People use "prefab" and "pre-approved" as if they mean the same thing. They do not. One is about a design that the city has already reviewed. The other is about where and how the unit is built. You can have one, the other, both or neither.
LA ADU Guide is an information site. We do not design, sell or build ADUs. Confirm details with your building department.
Two different ideas
Pre-approved plans are a paperwork shortcut. A designer submits a standard design to the city once. The city reviews it for code compliance. Later, many owners can reuse it.
Prefab, modular or manufactured ADUs are a construction method. Most of the unit is built in a factory and delivered to your lot. Pre-approved plans may or may not be involved.
| Pre-approved plan | Prefab or factory-built unit | |
|---|---|---|
| What it speeds up | City plan review | On-site construction time |
| Where the work happens | Your lot, as usual | Mostly in a factory |
| Still needs a permit? | Yes | Yes |
| Still needs site work? | Yes | Yes |
How the LA City Standard Plan Program works
The Los Angeles Department of Building and Safety (LADBS) runs a Standard Plan Program. LADBS describes standard plans as "privately owned, pre-approved" architectural and engineering designs. ADUs are one of the project types it covers. LADBS says that because the design is already reviewed, "Permits can therefore be issued much faster."
The steps LADBS lists for a homeowner are, in short:
- Choose a plan from the Standard Plan Catalog.
- Contact the plan owner or designer about access, price and terms.
- Adapt the design to your lot, and check with LADBS that it works for your site.
- Submit through the ePlanLA portal.
- Get the permit once LADBS approves.
- Build, and schedule inspections before the Certificate of Occupancy.
Some points that surprise people:
- The designer owns the design. LADBS says you must contract with the designer to use it. Whether the plan can be changed depends on that designer.
- Pricing varies. Plans are bought from the plan owner. LADBS has said a plan owned by the City of Los Angeles may be used by property owners free of charge. Check the current catalog to see which plans are city-owned, since we could not confirm that list as of this review.
- Fees still apply. LADBS says that once you pick a pre-approved plan, "building plan check and permit fees" are based on the total construction valuation.
What pre-approval does not cover
LADBS is clear on this. Approval of a design is not approval of your lot. The program page says soil or other site conditions "may require site-specific foundation, structural, or engineering work." LADBS staff still review site factors such as zoning and foundation requirements. A pre-approved design may not be allowed in certain situations or site conditions.
Things that can change a standard plan once it meets your property:
- Slope, soil and foundation needs.
- Setbacks, easements and where existing buildings sit.
- Fire hazard zone requirements. See ADUs in fire hazard zones.
- Utility connections, which depend on where your lines are. See impact fees and utility connections.
- Overlay zones such as hillside, coastal or historic areas.
LADBS advises you to verify with it that a design works for your property before you contract with a professional. That is the single most useful step on this page.
State law and pre-approved plans
California's ADU law is at Government Code sections 66310 to 66342. It was renumbered in 2024 (SB 477, effective March 25, 2024) from the old section 65852.2. Cities may create their own pre-approved ADU plan programs, and state law sets review time limits for ADU applications. According to the ABAG summary of state law, a permit application for a detached ADU using a pre-approved plan has a shorter review clock than the normal 60 days. A 2026 change (SB 543) also added a completeness check deadline. Read the current statute or ask LADBS how these clocks apply, because we could not confirm the exact current text.
Prefab, modular and manufactured: know the type
Factory-built ADUs come in a few forms, and the type changes who regulates it.
- Modular or panelized units are built in sections, shipped and assembled on a foundation. They are generally permitted by your local building department, like a site-built unit.
- Manufactured homes are built to a federal HUD standard. CSLB notes that for a manufactured ADU, permits "may come from the California Department of Housing and Community Development." Check which agency applies before you sign.
Either way, you still need a foundation, utility hookups, a permit and inspections. Delivery, crane work and hookups are separate from the unit price. For cost ranges, see ADU cost in Los Angeles, which sources its figures.
Who can install one
CSLB says a B (General Building) contractor can build or install an ADU. It adds that "C-47 General Manufactured Housing Contractors can install an ADU if it is a manufactured home that is constructed off site and transported to the installation location." Other specialty contractors, like electricians, generally cannot run a full ADU build but can be subcontracted.
Building an ADU is a home improvement under state contractor law. CSLB says that means a written contract, and its 2024 ADU payments release says a down payment "cannot exceed $1,000 or 10 percent of the contract price." Be careful with payment schedules for factory orders, where sellers sometimes ask for large deposits. See hiring an ADU builder and checking a CSLB license.
Who can install which kind of unit
| Unit type | Who permits it | Who can install it |
|---|---|---|
| Modular or panelized | Generally your local building department | B (General Building) contractor |
| Manufactured home | May be the California Department of Housing and Community Development, per CSLB | C-47 General Manufactured Housing Contractor, or a B contractor |
Checklist before you buy any plan or unit
- Is the plan or unit allowed on my lot, per my zoning? Ask LADBS or your city.
- Who owns the design, and what can I change?
- What is included: plans only, the unit, delivery, foundation, hookups, permit help?
- Who pulls the permit, and who is licensed to do it?
- What deposit is requested, and is it within the limit for a home improvement contract ($1,000 or 10 percent of the price, whichever is less)?
- Is the seller's or installer's license active at cslb.ca.gov?
Bottom line
A pre-approved plan can save plan review time. A prefab unit can save construction time. Neither removes the need for a permit, site work, utility connections or inspections. Start by asking your building department whether a specific plan fits your lot.