, new development must be sited to avoid oak woodland, and where removal cannot be avoided it may not exceed 10 percent of the oak woodland on the property. Where new residential development is approved in H2 or H3 habitat, the building site, counting graded slopes, structures, decks, patios and other hard surfaces, is capped at 10,000 square feet or 25 percent of the parcel, whichever is less.
One local condition that shapes the work: CAL FIRE's State Responsibility Area map places nearly all of Topanga in a Very High Fire Hazard Severity Zone, and the coastal program asks new construction there for fire-resistant exterior materials, windows and roofing, with eaves and vents that resist flame and embers; County Fire fuel modification can also reach beyond the approved building site.
Which parts of Topanga do you work in?
All of it, including Fernwood, Old Topanga, Topanga Oaks and Topanga Woods. Our crews run every job out of our Cahuenga Pass base in Los Angeles, with our own people on site.
What does an ADU or addition cost in Topanga?
Detached ADU: generally $150,000 to $400,000+ across Southern California. What moves it is square footage, finish level, utility runs, and site access; garage conversions typically run well below a ground-up detached build. We give line-item pricing after seeing the site rather than quoting a number over the phone.
How long does an ADU take start to finish?
Plan a realistic year. Design and engineering typically take one to three months, city plan check and corrections add several more, and construction usually runs four to eight months depending on size and site access.
Is a garage conversion cheaper than a detached ADU?
Usually yes, and often substantially, because the foundation, walls and roof already exist. The trade-offs are that you lose the garage, and older slabs and framing sometimes need upgrading to meet current code.
Can I rent the ADU out?
California ADU law is designed to allow long-term rental, and an ADU is one of the few ways to add rentable square footage to a single-family lot. Short-term rental rules vary by city, so confirm your local ordinance before counting on that income.
Do you handle the permits?
Yes. Plans, engineering, Title 24 and the city submittal are all handled in-house, including responding to plan-check corrections. That is a service we sell on its own, so it is not something we hand off.
More Services in Topanga
Pool & Spa Remodeling in Topanga Backyard & Outdoor Living in Topanga Everything we build in Topanga
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Planning Guides
Before You Build in Topanga
Costs, materials and process notes from our own projects.
ADU Cost Breakdown by Size in Los Angeles A detached ADU in Los Angeles runs $150,000 to $400,000-plus. Here is the real cost broken down by unit size, with the soft costs, utility work and finish decisions that move the number. Read guide Garage Conversion to ADU: The Step-by-Step Build A garage conversion runs $80,000 to $180,000 in Southern California. Here is the real sequence, from slab testing and plan check to the raised floor, the mini-split and final sign-off. Read guide ADU Rental Income Math in the San Fernando Valley A one-bedroom ADU in the San Fernando Valley rents for roughly $2,100 to $2,900 a month. Here is the honest arithmetic on payback, carrying costs and what the unit adds to the property. Read guide Second Story Addition vs Building Out: Which Costs Less Building out runs $350 to $500 per square foot. Going up runs $450 to $700. Here is when the cheaper option is actually the wrong one, and what drives each number. Read guide
Planning an ADU in Topanga?
Sit down with your project manager and get an accurate estimate. We call you back within one business day.
Book a Consultation 818-300-2393
Licensed, bonded & insured · CSLB license #1119054
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