How Much Does an ADU Cost?
If you’re thinking about building an Accessory Dwelling Unit (ADU) on your property, one of the first questions is always cost. The truth is, ADU budgets can vary widely but understanding the main cost factors will help you plan realistically from the start.
For most detached ADUs in Orange County, total build cost ranges from $250 to $450 per square foot.
Smaller garage conversions may start closer to $200/sf, while high-end detached ADUs with custom finishes can reach $500/sf or more.
Example:
400 SF garage conversion → around $100K
800 SF detached ADU → $250K–$350K range
Design & Permit Costs
Before construction, you’ll need architectural, structural, and energy compliance plans.
At Backyard Project, our Permit-Ready ADU Plan Package typically ranges from $6,200–$9,500, depending on complexity and scope. We also offer a $99 ADU Fit Report and optional Concept Design Plan ($299) — low-cost ways to confirm feasibility and visualize your layout before committing to full design.
Utility Upgrades & Site Work
Utility tie-ins often surprise homeowners.
Expect costs for:
Electrical panel upgrades: $2K–$5K
Sewer connection or line extension: $3K–$10K
Grading, retaining, or new foundation work (for sloped lots): varies widely
Finishes & Fixtures
Finishes have one of the biggest impacts on total cost.
Choosing builder-grade materials can keep budgets tight, while premium kitchens or high-end baths add tens of thousands. A good rule: reserve 15–20% of your budget for finishes and contingencies.
How to Plan Smart
Start small.
Begin with feasibility and planning before hiring a builder.
Architect-led ADU design ensures your project meets building codes, avoids redesign costs, and moves efficiently through city review.
Final Thoughts
Building an ADU is one of the best investments you can make in your property — but only when you understand the numbers early.
Backyard Project helps you plan accurately with transparent pricing and step-by-step guidance.
👉 Curious what your ADU might cost?