What drives ADU cost in Los Angeles: unit type, sewer and utility runs, site access, fees and finishes. A licensed LA ADU builder explains each factor.
What an ADU costs in Los Angeles depends first on the type of unit (a garage conversion, a junior ADU, an attached unit or a detached new build) and then on site work: sewer route, electrical capacity, foundation and access to the backyard. Size and finishes matter less than most people expect, because even a small unit needs a full kitchen, a bathroom, a foundation and utility connections. Here is how the budget actually breaks down on the ADUs we build across the Valley.
The type of ADU sets the baseline
Before we talk square footage or finishes, we settle what kind of unit the lot supports. The size and setback rules for each type are covered in our ADU rules guide for Los Angeles. The cost picture looks like this:
ADU typeWhat you are paying forRelative costMain cost riskJunior ADUConverting space inside the existing house, sharing some systemsLowestFire separation and a separate entranceGarage conversionReusing an existing shell: slab, walls and roof if they are soundLow to moderateSlab condition, footings, roof framing, ceiling heightAttached ADUNew foundation, walls and roof tied into the main houseModerate to highRoof tie-in and matching the existing houseDetached new buildEverything new, from foundation to roof, plus long utility runsHighestSite work, sewer route and access
A garage conversion is usually the least expensive way to get a full, independent unit, but only if the garage is worth keeping. When the slab is cracked and out of level or the walls sit on no real footing, the savings shrink fast. Our ADU and garage conversion page covers how we evaluate an existing garage.
Site work and utilities: the costs you cannot see on a floor plan
Two ADUs with identical floor plans can land far apart in cost because of what happens between the unit and the street. On jobs like this we usually spend the first site visit on these items, not on finishes:
- Sewer. Where the existing sewer line runs, how deep it is, and whether the new unit can drain by gravity. A unit at the back of a lot that sits lower than the main line may need an ejector pump.
- Electrical capacity. Many older LA houses still have small panels. Adding a unit with a kitchen, laundry and heat pump often means a panel upgrade and sometimes a service upgrade coordinated with LADWP.
- Water and gas. Tapping the existing water line is common. Many new ADUs go all-electric, which avoids a new gas run but puts more load on the electrical service.
- Access. A narrow side yard means materials are carried by hand and excavation uses small equipment. Concrete may need to be pumped. All of that adds labor.
- Slope and soil. Hillside lots can require a soils report, deeper foundations, grading and retaining walls.
- Trees and obstacles. Protected trees, overhead power lines and easements can push the unit into a less convenient spot.
Size, design and finishes
Kitchens and bathrooms are the most expensive rooms per square foot in any building, and every ADU has at least one of each. That is why a small ADU does not cost proportionally less than a larger one: the fixed costs of foundation, utilities, kitchen and bath stay, while only the cheaper bedroom and living space shrink.
Design costs vary with the route you choose. LADBS runs a Standard Plan Program with pre-approved detached ADU designs, which can reduce design time and plan check review for lots that fit. Custom plans cost more to prepare but let the unit fit an awkward lot or match the main house. Finish level is the last lever: rental-grade finishes and owner-occupant or family finishes are very different budgets, and both are legitimate choices.
Fees, permits and state rules that affect cost
- Impact fees. California law, summarized in the California HCD ADU Handbook, exempts ADUs under 750 square feet from impact fees; larger units can be charged proportionally to the main house.
- School fees. School district developer fees generally apply once an ADU is larger than 500 square feet.
- Plan check and permit fees. LADBS fees scale with the project's valuation and scope.
- Energy code. New ADUs must meet the California Energy Code (Title 24). New detached units are generally subject to the solar requirement; conversions of existing space usually are not. Your Title 24 report confirms what applies.
For the permit steps themselves, see our garage conversion permit guide, which covers the conversion route through LADBS.
Where ADU budgets go wrong
- Pricing the building without pricing the sewer and electrical path to it.
- Assuming an old garage slab can be reused without testing moisture and level.
- Choosing finishes before the site work is known.
- Leaving out landscaping, fencing, a path to the entrance and exterior lighting.
- Not budgeting time: carrying costs grow while plans sit in correction.
How to get a real number for your lot
An ADU estimate is only as good as the site visit behind it. We look at the sewer cleanout and route, the panel, the path from the street to the build area, the slope and the trees, then tell you which ADU type fits and what drives the cost on your lot. We build across the Valley, including Chatsworth, where we are based, and Woodland Hills. Request a free written estimate and we will put the scope and assumptions in writing.
Frequently asked questions
Is a garage conversion cheaper than a detached ADU?
Usually, yes, because the slab, walls and roof already exist and the setbacks and parking are already settled. The savings depend on the garage's condition. A cracked, out-of-level slab, missing footings or undersized roof framing can mean rebuilding much of the shell, which narrows the gap. We check the structure first and tell you honestly whether converting or building new is the better use of your budget.
Why doesn't a smaller ADU cost much less than a larger one?
Because the most expensive parts of an ADU are fixed. Every unit needs a foundation, utility connections, a kitchen and a bathroom, regardless of size. Shrinking the unit mostly removes bedroom and living space, which are the least expensive square feet to build. Going smaller can still make sense for fees, setbacks or yard space, but it rarely cuts the budget in proportion to the square footage removed.
Do I have to pay impact fees on an ADU in Los Angeles?
Not on smaller units. Under state law, summarized in the California HCD ADU Handbook, ADUs under 750 square feet are exempt from impact fees, and larger units can only be charged proportionally to the main house. Other fees still apply, including LADBS plan check and permit fees, and school district fees for units over 500 square feet. We list expected fees separately in our estimates.
Does a pre-approved standard plan save money?
It can. LADBS pre-approved standard plans reduce design time and focus plan check on site-specific items, which saves weeks and some design cost. The savings disappear if the plan does not fit the lot well and needs heavy changes. We use standard plans when the lot allows and custom plans when access, slope or the shape of the lot calls for them.
What site conditions raise ADU cost the most?
Sewer route, electrical capacity, access and slope. A unit that cannot drain by gravity may need a pump. An old, small panel often needs an upgrade. A narrow side yard means hand-carrying materials and pumping concrete. Hillside lots can require soils reports, deeper foundations and retaining walls. These items are invisible on a floor plan, which is why we price them on site before anything else.