In Orange County in 2026, a Junior ADU starts around $85,000, a garage conversion runs $145,000 to $200,000, and a ground-up detached unit runs $245,000 to $440,000.
Before any of that, the unit's size decides how impact fees are treated. It is worth understanding early, because it is settled on the plan set rather than on site.
The 750 square foot line
State law exempts an ADU of 750 square feet or less of interior livable space from impact fees, and a junior ADU of 500 square feet or less. This sat in Government Code section 66324 and was renumbered to 66311.5 by SB 543.
Above 750 square feet the exemption ends, but it does not become a cliff. Where fees are charged they have to be proportionate to the primary dwelling by square footage, so a unit slightly over the line is charged on its size rather than handed a full fee.
What that changes is the question worth asking. Not "how do I stay under 750" but "what do the fees on this specific unit come to, from this specific agency". Ask for the fee categories by name during feasibility: permit review, utility connection, school, and impact. They are separate charges from separate bodies, and an exemption in one says nothing about the others.
Garage conversion
The cheapest path to a real unit, and the one with the most variation in what people are quoted.
Published ranges disagree. Some price a standard 400 square foot two-car garage at $200 to $300 per square foot, or $90,000 to $120,000. Others put an Orange County garage conversion at $145,000 to $200,000. Both are honest numbers describing different scopes. The low figures assume an attached garage on a slab that passes inspection. The high ones carry a foundation fix, a full kitchen, and a bathroom where no plumbing ran before.
Four things move it:
- Attached or detached. A detached garage needs new utility lines run underground to it, which adds 20 to 30 percent.
- The slab. Garage slabs slope to the door and sit lower than the house. Levelling it, or adding a subfloor over it, is common and seldom appears in the first quote.
- Ceiling height. Many garages are close to the minimum for habitable space. Raising a roof turns a conversion into something closer to new construction.
- The panel. A second dwelling adds load. An older 100-amp service often will not carry it, and a load calculation is what establishes whether yours does.
Permit fees on a garage conversion run $1,200 to $2,000 depending on the city.
Detached new construction
$245,000 to $440,000 and up in Orange County. You are building a small house: foundation, framing, roof, envelope, all trades, all finishes, plus utility runs from the main house and site work for access.
The advantages are that you choose the dimensions, the orientation and the light, none of which a garage lets you do. It also leaves you the garage.
Cost per square foot drops as the unit grows, which pulls against the 750 square foot fee threshold. That tension is the real design conversation on a detached ADU, and it is worth having with actual numbers rather than instinct.
Junior ADU
From about $85,000. You carve a JADU out of the existing house. It caps at 500 square feet and shares utility connections with the main dwelling. It can have a small kitchen and may share a bathroom.
Cheapest to build and the hardest to rent. Worth considering for family. Worth modelling twice as an income unit.
Rules change by city
Orange County is thirty-four cities, and the ADU ordinances differ. Setbacks, height limits, parking, owner-occupancy language, design review, and how long the planning counter takes to respond are all local.
State law sets a floor cities cannot go below, and the state has pulled several Orange County cities back into line with it over the past few years. Keep that in mind reading older advice. An article from 2021 about what your city allows is out of date.
Check your own jurisdiction before you fall in love with a plan.
What the budget covers
A complete ADU number should include design, structural engineering, Title 24 energy compliance, permit fees, utility connections, construction, and finishes. It excludes landscaping restoration, furniture, and the fence you will want once the unit is there.
Ask any bid whether the utility work is inside the number. On a detached unit, trenching for water, sewer, gas and electrical is real money, and it is the line most often left as an allowance.
Timeline
Design and engineering take one to three months. Permitting varies by city and can run two to six months in Orange County. Construction takes three to six months for a conversion and four to eight for a detached build.
Plan on a year from first drawing to occupancy for a detached unit, and less for a conversion. Anyone promising much faster is either skipping permits or has not met your city yet.
Common questions
Do I need to live on the property? State law removed the owner-occupancy requirement for most ADUs, though the rules for JADUs differ and cities have their own language. Confirm with your jurisdiction rather than with an article.
Will an ADU raise my property taxes? The county reassesses the new construction, not the whole property. You pay tax on the added value of the unit, not on a fresh valuation of the house.
Can I build an ADU and a JADU on the same lot? In many cases yes, on a single-family lot. State law sets the combinations and each city refines them.
What is the cheapest real ADU? A JADU carved from existing space, then an attached garage conversion. The moment a project needs new foundation, new roof, or utility trenching, the floor of the budget moves up.
The figures here are Orange County market ranges for planning, not a quote. Pricing for a specific property comes from a site visit and a written scope.
VARENTO is a design-build firm working across Orange County and Los Angeles. Our ADU work covers feasibility, design, permitting and construction with one team. If you are weighing Los Angeles instead, we have a separate breakdown of ADU costs in Los Angeles.