As of October 2026, published cost guides put a new detached ADU in the Los Angeles area at roughly $300,000 to $450,000 or more. Garage conversions usually cost the least, because the slab, walls and roof already exist. Design, engineering, permits and utility work add to the build price. Your lot sets the real number.
What do published cost guides say?
We did not find a published source that tracks Long Beach ADU prices alone. These are the most relevant published figures we found, as of October 2026:
- Detached ADU, Los Angeles: about $300,000 to $450,000 or more (Block Renovation, 2026).
- Average ADU, Los Angeles: about $150,000 to $375,000 (Angi).
- Garage conversion, national: about $50,000 to $150,000 (Block Renovation, 2026); $60,000 to $150,000 before permits and fees (Angi).
- Attached ADU, national: about $100,000 to $250,000 (Block Renovation, 2026).
Treat these as a starting point. National averages include lower-cost regions, and Los Angeles averages blend many very different projects.
Is a garage conversion cheaper than a backyard cottage?
Usually. A garage conversion reuses the existing slab, walls and roof. A backyard cottage needs a new foundation, framing, roof and longer utility runs. A garage with a failing slab or damaged framing can narrow the gap, so we inspect before quoting.
What drives the price of an ADU?
- Type and size. New construction costs more than conversion, and every bathroom and kitchen adds cost.
- Utilities. An electrical service upgrade, a new subpanel, or upsized water and sewer lines can be significant line items.
- Site. Access for materials, slope, soil, easements and the distance to the street all matter.
- Overlays. Coastal zone permits, historic district review and, in Signal Hill, oil well review add work before construction.
- Finishes. Cabinets, flooring, windows and fixtures can move the total either way.
How much do ADU design and engineering cost?
One 2026 guide (Block Renovation) estimates design and engineering at about 10 to 15 percent of construction cost. Pre-approved plans can reduce that where a city offers them and they fit your lot. See ADU design and permits.
What fees does state law limit for ADUs?
As of October 2026, California law:
- Bars impact fees on an ADU of 750 sq ft or less of interior living space, and on a junior ADU of 500 sq ft or less.
- Allows impact fees on larger ADUs only in proportion to their size.
- Exempts ADUs and junior ADUs of 500 sq ft or less from school impact fees (SB 543, effective January 1, 2026).
- Stops utilities from treating an ADU or junior ADU as a new residential use for connection fees or capacity charges, unless it is built with a new single-family home.
Plan check and permit fees still apply. The same 2026 guide puts permit and impact fees at about $1,500 to $25,000 depending on the city, the ADU's size and the scope.
When does the cost depend most on your lot?
It depends on what the site hides. Two similar ADUs can differ widely if one needs a new electrical service, a sewer line replacement or foundation work. That is why we measure, check the panel and look at the garage or yard before we give a written price. Size under 750 sq ft is one lever you control, since it avoids impact fees under state law.
Related
Sources, as of October 2026: Block Renovation, "A Deep Dive Into ADU Construction Costs" (2026); Angi, "How Much Do ADUs Cost?"; California Government Code §66311.5; Best Best & Krieger summary of 2025 ADU bills. Figures are published estimates, not our quotes.
Updated October 2026