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ADU Cost Calculator for Orange County
ADU costs in Orange County vary enormously by type. A junior ADU built inside existing walls can run $160 to $290 per square foot, while a new detached unit typically runs $360 to $620 per square foot once site work, foundation and utility trenching are included. California law requires cities to process qualifying ADU applications ministerially.
Your project
Impact fees are generally not charged on smaller units, which is one reason cost per square foot varies by size.
Not always required. Shared utilities are usually less expensive and often permitted.
Estimated range
$273,000 to $506,000
What makes up this range
- 700 square feet at $360 to $620 per square foot$252,000 to $434,000
- Design, Title 24 and plan check$6,000 to $16,000
- Permits and city fees$3,000 to $14,000
- Site work, foundation and utility trenching$12,000 to $42,000
Worth knowing
- California ADU law has been expanded repeatedly, and cities must process qualifying applications ministerially, meaning no discretionary hearing.
- Impact fees are generally not charged on smaller ADUs, which is one reason cost per square foot varies so much by size.
- A junior ADU must be created within the existing walls of the primary home, which is why it is the least expensive path.
This is an estimate for planning purposes only, not a quote. Real pricing depends on your specific site conditions, material selections and city requirements. We provide a written itemized proposal after walking the space.
Get a written quoteRead the full ADU guideAnswers
Frequently asked questions
What is the difference between an ADU and a junior ADU?
A junior ADU is created within the existing walls of the primary home, is limited in size, and may share a bathroom with the main house. A standard ADU is a fully independent dwelling with its own kitchen, bathroom and entrance, and can be detached or attached.
Why does a garage conversion cost so much less per square foot?
The foundation, walls and roof already exist. You are adding insulation, drywall, electrical, plumbing, HVAC and finishes rather than building a structure from the ground up. The main added costs are slab moisture treatment and replacing the garage door with a rated wall.
How long does ADU permitting take in Orange County?
California law requires cities to act on a complete ADU application within a defined statutory window, and qualifying applications are approved ministerially without a discretionary hearing. In practice, the design and correction cycles before an application is deemed complete are what take the most time.
Can my HOA stop me from building an ADU?
California law limits the ability of homeowners associations to prohibit ADUs outright on single family lots. HOAs may still apply reasonable architectural standards. Rules have shifted repeatedly, so confirm current requirements with your city and your association before designing.
Next step
Turn your estimate into a real number
An estimate is a starting point. We walk the space, confirm what is actually involved, and send a written itemized proposal so you know exactly what you are buying.
Licensed and insured. Written scope before work begins. Weekly progress updates with photos.
What happens next: we reply the same business day, schedule a walkthrough, then send your written proposal.