How Much Does an ADU Cost in San Gabriel Valley, CA? (2026 Guide)

Building an ADU in the San Gabriel Valley typically costs between $150,000 and $400,000 in 2026, depending on whether you convert an existing garage, attach to your main home, or build a fully detached unit. Permit and impact fees add roughly $6,000–$15,000 on top of construction costs, though units under 750 square feet are exempt from most impact fees.

What Determines ADU Cost

Every ADU project is different, and the final price depends on a handful of factors: the size and layout of the unit, whether you’re converting existing space or building new, site conditions like slope and utility access, finish level (standard vs. custom), and local permit and impact fees. At Potato Construction, we walk every client through a free consultation to scope these variables before giving a quote.

Typical Cost Ranges by ADU Type

• Garage conversion ADU: $150,000–$250,000 — the most budget-friendly option since you’re working with an existing structure and foundation.
• Attached ADU: mid-range cost, benefits from sharing a wall and utility connections with the main house.
• Detached ADU: $225,000–$400,000+ — highest cost due to new foundation, utility runs, and standalone construction, but offers the most privacy and design flexibility.

Permit and Impact Fees in the San Gabriel Valley

California law exempts ADUs under 750 square feet from most local impact fees, and fees for larger units must be charged proportionately based on square footage. In practice, most San Gabriel Valley cities charge somewhere between $6,000 and $15,000 in combined plan check, building permit, and school impact fees for a typical ADU project. Fees vary by city, so we recommend confirming exact numbers with your local building department or letting our team handle that research as part of your project.

Ways to Manage Your ADU Budget

• Consider a garage conversion if you already have an underused garage — it’s the fastest path to a lower total cost.
• Keep your unit under 750 square feet to avoid most impact fees.
• Get a complete, code-compliant plan set on the first submission to avoid costly resubmission delays.
• Work with a single design-build team so design, permitting, and construction stay coordinated and on budget.

Get a Free ADU Quote

Potato Construction has helped San Gabriel Valley homeowners design, permit, and build ADUs from concept to move-in. Contact us or call (626) 777-6666 for a free consultation and a project-specific quote, or learn more about our ADU services.

Frequently Asked Questions