How Much Does It Cost to Build an ADU in California?

That range is wide because two different projects can both be called “an ADU.” A 400-square-foot garage conversion with builder-grade finishes shares almost nothing with a 1,000-square-foot detached unit in Manhattan Beach with quartz counters, mini-split HVAC, and custom millwork. This guide breaks down what’s actually inside the price — hard construction, soft costs, fees, and the site-specific variables that explain why two neighbors with identical lots can pay very different numbers.

2026 cost ranges by ADU type

The wide spreads aren’t padding — they reflect real variability. Foundation work on a sloped lot, an electrical service panel that can’t carry the new load, a sewer lateral that has to be replaced, or the gap between stock cabinets and custom millwork can each shift a budget by $30,000 or more.

Cost-per-square-foot can also be misleading. A full kitchen and bathroom in a 400-square-foot ADU costs roughly the same as a kitchen and bath in an 800-square-foot ADU, which makes the smaller unit more expensive per square foot. Plan with total project cost in mind, not the rate.

What’s actually inside that number

Hard costs (the build itself)

Hard costs are the actual construction work — labor, materials, and trades. On a typical detached ADU in California, hard costs account for roughly 80–85% of the total budget. The major line items:

• Foundation and site prep. Concrete slab foundation runs $5,000–$15,000 on flat ground. Sloped lots, expansive soils, or removal of existing structures can double this.
• Framing, roofing, exterior. Walls, roofing, siding, windows, and doors typically run $50,000–$120,000 depending on size and finish level.
• Electrical, plumbing, HVAC (“MEP”). Running utilities to a detached ADU, panel upgrades, plumbing rough-in, and HVAC together typically run $25,000–$45,000.
• Interior finishes. Cabinetry, countertops, flooring, fixtures, paint, and trim. This is where finish-level swings the budget most — anywhere from $30,000 to $80,000+.

Soft costs (everything that isn’t the build)

Soft costs are 10–20% of the total project. They include:

• Architectural plans and engineering. $3,500–$14,000 depending on complexity.
• Plan check and permits. Highly jurisdiction-dependent. In Manhattan Beach, expect a plan check fee in the low thousands plus separate building permit fees.
• Impact fees. California state law exempts ADUs under 750 square feet from impact fees entirely. Above 750 square feet, fees are charged proportionally to the size of the primary residence.
• Surveys, soils reports, Title 24 energy compliance. $1,500–$5,000.

Site-specific costs

The variables that change projects most:

• Setbacks, easements, and lot geometry — a complex lot can require expensive workarounds.
• Soil conditions — expansive soils, organic fill, or a high water table can require engineered foundations.
• Utility connections — if your electrical panel is at capacity, you’ll need a new service. New service installation can run $4,000–$15,000+.
• Solar panels — California Title 24 generally requires new ADUs to include solar where feasible, with some exceptions for conversions and retrofits.

How California’s 2026 ADU laws affect costs

California has spent the last several years steadily reducing the financial and procedural friction around ADUs. The biggest changes effective January 1, 2026:

• SB 543 — 15-day completeness review. Local agencies must determine permit application completeness within 15 business days and provide a written list of any deficiencies. After completeness, the state-mandated 60-day full review applies. A well-prepared application can now move from submission to permit in roughly 75 business days, down from the six-plus months that was typical in 2021.
• AB 1154 and SB 543 — JADU rules clarified. Junior ADUs are now explicitly capped at 500 square feet. Owner-occupancy is only required if the JADU shares sanitation facilities with the primary home. If the JADU has its own bathroom, owner-occupancy is not required.
• Impact fee exemption. Under California Government Code § 65852.2(f)(3), ADUs of 750 square feet or less are exempt from impact fees, including school district fees per Education Code § 17620. Above 750 square feet, fees must be charged proportionally to the primary dwelling’s square footage. For most South Bay homeowners, this exemption alone can save thousands to tens of thousands of dollars, depending on jurisdiction.
• State law overrides conflicting local rules. Local agencies cannot impose stricter setbacks, design review, or owner-occupancy than state law allows. If a local ordinance conflicts with state ADU law, state law wins.

The authoritative reference on all of the above is the California Department of Housing and Community Development’s ADU Handbook, updated January 2025 with a December 2025 addendum covering the 2026 law changes.

South Bay and Manhattan Beach specifics

ADU costs in the South Bay run at the higher end of California ranges for three reasons: labor premium, finish expectations, and coastal-zone permitting where applicable.

• Labor. South Bay contractor rates typically run higher than inland California averages, reflecting both cost of living and demand.
• Finish quality. Buyers and renters in Manhattan Beach, Hermosa Beach, and Palos Verdes expect finish quality comparable to the primary residence — quartz or natural stone counters, mini-split HVAC, custom cabinetry. This shifts per-square-foot costs from $300 toward $500+.
• Coastal zone. Properties within the California Coastal Zone may require California Coastal Commission review on top of city review. SB 1077 directed the Coastal Commission to publish updated ADU guidance by July 1, 2026, which should standardize this — but until that takes effect, coastal-zone ADUs require more permitting steps and time.
• Manhattan Beach specifically. The city follows state ADU law plus MBMC Chapter 10.74. The city’s published ADU handout covers the local-specific requirements; expect a total permitting timeline in the 3.5–4 month range for a typical detached ADU.

For a clear picture of how AGR Homes Inc. handles ADU projects in the area, see our detached ADU service page and our garage conversion and junior ADU page. We also serve Manhattan Beach and the surrounding South Bay directly.

Where you can actually save money

The legitimate ways to reduce ADU costs without sacrificing quality:

1. Convert an existing garage rather than building detached. Garage conversions skip foundation, framing, and roofing for the basic structure and run roughly half the cost of detached new-build for the same usable square footage.
2. Stay under 750 square feet. This single threshold unlocks the state impact fee exemption. For most homeowners building a one-bedroom rental, 750 square feet is more than enough.
3. Skip the JADU + ADU combo if you don’t need both. Pairing them adds complexity to setbacks, parking, and review.
4. Use a pre-approved plan set if your jurisdiction offers one. Pre-approved plans skip the architectural design step and shorten plan check. Sacramento County and some other jurisdictions offer free pre-approved detached ADU plans; check your local planning department. Manhattan Beach does not currently publish pre-approved plans.
5. Hire a licensed general contractor with ADU experience. Cost-cutting at the contractor level — unlicensed help, owner-builder, design-only firms with no construction arm — usually adds 20–50% in cost overruns from permit corrections, inspection failures, and rework. License status is checkable at the CSLB License Check tool.

What doesn’t actually save money: skipping permits (unpermitted ADUs disqualify you from refinancing and create resale problems), DIY mechanical and electrical work (against code in California unless you’re licensed), or hiring the cheapest bidder without checking CSLB license status.

Realistic 2026 timeline

For a typical detached ADU in the South Bay, with a competent design + build team and no surprises:

• Design and engineering: 4–8 weeks (concept, plans, structural calcs, Title 24).
• Plan check and permits: 10–14 weeks (15 business days for completeness + 60 days for full review, plus revision cycles).
• Construction: 16–24 weeks for a typical detached ADU. Garage conversions: 8–14 weeks.
• Inspections and final sign-off: 2–4 weeks.

Total: 9–14 months from first design meeting to final certificate of occupancy. Faster is possible with pre-approved plans or aggressively scheduled trades; longer is common when scope changes mid-project, coastal-zone review applies, or unforeseen site conditions surface. Our build process walks through each phase in detail.