An accessory dwelling unit is the one addition California lets you build by right on nearly any single-family lot, and homeowners here use that right for granny flats, casitas, in-law suites and long-term rentals. Master Construction designs, permits and builds them under a single contract, Lic. #1027281, family owned since 1994, and that one contract is what separates ADU builders in Ventura County from framers who leave you to find an electrician. As of 2026 the state guarantees an 800 sq ft detached unit and a 15-business-day completeness check. The hard parts of an ADU here are the sewer lateral, the electrical panel and whichever overlay your lot sits in, and those get answered at the first site walk.
Four Ways to Add an ADU in Ventura County
A detached new build is the unit most people picture: a stand-alone casita in the back yard with its own door, kitchen and bath, for an owner who wants real separation. The state floor is 800 sq ft with 4 ft side and rear setbacks. Distance decides its cost: how far sewer, water and power travel from the house to the new slab.
An attached ADU shares a wall with the house and has its own exterior entrance. It suits narrow lots where a detached unit would eat the yard. State law caps it at half the floor area of the existing house, with the same 800 sq ft floor protecting smaller homes. Structure decides its cost. Tying a new roof and foundation into a 1960s house is where the money goes, which is residential construction work.
A garage conversion turns existing square footage into the unit and is the lowest-cost route on most tract lots. No setback applies, no replacement parking can be required, and the structure may grow by up to 150 sq ft for an entrance. The slab decides its cost: a garage floor sits low and is rarely insulated, so our home renovations crew reads it before anyone prices a kitchen.
A junior ADU is no more than 500 sq ft carved out of the house itself, with its own outside door and an efficiency kitchen. It can share a bathroom with the house and suits an in-law suite or a first-time landlord. The state exempts a JADU from parking, separate utility connections and impact fees. The kitchen decides its cost: its own bath and a real cooking wall make it a small remodel; a shared bathroom makes it mostly a door and a counter.
What the 2026 Rules Let You Build in Ventura County
State law, as of 2026, guarantees two units on any lot with a single-family house: one conversion unit inside existing space, and one new detached ADU of up to 800 sq ft with 4 ft side and rear setbacks, 16 ft tall or 18 ft near major transit. No city can refuse that pair. Beyond it a city sets its own maximums, never below 850 sq ft of interior livable space, or 1,000 sq ft for more than one bedroom, and a detached unit tops out at 1,200 sq ft of total floor area. No impact fees apply at 750 sq ft of interior livable space or less.
The clocks got shorter on January 1, 2026. Under SB 543 every permitting agency, water district and fire department included, has 15 business days to say whether the application is complete; miss it and the application is deemed complete. The older 60-day decision clock still runs from there, with 30 days for a detached unit on a pre-approved plan. No owner-occupancy requirement applies to an ADU, and under AB 1154 none applies to a JADU with its own bathroom. Cities can hold rentals to 30 days or longer.
Cities still decide design standards on units beyond the by-right pair. In the coastal zone, which covers parts of Ventura, Oxnard and Port Hueneme, an ADU still needs a coastal development permit, though since AB 462 it runs on the same 60-day clock with no hearing. Inland, the 2025 fire maps put more of Moorpark, Simi Valley, Thousand Oaks and Ojai in Very High severity zones, where a new detached unit is built to the WUI code (formerly Chapter 7A): Class A roofing, ember-resistant vents, tempered glass.
Ventura County and the City of Ventura offer the same three free detached plans at 700, 900 and 1,188 sq ft, and Moorpark posts the same three sizes as structural-only sets. Oxnard accepts the county set. Thousand Oaks runs a gallery of 17 plans, ten of them prefabricated units already reviewed for code. Ojai offers a free one-bedroom. Camarillo and Simi Valley publish no plan library, we found none for Fillmore or Port Hueneme, and Santa Paula's are still marked draft. A pre-approved plan trims plan check, not site work. Site work is what ADU builders in Ventura County are hired for, and our design and planning team starts there.
Utilities Are the Number People Miss on an ADU
Sewer first. A backyard unit normally ties into the house's existing lateral, and plan check looks at its capacity and cleanout, so we camera the line at the site walk. On septic, in Somis, the Ojai Valley outside the sewer and the hills above Santa Paula, Environmental Health sizes the system by bedrooms and fixtures, and the ground around the tank and leach field must stay open to the sky, which can decide where the unit goes. A conversion or JADU stays on the house water meter; a new detached unit may owe its own connection, and our plumbing crew runs it either way.
The main panel is the threshold question on an older house. Many homes here still carry 100-amp service, and an all-electric unit with a heat pump and induction range often does not fit on it. A subpanel off the main is the usual answer; when it is not, an SCE service upgrade is, and SCE sets its own field schedule, so we order it early. Our electrical crew runs the load calc at the site walk. A separate meter is optional, and so is gas; many new detached units go all-electric.
Solar comes with the building. For permits pulled since January 2026 a newly built detached ADU needs its own new PV system; the house's array does not count, and our solar crew sizes it with the load calc. Attached units and garage conversions carry no solar mandate. Sprinklers cannot be required unless the house has them. Then the dirt: the trench from house to unit, grading that keeps runoff off the slab, and the concrete itself.
One Contract From ADU Drawings to Keys
As ADU builders in Ventura County we run every unit through five steps under one contract.
- Feasibility read. We walk the lot, camera the lateral, read the panel and check the zone, coastal, fire and septic maps, then tell you which unit type fits.
- Design and drawings. We draw the unit or adapt a pre-approved plan, with structural, Title 24 and solar sheets and a site plan the city and fire department can approve.
- Permit and plan check. We file with the city or County RMA plus any coastal, fire or Environmental Health desk and answer every correction inside the state clocks.
- Build. Foundation, framing, roof, rough trades, insulation, drywall and finishes on one schedule; trenching and any SCE upgrade go first.
- Inspections and keys. Final inspection, certificate of occupancy and a walkthrough with the punch list closed before keys change hands.
Property managers, HOA boards and investors get the same process. On an existing multifamily lot the state allows conversion units up to a quarter of the unit count, plus up to eight detached units and never more than the units already there. Investors get a written fixed-price scope and a schedule for the pro forma. Boards get a review package built around Civil Code 4751 and standing certificates of insurance, like our HOA and property maintenance clients. Request a quote or call (805) 667-8800.
- Detached backyard ADUs & casitas
- Garage conversions
- Attached ADU additions
- Junior ADUs inside the house
- Pre-approved plan adaptation
- Design, permits & plan check
- Panel upgrades & utility trenching
- Multifamily & investor ADU programs
Proudly serving Ventura, Oxnard, Camarillo, Thousand Oaks, Newbury Park, Simi Valley, Moorpark, Agoura Hills, Westlake Village, Santa Paula, Fillmore, Ojai, Port Hueneme, Somis, and Calabasas, plus all of Ventura & Los Angeles Counties.