ADU Cost in Simi Valley: Budget and Permit Guide

Compare the same scope before comparing ADU prices

A low ADU price is hard to judge without knowing what it includes. For a Simi Valley homeowner comparing a garage conversion with a detached unit, the structure, utility work and design scope matter as much as square footage. The same is true in nearby Moorpark: two similar floor plans can involve very different site work.

Start by identifying the intended use. Housing a parent, creating space for an adult child and building a long-term rental can lead to different bathroom layouts, privacy needs and finish choices. The useful budget is the one attached to a design that serves that purpose.

We use the scenarios below to show how an early model changes by type and size. They are not a claim about completed Safeway jobs, neighborhood averages or a price available without a site review. A written estimate needs a defined scope, actual site conditions and confirmed fees.

Three calculator scenarios for an initial budget

As of September 16, 2026, our calculator models a 400-square-foot Simi Valley garage conversion at roughly $130,000 to $180,000. The scenario assumes standard finishes, one bedroom, one bathroom, laundry and a private entrance. It assumes a flat site and does not select a new separate meter.

With the same basic selections, a 600-square-foot detached ADU is roughly $240,000 to $330,000. A 1,000-square-foot detached unit is roughly $355,000 to $485,000. These rounded ranges reflect the calculator’s assumptions; the larger example is a size comparison, not a recommended one-bedroom layout.

The model includes construction categories and provisional permit allowances. Architectural design, engineering, surveys, actual utility charges, difficult soils, major retaining work and other project-specific costs need separate verification. Do not combine the low end of one scenario with the inclusions of another.

What a garage conversion still needs

An existing garage can avoid some new structural work, but its condition determines the savings. The slab may need moisture or elevation work. Walls and roof may need repairs or insulation. The plumbing route, ceiling height, openings and required separation from other spaces also affect the scope.

Ask the team to inspect the structure before treating it as a ready-to-finish shell. A conversion estimate should explain which existing elements stay, what needs alteration and what remains unknown until inspection or selective opening. Photograph those conditions so competing proposals address the same work.

Consider what happens to storage and daily access as well. State parking exemptions can affect formal requirements, but losing the garage still changes how the household uses the property. Include any separately approved storage or exterior work in the overall budget rather than assuming it is part of the ADU price.

Why a detached unit changes the cost

A detached ADU needs a foundation, building envelope, roof and utility routes to a separate structure. Equipment access, trenching distance, drainage and the relationship to the existing home can add work that a floor plan does not show. Have those routes drawn before comparing final bids.

The smaller unit does not cost exactly half as much as one twice its size. Both can need a bathroom, kitchen, electrical equipment, permitting and mobilization. Adding square footage may change the budget differently from adding a second bathroom or moving the unit farther from the sewer connection.

A hillside or constrained site needs an actual review. We do not assign a fixed percentage premium simply because a property is in Wood Ranch, Big Sky or another named neighborhood. The survey, soils information, access and required engineering determine whether that parcel carries extra work.

Simi Valley permits and school district clearance

The city’s Planning Division provides the current ADU resources and notes that its ordinance and development standards are undergoing updates. Start with those current links and verify requirements with Planning. Older handouts can remain online after state law changes.

For 2026, Government Code section 66311.5 exempts ADUs with 750 square feet of interior livable space or less from development impact fees. It separately provides a school-fee exclusion for units below 500 square feet of interior livable space. A 600- or 750-square-foot unit is not automatically school-fee exempt under that statewide provision.

Local school district guidance may provide additional exemptions or require a clearance document even where no payment is due. Request the district’s current rate and clearance requirements in writing for your address and proposed unit size.

Permit review, inspections and utility charges also need their own assessment. A quote should identify allowances versus confirmed amounts and state who pays each agency. The calculator cannot establish the district boundary, prove eligibility for an exemption or replace the final invoice.

Budget the time in separate phases

For planning, the calculator uses about 4 to 5 months of active construction for a garage conversion and about 6 to 11 months for a detached unit, depending on its size and scope. These are indicative model assumptions. They are not the full time from the first design conversation to move-in.

Feasibility, design, engineering and agency review come first. State law generally requires a written completeness decision within 15 business days and a decision on a qualifying complete ADU application within 60 days, with statutory exceptions. That does not promise a permit 60 days after an unfinished concept is submitted.

Final inspections, utility activation and any required occupancy approval matter at the other end. Before committing to a lease date or a relative’s move, review the critical items with the team. Keep the schedule updated when the scope, material selections or agency requirements change.

JADUs have different design and occupancy rules

A junior accessory dwelling unit is not simply any small cottage. It is a qualifying unit within a single-family residence, with a maximum of 500 square feet of interior livable space. Confirm the category with the designer and city before applying a JADU budget or checklist.

Under the 2026 rules, sharing sanitation facilities with the primary home triggers the JADU owner-occupancy requirement. A JADU with separate sanitation facilities does not have that same requirement. If rented, JADUs must have rental terms longer than 30 days.

The best option depends on the existing layout and how the household will use it. A shared bathroom can reduce independence even when it is lawful. Compare privacy, sound separation, circulation and the practical effect on the main residence before choosing a smaller unit solely for its starting cost.

Keep rental, tax and financing assumptions separate

An ADU may create rental income or improve flexibility for family, but neither a rent amount nor resale value follows directly from its build cost. Use current comparable units and advice from a local leasing or valuation professional. Account for the features of the actual proposed unit.

As an arithmetic example, $2,400 in monthly rent collected for 11 months produces $26,400 in gross annual income. It is not a Simi Valley rent forecast. Subtract vacancy beyond that assumption, maintenance, insurance, taxes, utilities, management and financing before considering cash flow.

Ask the Ventura County Assessor how the new construction affects the assessment and discuss the resulting tax estimate with a qualified adviser. A construction calculator cannot predict a household’s tax bill. Review loan terms, fees and payments with the lender using the full project budget.

CalHFA currently states that its ADU grant funds were fully reserved as of December 28, 2023. An old page describing a $40,000 grant does not establish money available for a new project. Verify a current award before using grant funds in a financing plan.

How to compare written ADU proposals

Give each bidder the same plans and scope. Ask whether the price includes demolition, foundation work, utility trenching, finishes, appliances, site protection and cleanup. Identify the party responsible for plans, engineering, permits and utility-provider coordination.

Check allowance amounts for items you have not selected. Record what happens if concealed damage, unsuitable soils or a required service upgrade changes the work. An agreed written change should describe the added or removed scope, its price and its schedule effect before the changed work proceeds.

Finally, keep a separate contingency for unresolved conditions and avoid spending it on optional upgrades at the outset. Once the design, inspections and agency assessments close those questions, the budget becomes more dependable. That process is more useful than treating a broad online range as a fixed-price commitment.

Common homeowner questions