2026 ADU Laws: A Ventura County Homeowner Guide

Start with the fee rules that affect your design

An ADU budget can change when a plan crosses a size threshold. In Simi Valley and Thousand Oaks, the same statewide fee rules apply, but the school district, utility provider and permit office still need to review your particular address. A smaller unit may reduce certain fees without eliminating the cost of permits.

The useful distinction in 2026 is between school fees and development impact fees. An ADU with 750 square feet of interior livable space can qualify for the state development impact-fee exemption. That does not make it exempt from school fees. The specific statewide school-fee exclusion applies to units with less than 500 square feet of interior livable space.

This guide focuses on the rules that change your planning decisions. It does not certify that a particular lot, floor plan or rental arrangement qualifies. Use the official sources at the end alongside a written response from the agencies serving your property.

SB 543: school fees and impact fees have different thresholds

Government Code section 66311.5 distinguishes the two fee categories. For development impact fees, the exemption covers ADUs with 750 square feet of interior livable space or less and JADUs with 500 square feet or less. For an ADU larger than 750 square feet, applicable impact fees must be proportionate to its size relative to the primary dwelling.

The school-fee provision uses a different boundary: less than 500 square feet of interior livable space. At exactly 500 square feet, do not assume that exclusion applies. A district may have an additional exemption or a clearance process, so request its written determination. A JADU being 500 square feet or less does not, by itself, prove all school fees are waived.

For example, a 600-square-foot ADU falls within the development impact-fee exemption but may still have school fees. An exactly 750-square-foot ADU also falls within that impact-fee exemption. At 751 square feet, the proportional impact-fee rule becomes relevant. These are size examples, not quoted savings or a city fee invoice.

An exempt impact fee does not mean a permit-free project

Building permits, plan review and inspections are separate from the development impact-fee exemption. Water and sewer connection or capacity charges also have their own rules. Some conversions within existing space qualify for additional utility-connection protections; a new detached unit may involve a different calculation.

Ask for a budget with separate lines for design and engineering, building and plan-check fees, school district clearance, utility work, any applicable impact fees and construction. Do not subtract a generic fee-savings figure from an online construction estimate. The relevant agencies must establish what the proposed project actually owes.

Changing a floor plan solely to avoid one fee can be a false economy. Compare the actual assessment with how the smaller layout works for a parent, adult child or future tenant. A practical bathroom, usable kitchen and comfortable circulation matter long after the permit is issued.

Permit clocks start at defined application stages

SB 543 requires a written completeness determination within 15 business days after an ADU or JADU application is received. If the agency finds the application incomplete, it must identify the missing items. That intake deadline is different from the later approval decision.

For qualifying applications on a lot with an existing single-family or multifamily dwelling, state law generally requires an approval or denial within 60 days of receiving a complete application. There are provisions for applicant-requested delays and applications submitted with a new primary dwelling. A complete application is not the same as an automatically approved design.

Design work, a survey, engineering, missing documents, utility coordination and construction sit outside that simple 60-day statement. Keep a dated copy of each submittal and correction response. A project schedule should identify the design phase, agency review, construction and final occupancy approval separately.

AB 462: coastal review still matters in Oxnard

AB 462 took effect October 10, 2025. Government Code section 66329 requires a decision on a completed coastal development permit application for an ADU within 60 days, with coastal and ADU review running concurrently under the statute. The responsible agency depends on whether a certified local coastal plan or program is in place.

The law does not remove the California Coastal Act. An Oxnard coastal property still needs a parcel-specific review of the coastal boundary, hazards, access and applicable standards. A street name, ocean view or a rough line west of a highway is not enough to determine the permit path.

The statute also limits appeals of the specified local decisions under Public Resources Code section 30603. That is narrower than promising that a project can never face a legal challenge. Ask Planning which approvals apply before treating a coastal unit as a straightforward inland build.

AB 1154: JADU bathroom and occupancy choices

A JADU is a distinct category with a maximum of 500 square feet of interior livable space within a single-family residence. Under the 2026 rules, a JADU that shares sanitation facilities with the primary home requires owner occupancy. When it has separate sanitation facilities, that particular owner-occupancy requirement does not apply.

JADU rentals must be for terms longer than 30 days. Do not base the design or financing on a short-term rental plan. Confirm the required recorded documents and intended occupancy with the permit office before choosing between an interior JADU, a garage conversion and a detached ADU.

AB 1154 is not a blanket exemption from fire or building requirements. Construction still needs a compliant design. If a property has fire exposure, access limitations or an older structure, have the appropriate professionals address those conditions in the plans.

SB 9: distinguish the 2025 bill from the earlier lot-split law

Bill numbers repeat in different legislative sessions. SB 9 enacted in 2025 addresses local ADU ordinance submission and response requirements involving HCD. It is not the 2021 SB 9 law commonly discussed for urban lot splits.

Do not assume an ADU approval automatically authorizes subdivision, separate sale or a specific number of new homes. Those are different questions. If the project involves splitting a Simi Valley or Moorpark property, ask Planning to identify the applicable process and the interaction with any proposed ADU or JADU.

Likewise, the available combination of converted, detached and junior units depends on the state-law category and qualifying conditions. Start with a site plan showing the existing legal structures. A promise of three additional units on every single-family lot is too broad.

Build a realistic budget before choosing the final size

For scale, our calculator scenario for a 600-square-foot detached ADU in Simi Valley produces roughly $240,000 to $330,000 as of September 16, 2026. That scenario uses standard finishes, one bedroom, one bathroom, laundry, a private entrance and a flat site without a selected new meter. It is a model output, not an average of completed jobs or a binding proposal.

Architectural and engineering services, address-specific agency charges, major utility upgrades, retaining walls and unusual soil conditions need separate verification. Compare bids against the same scope and reserve money for unresolved conditions. The estimate should explain who handles plans, permit fees, inspections and corrections.

CalHFA currently states that its ADU grant funds were fully reserved as of December 28, 2023. Do not reduce your budget by a presumed $40,000 award. Verify any new funding announcement and your eligibility directly with the program before counting it as available money.

Bring these questions to the first property review

First, identify the parcel and permitting jurisdiction. A mailing address does not always establish whether the city or unincorporated county handles the application. Then confirm existing permits, lot boundaries, easements, sewer or septic service, utility capacity and any coastal or hazard overlay.

Next, write down the intended use: family housing, a long-term rental or flexible space for later. Choose a size and layout that suit that use, then compare the cost consequences. Rental income and resale value depend on the property and market; neither follows automatically from construction cost.

Finally, ask for a written scope and schedule with assumptions. Clear allowances and exclusions are more useful than a low headline price. They give you a way to compare estimates and understand what must be decided before work starts.

Common homeowner questions