Written by Pro Value Contracting | Published on July 2025
An ADU, or Accessory Dwelling Unit, is a secondary housing unit on a residential lot, and yes, it is completely legal to build one in California. State law has made it easier than ever, especially since sweeping ADU reform laws took effect in 2020. This article covers what qualifies as an ADU, the rules specific to San Diego, CA, typical costs, and what steps you need to take to get one built.
Understand What Counts as an ADU
An ADU is a self-contained living unit that includes its own kitchen, bathroom, and sleeping area. It sits on the same parcel as a primary residence. Depending on the layout, it can be attached, detached, or converted from existing space like a garage or basement.
California recognizes three main types of ADUs:
- Detached ADU: A fully separate structure built in the backyard or side yard of the primary home.
- Attached ADU: An addition connected to the main house with its own entrance.
- Garage or interior conversion: Converting an existing garage, basement, or unused room into a livable unit.
There is also a Junior ADU (JADU), which is a smaller unit of up to 500 square feet created within the existing footprint of the primary home. JADUs are popular in San Diego because they require less construction and lower permit fees.
The key requirement for any ADU is independent livability. It must function as a complete home on its own, separate from the main house.
Learn How California State Law Makes ADUs Legal
California has some of the most permissive ADU laws in the country. A series of bills passed between 2017 and 2020 removed major barriers that cities and counties previously used to block or delay ADU construction.
Here is what California state law now requires local jurisdictions to allow:
- At least one ADU and one JADU per single-family residential lot.
- Up to two detached ADUs on multi-family lots.
- Ministerial approval (no public hearing required) for most ADU applications.
- Permit decisions within 60 days of receiving a complete application.
- No owner-occupancy requirement (as of 2020, through at least 2025).
Cities and counties can still set local standards for setbacks, height limits, and design, but they cannot outright ban ADUs or impose rules that make them effectively impossible to build. This is a major shift from the rules that existed before 2017.
Review San Diego’s Specific ADU Rules
San Diego, CA has adopted local ADU regulations that align with and, in some cases, go beyond state minimums. The City of San Diego updated its ADU ordinance to reflect state law and encourages ADU development as a strategy to address the region’s housing shortage.
| ADU Type | Max Size (San Diego) | Max Height | Min Setback |
|---|---|---|---|
| Detached ADU | 1,200 sq ft | 16 ft (up to 25 ft in some zones) | 4 ft rear and sides |
| Attached ADU | 50% of primary home or 1,200 sq ft | Matches primary structure | Matches primary structure setbacks |
| Garage Conversion | Size of existing garage | Existing structure height | No additional setback required |
| JADU | 500 sq ft | Matches primary structure | Matches primary structure setbacks |
San Diego also waives parking requirements for ADUs located within half a mile of public transit, which covers a large portion of the city. This makes garage conversions especially attractive in urban San Diego neighborhoods where parking is already limited.
If your property is in a coastal zone, historic district, or has a homeowners association, additional restrictions may apply. Always verify with the City of San Diego Development Services Department before starting your project.
Compare Typical ADU Construction Costs in San Diego
Building an ADU in San Diego is not cheap, but it is often one of the highest-return investments a homeowner can make in Southern California. Costs vary widely based on ADU type, size, finishes, and site conditions.
| ADU Type | Estimated Cost Range | Typical Timeline |
|---|---|---|
| Detached new construction (400-800 sq ft) | $150,000 to $350,000 | 9 to 18 months |
| Attached addition (400-800 sq ft) | $100,000 to $250,000 | 6 to 14 months |
| Garage conversion | $60,000 to $150,000 | 3 to 8 months |
| JADU (interior conversion) | $40,000 to $100,000 | 2 to 6 months |
| Prefab or modular ADU | $80,000 to $200,000 | 4 to 10 months |
These ranges reflect San Diego’s higher labor and material costs compared to inland California markets. Soft costs like architectural drawings, engineering reports, and permit fees can add $10,000 to $30,000 on top of construction expenses.
The City of San Diego has reduced ADU permit fees for units under 750 square feet, which is worth factoring into your budget. In some cases, state-approved standard ADU plans can further reduce design costs and speed up permit approval.
Know the Permitting Process Step by Step
Getting your ADU permitted in San Diego follows a predictable process, though timelines can stretch depending on application volume and project complexity. Here is the general sequence:
- Pre-application research: Confirm your zoning, lot size, and any deed restrictions. Check the City of San Diego’s website or call Development Services.
- Hire a designer or architect: You will need stamped construction documents for most ADU types. Some firms specialize in ADU design in San Diego.
- Submit building permit application: Include site plans, floor plans, elevation drawings, and energy calculations. Applications go to the City’s Development Services Department.
- Plan check review: The city reviews your plans, typically within 30 to 60 days. You may receive correction comments requiring revised drawings.
- Permit issuance: Once approved, you pay permit fees and receive your building permit.
- Construction: Work begins. Inspections are required at key milestones, including framing, rough mechanical work, and final inspection.
- Certificate of occupancy: After passing final inspection, the city issues a certificate of occupancy, making your ADU legal to occupy or rent.
Pro Value Contracting works with homeowners across San Diego County and understands the local permitting landscape. Having a contractor familiar with city processes can prevent costly delays.
Understand How ADUs Affect Water Systems and Moisture Risk
Adding a kitchen, bathroom, and laundry area to a new ADU significantly increases water demand and drain load on your property. This is an area that many homeowners overlook during the planning phase.
Older San Diego homes, especially those built before 1980, may have aging plumbing that was never designed to support a second full household. Adding an ADU without upgrading the main water supply line or sewer lateral can lead to low water pressure, slow drains, or sewage backups.
Garage conversions carry particular water damage risk. Many garages have concrete slabs without waterproofing or vapor barriers. Once you add living space, moisture can migrate up through the slab and cause mold growth inside walls and flooring.
Pro Value Contracting provides water damage assessments and moisture mitigation services that are especially valuable during ADU conversions. Addressing water intrusion risk before you finish the interior saves you significant money down the line.
San Diego’s coastal climate, with marine layer humidity and occasional winter rains, creates conditions where moisture problems can develop quickly in improperly sealed spaces. Do not skip moisture barriers, proper ventilation, and drainage planning during your ADU build.
FAQs on ADUs in California
Can I rent out my ADU in San Diego?
Yes, you can rent your ADU to long-term tenants without any restriction on who the owner is, thanks to California’s 2020 ADU law changes. Short-term rentals through platforms like Airbnb are subject to San Diego’s separate short-term rental ordinance, which requires a permit and limits the number of days you can rent.
Do I need to live on the property to build an ADU in California?
No. California suspended the owner-occupancy requirement for ADUs through at least 2025. This means you can build an ADU on a rental property you do not personally live in. Check current state law for any updates to this rule, as it is subject to legislative renewal.
How long does it take to get an ADU permit in San Diego?
San Diego’s Development Services Department is required by state law to process complete ADU applications within 60 days. In practice, timelines vary based on workload and whether your application requires corrections. Simple garage conversions using pre-approved plan sets can move faster than custom new construction.
Will an ADU increase my property taxes in California?
Yes, but only the value of the new ADU will be reassessed, not your entire property. California’s Proposition 13 limits annual property tax increases to 2% for existing assessed value. Your tax bill will go up, but only incrementally based on the added value of the ADU.
Can I build an ADU if I have an HOA?
California law prohibits HOAs from outright banning ADUs, but they can impose reasonable design standards. Your HOA may require architectural review and compliance with community guidelines on materials, colors, and aesthetics. Always submit your ADU plans to your HOA before breaking ground to avoid disputes.
What is the difference between an ADU and a JADU?
A JADU is a Junior ADU, limited to 500 square feet and created entirely within the existing walls of the primary home. A standard ADU can be larger, detached, or attached to the home. JADUs require the owner to occupy either the main home or the JADU, while standard ADUs have no occupancy requirement through at least 2025.
Wrap Up and Next Steps
ADUs are legal, encouraged, and increasingly common in San Diego, CA. California’s state laws have removed most barriers, and the city’s updated ordinances make the process more accessible than ever for homeowners ready to add rental income or multigenerational living space.
If your ADU project involves a garage conversion, basement finishing, or any work near plumbing, do not overlook moisture protection. Water damage during or after construction can cost far more than prevention. Pro Value Contracting is here to help San Diego homeowners protect their investment at every stage.
Call Pro Value Contracting today at (888) 508-6580 to schedule a consultation or request a quote. Our team serves San Diego County and is ready to assess your property, flag any water damage risks, and help you move forward with confidence.