The honest answer is that an ADU's cost is set by three things: how much new structure you're building, what your parcel and jurisdiction demand, and the finish level you choose. Two identical 700 sq ft ADUs on two different lots can land far apart once utilities, grading, and plan-check requirements come into play.
The numbers below are preliminary planning ranges from HomeAdd, not bids. They exist to help you decide whether a project is worth designing. A detailed construction proposal with defined scope and itemized allowances follows design and engineering.
Preliminary planning ranges by ADU type
These are the starting points we see most often across Los Angeles, Orange, and Ventura counties for mid-range finishes on a straightforward lot.
| Project type | Planning range | What drives it |
|---|---|---|
| Garage conversion ADU (400–500 sq ft) | starting at $85K | Existing slab and walls reused; cost driven by insulation, egress, plumbing runs, and panel capacity. |
| Junior ADU inside the house (300–500 sq ft) | starting at $60K | No new foundation. Efficiency kitchen, private entry, and whether the bath is shared or private. |
| Detached studio ADU (400–500 sq ft) | starting at $175K | New foundation, framing, roof, and full utility trenching from the main house. |
| Detached 1-bedroom ADU (600–800 sq ft) | starting at $240K | Larger envelope, separated bedroom, dedicated laundry, upgraded finishes. |
| Detached 2–3 bedroom ADU (800–1,200 sq ft) | starting at $330K | Bigger structure plus more MEP, more site work, and often a service upgrade. |
| Above-garage or two-story ADU | starting at $240K | Structural retrofit of the garage below, stairs, and shear/foundation strengthening. |
Want to model your own configuration? Use the HomeAdd cost estimator to adjust size, finish level, and region.
The seven things that move the number most
1. New structure vs. reusing what exists
A conversion reuses a slab, walls, and roof, so it starts materially lower than a detached build. A detached ADU pays for excavation, foundation, framing, roofing, exterior finish, and a full utility run before a single interior finish is chosen.
2. Utilities and electrical capacity
An older 100-amp service often needs upgrading to support a second dwelling with its own kitchen, HVAC, and laundry. Sewer lateral condition, water line sizing, and gas runs matter just as much. On deep lots, trench length alone can be a meaningful line item.
3. Site conditions
Slope, drainage, retaining walls, soils, tree protection, and access for equipment all change the cost of the same building. Hillside parcels in the Santa Monica Mountains, Simi foothills, and south Orange County canyons routinely carry extra structural and grading scope.
4. Jurisdiction and fees
Plan-check fees, permit fees, and impact or school fees vary by city and county. Under California law, ADUs under 750 sq ft are exempt from impact fees, and larger ADUs are charged proportionally to the primary dwelling — see the California HCD ADU Handbook for the statewide baseline. Confirm the current fee schedule with your own jurisdiction, such as LADBS, OC Public Works, or Ventura County RMA.
5. Design and engineering
Architectural design, structural engineering, and Title 24 energy compliance are real costs that some quotes quietly exclude. Standard or pre-approved plan programs, where a jurisdiction offers them, can reduce design time — but they never remove the need for a site-specific plan, foundation design, and utility design.
6. Finish level
Cabinetry, countertops, tile, flooring, windows, and fixtures can swing a small ADU's budget substantially. The most useful thing a proposal can give you is an itemized allowance per category, so you can see exactly where an upgrade costs more.
7. Overlays and special review
Coastal Zone parcels, Very High Fire Hazard Severity Zones, historic districts, and HOA architectural review all add time, and time usually adds cost. Identifying them during feasibility is far cheaper than discovering them in plan check.
Hard costs vs. soft costs
- Hard costs — site work, foundation, framing, roofing, exterior, MEP rough-in, insulation, drywall, cabinetry, tile, flooring, fixtures, and finish carpentry.
- Soft costs — architectural design, structural engineering, Title 24, survey or soils reports when required, plan-check and permit fees, and applicable impact or school fees.
- Contingency — a reserve you set aside for unknowns found once walls and ground are opened. Every honest budget has one.
County-by-county context
Costs track the parcel more than the county line, but each area has patterns worth planning around.
- Los Angeles County — the City of Los Angeles reviews through LADBS, while unincorporated areas and the many independent cities each run their own process. Hillside and fire-zone parcels are common. LA County service area
- Orange County — incorporated cities review their own permits; OC Public Works covers unincorporated areas. HOA architectural review and Coastal Zone jurisdiction are frequent factors. Orange County service area
- Ventura County — Thousand Oaks, Simi Valley, Camarillo, and Moorpark each review their own projects; Ventura County RMA handles unincorporated parcels. Wildfire and hillside requirements are common. Ventura County service area
How to compare two proposals fairly
- Ask what is excluded — that's where budgets diverge.
- Confirm design, engineering, and Title 24 are in or out.
- Confirm permit, plan-check, and impact fees are in or out.
- Compare allowances per finish category, not one lump sum.
- Ask how utility upgrades and site work are handled if conditions differ.
- Ask for the written change-order process before you sign anything.
- Confirm the payment schedule ties to completed milestones.
Frequently asked questions
Why do ADU cost estimates vary so much between contractors?+
Because proposals often cover different scopes. One may exclude design and engineering, permit and impact fees, utility upgrades, site work, or finish allowances. Compare line by line: design, engineering, permits and fees, site work and utilities, the structure itself, finishes, and allowances. A number without a defined scope is not comparable.
Are ADU costs higher in Orange County than in Los Angeles County?+
Construction labor and material costs are broadly similar across the region. The bigger cost differences come from parcel and jurisdiction: plan-check timelines, impact and school fees, coastal or hillside review, HOA architectural review, and how far utilities have to travel on your lot.
What are soft costs on an ADU project?+
Soft costs are everything that is not physical construction: architectural design, structural engineering, Title 24 energy compliance, surveys or soils reports when required, plan-check and permit fees, and any impact or school fees the jurisdiction charges. Budget for them separately from construction.
Does an ADU pay for itself?+
It depends on your rent expectations, financing terms, and how long you hold the property. Rental income and added property value vary by neighborhood and unit configuration. Model it with real local rent comparables and your actual financing cost rather than a generic rule of thumb.
What unexpected costs come up most often?+
Electrical service and panel upgrades, sewer lateral condition or capacity, drainage and grading, long utility trench runs across a deep lot, fire-access or defensible-space requirements in high fire severity zones, and soils conditions that change the foundation design.
How firm is a preliminary range?+
A planning range is meant to tell you whether a project is worth designing. A detailed construction proposal with defined scope and itemized allowances comes after design and engineering, when the drawings show exactly what is being built.
Get a realistic budget for your address.
Send us the property and what you have in mind. We'll come back with zoning, buildable envelope, and an honest starting range.