Building an ADU in Los Angeles County: Your Every-Step Guide
If you own a property in Los Angeles County and you’ve been thinking about adding an extra dwelling unit — whether for a family member, rental income, or long-term value — now is a good time to act. ADU rules have been streamlined in recent years, and the county (and many of its cities) offer standard plans, expedited review paths, and generous size/height allowances. But there are still many moving parts. This post walks you through the key guidelines and process so you can plan your project with confidence.
What is an ADU (and what’s the difference between ADU/JADU)
An ADU (Accessory Dwelling Unit) is a self-contained living unit on the same lot as a primary residence. It has its own kitchen, bath, sleeping area, etc.
In Los Angeles County, you’ll also often hear about a JADU (Junior Accessory Dwelling Unit) — a smaller unit located within the main home (or attached) that shares some utilities or space. LA County Public Works+2LADBS+2
JADUs typically:
Are up to about 500 sq ft. LADBS+1
Are contained within the existing home (so less new footprint)
May share sanitary facilities or utilities with the main home. LA County Public Works+1
By comparison, a full ADU (detached or attached) will have full independent facilities (kitchen, bath, entrance). Planning for your project means first choosing the model that makes sense for your lot, goals, budget, and timeline.
Key Development Standards and Guidelines
Here are some of the main rules you’ll need to follow in Los Angeles County (unincorporated and many cities) when building an ADU:
Maximum size: Many county guidelines allow a detached ADU up to 1,200 sq ft in size. LA County Public Works+2California ADU Experts+2 For attached or conversions, there are other limits (for instance, up to 50% of existing home’s floor area). Gather ADU+1
Setbacks / building separation: For detached ADUs, the side and rear setbacks are often around 4 ft minimum in some zones; the building-to-building separation might be required (e.g., 6 ft) depending on local code. California ADU Experts+1
Zoning / lot eligibility: The property must be zoned for residential use (single-family or multi-family depending on your plan). There must be or be proposed an existing dwelling. Gather ADU+1
Owner occupancy / sale restrictions: In many jurisdictions, the ADU cannot be sold separately from the lot (i.e., you’re not subdividing just to sell off the ADU). California ADU Experts+1
Parking: Many local ordinances waive parking requirements if the ADU is near transit, or allow fewer off-street parking spaces for the ADU. LADBS+1
Fire, safety and utilities: If the property sits in a fire-hazard severity zone (common in LA County), additional requirements (fire sprinklers, access roads, fuel modification) may apply. LA County Public Works+1
Pre-approved standard plans: Both the City of Los Angeles and the County offer “standard plan” programs — pre-approved ADU designs that speed up review. LA County Public Works+1
Understanding these baseline rules helps you make early decisions: Will you convert a garage? Build detached? Use a stock plan? What setbacks apply? What costs and timing are involved?
Step-by-Step: The ADU Process in Los Angeles County
Here’s a typical workflow for building an ADU in Los Angeles County (for unincorporated areas; city processes may be very similar) based on official guides. LA County Public Works+2LA County Public Works+2
1. Preliminary check & eligibility
Confirm your property allows an ADU and meets zoning and lot size rules. LA County Public Works+1
Assess existing structures: are you converting a garage, basement, or building new? If converting, check if standard notes apply (e.g., no change to footprint). LA County Public Works+1
Evaluate whether additional requirements apply (fire zone, hillside, environmental restrictions).
2. Design & plan preparation
Hire an architect or designer (or select a pre-approved standard plan) to create architectural and structural drawings. LA County Public Works+1
Prepare required calculations (e.g., Title 24 energy compliance). LA County Public Works+1
Coordinate utilities, sewer/water, grading, drainage if required. LA County Public Works
3. Submit application to Planning / Regional Planning
For unincorporated areas, submit the “Base Application for ADU” to county planning. This review determines compliance with zoning, setbacks, height, architecture. LA County Public Works
Receive approval (and site-specific plot plan) from Planning. Some jurisdictions may require ministerial approval within a set timeframe. Los Angeles County Planning
4. Submit Building Permit Application
Once planning approval is secured, submit building permit application at the local Building & Safety Department. Upload plans, structural, energy, agency referral forms. LA County Public Works
The department reviews for compliance with building codes, fire code, accessibility, etc. Corrections may be required. LA County Public Works+1
5. Agency referrals and approvals
Depending on location, you may have additional referrals: Fire Department (especially in fire hazard zones), Water Company, Sanitation, Drainage/Grading, Environmental Health (if septic). LA County Public Works
Address any fees, impact or connection requirements.
6. Permit issuance & construction
After plans are approved and permits issued, construction may begin. Inspections will be scheduled at key points (foundation, framing, systems, final). LA County Public Works+1
If using a pre-approved standard plan, the process may be faster and some checks streamlined. LA County Public Works
7. Final inspection & occupancy
Once construction is completed, schedule final inspection(s). Ensure all code requirements are met, utilities connected, proper certificate of occupancy or final permit sign-off obtained.
If you plan to rent the ADU, ensure local rental/tenant laws are followed (in cities that have them). Housing Authority of Los Angeles
Approximate Timing, Costs & Common Pitfalls
Timing: For a well-prepared project, many ADU permit approvals in the Los Angeles region take around 4-6 months (or longer if complexity or utility issues arise). Maxable
Costs: Design, permit fees, utility connections, construction all add up. Using a standard plan can reduce design costs.
Common pitfalls:
Incomplete submittals (missing calculations, missing referral items) → delays.
Utilities: if new meters or services are required, wait times from power/water companies add weeks.
Fire hazard zones/hillside lots: more restrictions, more cost, more review.
Conversions with existing non-conforming structures: additional review.
Misinterpretation of setbacks, building separation, tree or utility conflicts. Reddit threads frequently mention frustration around these exact issues. Reddit+1
Garage Conversions & Streamlined Paths
If you’re converting an existing garage or storage building to an ADU (or JADU), the county provides a streamlined process:
No major footprint addition required; limits on structural changes. LA County Public Works
Use of standard notes and details reduces engineering design costs and delays.
Step-by-step: planning approval → building permit application with county standard notes → pre-construction meeting → inspections. LA County Public Works
If eligible, this path can save months and reduce cost — particularly for homeowners who don’t need a full new structure.
Why this is a Good Time to Build an ADU
The state of California has encouraged ADUs for years (reducing barriers, limiting local parking requirements, relaxing discretionary review). Many jurisdictions have adopted these changes. NEO Builders ADU
The housing shortage, high rents, and desire for multi-generational living continue to drive demand for ADUs — meaning better value for homeowners and renters.
Standard plan programs and expedited processes exist now — you don’t have to reinvent the wheel.
You gain long-term value: adding an ADU increases usable living space and rental potential, while staying under the existing property umbrella (if you so choose).
Final Tips for Homeowners
Start with your lot: before spending on design, verify zoning, setbacks, utilities, and fire hazard status.
Budget realistically: construction costs in LA County are high; allow for contingencies (e.g., utility upgrades, unexpected foundation or site issues).
Consider occupancy and use: Will you live in it? Rent it? Host family? That decision affects design, code, and ongoing management.
Hire local experts: an architect or designer familiar with LA County ADU rules can help avoid surprises.
Use standard plans if possible: they save time and cost.
Communicate early with utilities and agencies: power, water, grading, drainage — those can be hidden delays.
Stay organized: track your plan reviews, corrections, agency referrals, submittal dates.
Expect inspections: when building, stay on top of scheduling and pass each stage to avoid costly rework.
Plan for your tenants or use: If you plan to rent it, check local rental regulations (especially in cities within LA County).
In Summary
Building an ADU in Los Angeles County is more achievable today than it was a decade ago—but it still demands careful planning, informed design, and diligent follow-through. If you respect the zoning and building rules, allow time for review and approvals, anticipate utility/agency steps, and budget for the full project, you have the opportunity to add meaningful living space, value, and flexibility to your property.
Whether you’re creating a place for aging parents, generating rental income, or simply building long-term value, this ADU could be one of the smartest additions you make — as long as you walk the process well. Good luck with your project!
Contact JN DAVIS and we will help guide your through every step and build your dream ADU you for you, today!
