In most of Ventura County, a detached ADU can be up to 1,200 sq ft — Simi Valley and Oxnard both allow it. State law guarantees every homeowner at least 850 sq ft for a studio or 1-bedroom and 1,000 sq ft for 2+ bedrooms, and no city can block an 800 sq ft ADU using lot coverage rules. Setbacks: 4 feet side and rear. Height: 16 feet baseline, up to 18–25 feet in specific cases. Attached ADUs are capped at 50% of your home's floor area. JADUs max out at 500 sq ft. A 1,000 sq ft detached unit runs roughly $280,000 to $420,000 all-in to build.
Two numbers decide almost everything about an ADU project: how many square feet you're allowed to build, and what each of those square feet costs. Homeowners usually ask us the second question first. It's the wrong order. Until you know your legal size ceiling, you can't price anything — and the ceiling depends on which side of a city boundary your lot happens to sit on.
The good news: California has spent a decade steadily overriding local size restrictions, and the 2026 rules are the most homeowner-friendly they've ever been. Cities can't ban ADUs, can't demand giant setbacks, and can't use lot coverage math to squeeze your unit down to a shed. But each Ventura County city still sets its own caps above the state floor, and the difference between an 850 sq ft ceiling and a 1,200 sq ft ceiling is an entire second bedroom — which is also the difference between $2,000/month and $2,800/month in rent.
We've built ADUs across Ventura County for over 20 years (CA License #1066117, 5.0 stars on Google), so we deal with these limits at the permit counter, not in the abstract. Here's the full 2026 picture: the state rules that protect you everywhere, the city-by-city caps, and how to pick a size that actually pencils.
The State Floors: What Every City Must Allow
Start with the rules that apply no matter where you live in Ventura County. California Government Code sets minimum ADU sizes that cities must permit — a city can be more generous, but never less:
- 850 sq ft minimum for a studio or 1-bedroom ADU
- 1,000 sq ft minimum for an ADU with 2 or more bedrooms
- 1,200 sq ft is the common upper cap cities adopt for detached units — the state default where a city has no compliant ordinance of its own
Then there's the single most important protection in the entire code: the 800 sq ft by-right rule. No city may use lot coverage, floor area ratio, open space, or minimum lot size standards to prevent you from building an ADU of at least 800 sq ft, with 4-foot side and rear setbacks, at 16 feet of height. Small lot, big house, tight coverage limits — doesn't matter. 800 sq ft is essentially guaranteed on any residential lot in the county.
Recent legislation keeps widening the lane. SB 1211 (effective January 2025) expanded detached ADU allowances on multifamily lots from two units to as many as eight, and barred cities from demanding replacement parking when you convert existing parking to an ADU. AB 2221 cleaned up definitions and forced cities to process ADU applications faster. For the full rundown of what changed and what it means locally, see our 2026 California ADU law changes guide.
Detached vs. Attached vs. JADU: Different Size Math
The size rules change depending on how your ADU relates to the main house:
Detached ADU — up to 1,200 sq ft
A standalone building. The 50% rule does not apply — you can put a 1,200 sq ft ADU behind a 1,400 sq ft house in cities that allow the full cap. This is why detached units dominate for rental income: they're the only path to a true 2-bedroom in most yards.
Attached ADU — 50% of the primary dwelling
Shares a wall with your house. Capped at 50% of the primary home's floor area — a 1,600 sq ft house allows an 800 sq ft attached ADU. The exception: cities must allow an 800 sq ft attached ADU even when 50% of your house would be less than that.
JADU — 500 sq ft max
Carved out of your existing home, with its own entrance and an efficiency kitchen. Capped at 500 sq ft. The kicker: state law lets you build a JADU and a full detached ADU on the same single-family lot — two rentable units from one property.
Garage conversions get their own advantage: an existing structure converted to an ADU keeps its footprint and setbacks, even if the garage sits right on the property line. If your garage is already there, that's often the cheapest square footage you'll ever add — our garage conversion ADU guide covers the details.
Height Rules: 16, 18, or 25 Feet
Height limits come in three tiers under state law:
| Height Limit | When It Applies |
|---|---|
| 16 feet | The baseline every city must allow for a detached ADU — a standard single-story unit |
| 18 feet | Detached ADUs within a half mile of major transit, or on multifamily lots with an existing multistory building (+2 ft allowed to match the primary home's roof pitch) |
| 25 feet | Attached ADUs — up to 25 feet or the underlying zone's height limit, whichever is lower. This is how second-story ADUs over garages get built |
Practical translation for most Ventura County backyards: plan on a single-story detached unit at 16 feet. Two-story detached ADUs are possible in some zones, but they trigger design review in several local cities and cost meaningfully more per square foot. If you need the space, going wider is almost always cheaper than going up.
Setbacks and Lot Coverage
State law caps side and rear setbacks at 4 feet for new detached ADUs — a fraction of the 10–20 foot setbacks that used to kill backyard projects. Front setbacks still follow local zoning, but a city can't apply them in a way that prevents an 800 sq ft unit.
- New detached ADU: 4 ft minimum from side and rear property lines
- Conversion of an existing structure: zero new setbacks required — the existing footprint is grandfathered
- Fire separation: building code still requires distance or rated construction between structures, typically 6–10 ft between the ADU and the main house for the simplest path
Lot coverage is where cities used to quietly shrink projects: a 40% coverage limit on a small lot with a big house could zero out your buildable area. That's exactly what the 800 sq ft by-right rule eliminates. Coverage limits can still trim a proposed 1,200 sq ft unit down on a tight lot — but they can never take you below 800.
City-by-City: ADU Size Caps Across Ventura County
Here's where the local caps stand in 2026. Ordinances get amended, so treat this as your starting map and confirm with the planning counter before you design — in Simi Valley that's the Building & Safety counter at 2929 Tapo Canyon Rd.
| Jurisdiction | Detached ADU Cap | Notes |
|---|---|---|
| Simi Valley | Up to 1,200 sq ft | 16 ft height for detached units; 1 ADU + 1 JADU allowed on single-family lots. Permit counter: 2929 Tapo Canyon Rd |
| Oxnard | Up to 1,200 sq ft | Among the county's most permissive; Coastal Zone parcels add review time |
| Thousand Oaks | 1,000–1,200 sq ft depending on zone and bedrooms | Published guidance varies; state floors of 850/1,000 sq ft always apply. Confirm with Community Development before designing above 1,000 sq ft |
| Camarillo | State defaults: 850 sq ft (studio/1BR), 1,000 sq ft (2+BR) | We couldn't verify a published cap above the state floor, so plan to the state defaults and ask Planning about larger units |
| Moorpark | State defaults: 850 sq ft (studio/1BR), 1,000 sq ft (2+BR) | ADU-friendly city with minimal design restrictions; confirm the current cap for units over 1,000 sq ft |
| Unincorporated Ventura County | Up to 1,200 sq ft (typical, non-coastal zones) | County RMA handles permits; hillside, septic, and fire-zone parcels carry extra site requirements |
One pattern worth noticing: the state floors do most of the work. Even in the most conservative jurisdiction, a 2-bedroom, 1,000 sq ft ADU must be approved on a code-compliant application. The city cap only matters if you want that last 200 sq ft.
What Size Should You Actually Build?
Legal maximum and smart size aren't the same number. ADU cost doesn't scale linearly with square footage — the kitchen, bathroom, utility connections, and foundation mobilization cost roughly the same whether the unit is 500 or 1,200 sq ft. That means small units carry the highest cost per square foot, and each added foot of bedroom or living space is comparatively cheap.
New-construction ADUs in Ventura County average $250 to $400 per square foot depending on finishes, site conditions, and complexity. Here's how that plays out all-in (construction plus permits, design, and utility connections) at common sizes:
| Detached ADU Size | Typical All-In Cost | What You Get |
|---|---|---|
| 500 sq ft | $150,000–$220,000 | Studio or tight 1BR; highest $/sq ft |
| 750 sq ft | $210,000–$320,000 | Comfortable 1BR with real living space |
| 1,000 sq ft | $280,000–$420,000 | True 2BR — the rental income sweet spot |
| 1,200 sq ft | $330,000–$500,000 | 2BR/2BA; only where the city allows the full cap |
Our rule of thumb after 20+ years of building these: if the lot and budget allow, get to two bedrooms. In Ventura County a 600–800 sq ft detached ADU rents for $2,000 to $2,800/month, and a 2-bedroom unit sits at the top of that range while renting faster and to a deeper tenant pool. If the budget stops short of 1,000 sq ft, a well-planned 750 sq ft 1-bedroom beats a cramped 850 sq ft unit trying to fake a second bedroom.
Full pricing detail — garage conversions ($80,000–$150,000), JADUs ($50,000–$80,000), and line-by-line cost breakdowns — is in our complete 2026 ADU cost guide.
Want a number for your specific lot and size? Our free ADU cost calculator gives you a personalized range in a couple of minutes, or run a full project estimate at SafewayQuickQuote.com. Prefer to talk it through? Call (805) 222-6544.
Three Size Mistakes That Cost Real Money
Designing to the cap before checking the lot
A 1,200 sq ft unit that ignores fire separation, easements, or a septic field comes back from plan check with a redesign bill. Measure the buildable envelope first, then size the unit.
Building small to save money
Dropping from 1,000 to 600 sq ft doesn't cut cost by 40% — the kitchen, bath, and utilities dominate small budgets. You give up a bedroom's worth of rent for a modest saving.
Forgetting the second story triggers
Going above 16 feet can mean design review, neighbor pushback, and structural cost that going wider avoids. Only go up when the lot truly can't go out.
Frequently Asked Questions
How big can an ADU be in Ventura County?
Detached ADUs can reach 1,200 sq ft in most local cities, including Simi Valley and Oxnard. At minimum, state law forces every city to allow 850 sq ft for a studio/1-bedroom and 1,000 sq ft for 2+ bedrooms. Attached ADUs are capped at 50% of the main home's floor area (with an 800 sq ft minimum allowance).
What ADU size can no city deny?
800 square feet. No city can use lot coverage, floor area ratio, open space, or lot size rules to block an 800 sq ft ADU with 4-foot side/rear setbacks at 16 feet of height. It's the by-right floor on essentially every residential lot in the county.
How tall can an ADU be?
16 feet is the detached baseline. 18 feet applies within a half mile of major transit or on multistory multifamily lots (+2 ft to match the primary roof pitch). Attached ADUs can go to 25 feet or the zone's height limit, whichever is lower.
What are ADU setback requirements?
4 feet from side and rear property lines for new detached units. Conversions of existing structures — like garage conversions — need no new setbacks; the existing footprint is grandfathered even at the property line.
How much does a 1,000 sq ft ADU cost in Ventura County?
Roughly $280,000–$420,000 all-in, based on new-construction averages of $250–$400 per square foot plus permits, design, and utility connections. Finishes and site conditions set where you land in the range.
What's the maximum size for a JADU?
500 sq ft, carved out of the existing home with its own entrance and efficiency kitchen. Typical cost is $50,000–$80,000 — and you can build a JADU in addition to a full ADU on the same single-family lot.
Can my ADU be bigger than 50% of my house?
Attached: no — 50% of the primary home's floor area is the cap, though 800 sq ft must be allowed regardless. Detached: yes — the 50% rule doesn't apply, so a 1,200 sq ft ADU behind a smaller house is legal where the city allows the full cap.
Find Out What Your Lot Allows
The size rules are on your side — the question is what your specific lot, budget, and city allow. We've designed and built ADUs across Ventura County for over 20 years, CA License #1066117, 5.0 stars on Google. Start with a free estimate from our ADU cost calculator or see our full ADU construction services.
Serving Simi Valley, Thousand Oaks, Moorpark, Camarillo, Oxnard, Ventura, Newbury Park, and Agoura Hills.