The short answer
Across Los Angeles, Orange, San Bernardino and Riverside counties, ADU construction generally runs $285 to $350 per square foot for hard construction costs. That rate varies by finish level, site conditions, and the contractor you choose — a stripped-down garage conversion with builder-grade finishes lands at the low end, while a custom detached unit with high-end finishes can run well above $350/sqft.
Multiply that per-square-foot rate by your unit's size and you get a construction-only estimate. Total project cost is higher once you add design, permitting, and site work — more on that below.
Typical total cost by ADU type
Because different ADU types are built differently — a garage conversion reuses an existing structure, while a detached ADU is ground-up new construction — the realistic total cost range looks different for each:
| ADU Type | Typical Total Cost | Typical Timeline |
|---|---|---|
| Junior ADU (JADU) | $70,000 – $140,000 | 2–5 months |
| Garage Conversion | $115,000 – $210,000 | 4–8 months |
| Attached ADU | $140,000 – $280,000 | 6–11 months |
| Prefab / Modular | $140,000 – $350,000 | 3–6 months |
| Detached ADU | $170,000 – $420,000+ | 8–15 months |
These are planning-level estimates, not quotes — see our full ADU type breakdown or use the cost calculator for a range based on your specific square footage.
What actually drives the price up or down
- Existing structure vs. new construction. Converting a garage or interior space reuses a foundation, roof, and walls that a detached unit has to build from scratch — this is the single biggest cost driver between ADU types.
- Site conditions. Sloped lots, poor soil, or long utility runs to reach a backyard structure can add real cost that a flat, easy-access lot won't have.
- Finish level. Builder-grade finishes versus higher-end fixtures, flooring, and cabinetry can swing the per-square-foot rate by a wide margin.
- Unit size. Cost per square foot often decreases slightly as size increases, since fixed costs (kitchen, bathroom, utility connections) get spread across more square footage.
- Contractor and market conditions. Labor and material costs vary by contractor and by how busy the local construction market is at the time you build.
What's usually not included in a per-square-foot number
Beyond hard construction costs
A per-square-foot rate typically covers labor and materials for the physical build. On top of that, most projects also carry design and engineering fees (roughly 10% of construction cost), permit and impact fees (roughly 8%), and separate site work like grading, utility trenching, or driveway modifications that aren't part of the core construction number. A licensed contractor can break down exactly what's included in a specific bid — treat any number that skips this breakdown with some skepticism.
How to get a number specific to your property
A per-square-foot rate and a table of typical ranges are useful for planning, but they can't account for your specific lot, existing structures, or design goals. The most reliable next step is a free property review — a first, no-obligation look at what your property can realistically support, followed by introductions to licensed contractors who can give you a firm written estimate.
Frequently asked questions
It's a realistic planning range for our service area, but individual projects can fall outside it. Difficult site conditions can push costs above $350/sqft, while a simple conversion with basic finishes can come in under $285/sqft. Treat it as a starting point, not a quote.
Not in the core per-square-foot construction rate. Permit and impact fees typically add roughly another 8% on top of hard construction costs, and vary by city — check with your local building department for exact fee schedules.
Not necessarily cheaper, but often faster. Prefab and modular units can carry similar total costs to site-built detached ADUs once foundation, delivery, and site work are factored in — the main advantage is typically a shorter build timeline, not a guaranteed lower price.