Prefabricated vs. Custom-Built ADUs: Navigating the Permit Process in California

Table of Content

1. What is an ADU?

2. Benefits of Building a Permitted ADU in California

3. Prefabricated vs. Custom-Built ADUs: What’s the Difference?

3.1 Prefabricated ADUs (Prefab ADUs)

3.2 Custom-Built ADUs

4. Navigating the ADU Permit Process: Prefab vs. Customs

5. Key Permit Considerations for Both Types

6. Which ADU Type Is Best for You?

6.1 Choose a Prefabricated ADU

6.2 Choose a Custom-Built ADU

7. FAQ

What is an ADU?

An Accessory Dwelling Unit (ADU) is a secondary residential unit located on the same property as a primary home. ADUs can be:
- Detached (standalone in the backyard)
- Attached (connected to the main house)
- Converted (from garages, basements, or other interior spaces)
These units provide flexible housing solutions and are essential to California’s strategy to increase affordable and sustainable housing.

Benefits of Building a Permitted ADU

Investing in a permitted ADU offers long-term value and legal peace of mind:

✅ Generate Legal Rental Income
✅ Boost Property Value
✅ Support Multigenerational Living
✅ Avoid Fines and Legal Trouble
✅ Access Financing and Insurance

Permitted ADUs are compliant with both state and local regulations, making them more secure and valuable than unpermitted structures.

Prefabricated vs. Custom-Built ADUs: What’s the Difference?

🔹 Prefabricated ADUs (Prefab ADUs)

Prefabricated ADUs are factory-built housing units, delivered and installed on-site. These include modular, panelized, or container-style ADUs. They are pre-designed and often come as part of a streamlined, bundled package.

🔹 Custom-Built ADUs

Custom-built ADUs are designed and constructed from scratch on-site based on homeowner preferences, architecture, and site-specific requirements. These allow for full customization and integration with existing property aesthetics.

Navigating the ADU Permit Process: Prefab vs. Custom

While both prefab and custom-built ADUs must go through permitting, the process, timeline, and complexity differ. Here’s how:

Category	Prefabricated ADUs	Custom-Built ADUs
Design Flexibility	🔸 Limited to manufacturer’s models	✅ Fully customizable design
Permit Timeline	✅ Often quicker due to pre-approved plans	🔸 Can take longer due to complex site-specific design
Permit Submission	✅ Provider often handles submittals	🔸 Homeowner or architect manages process
Foundation & Site Work	🔸 Requires separate grading and foundation permits	✅ Integrated into the full construction plan
Cost Predictability	✅ More predictable fixed costs	🔸 Higher variance based on finishes, materials, and labor
Building Code Compliance	✅ Built to CA residential code in factory	✅ Built on-site to meet all code standards
Zoning Compatibility	✅ Easier in cities familiar with prefab units	🔸 Requires local review and adaptation
Speed to Completion	✅ Faster (as little as 3–5 months)	🔸 Typically 6–12+ months

Key Permit Considerations for Both Types

Regardless of the method, California lawrequires the following for all ADUs:

Zoning Compliance: Your lot must allow ADUs under state/local rules.
Building Permit: You must submit architectural plans for approval.
Site Plan: Includes layout, setbacks (minimum 4 ft), and access.
Utility Connections: Sewer, water, and electricity must meet code.
Title 24 Energy Compliance: Required for all new residential construction.

Which ADU Type Is Best for You?

✅ Choose a Prefabricated ADU if:

You want a faster build timeline
You’re looking for an all-in-one solution
You have a flat, accessible lot
You prefer lower, more predictable costs

✅ Choose a Custom-Built ADU if:

Your property has unique constraints or terrain
You want full control over design and materials
You’re integrating the ADU with an existing structure
You’re building for high-end resale or architectural value

Check Common Zoning Challenges When Applying for an ADU Permit in California

FAQ

Do prefab ADUs require a permit in California?

Yes. All ADUs—prefab or custom—require permits. Prefab units must still meet local zoning, site prep, and utility hookup requirements.

Is it easier to get a permit for a prefab ADU?

In many cases, yes. Some prefab companies offer pre-approved plans that simplify the process with certain cities.

Can I modify a prefab ADU design?

Only slightly. Customization is limited to what the manufacturer allows. For full customization, choose a custom-built ADU.

Which is more cost-effective?

Prefab ADUs often cost less due to mass production and faster timelines. However, custom-built ADUs can offer more long-term value and design alignment.

Can prefab ADUs be installed anywhere?

Not always. Your local city must approve the site, and accessibility for delivery must be considered. Steep, narrow, or hillside lots may not be suitable.