Cost of ADU's

The average cost of constructing a detached/attached ADU in Orange County, CA ranges from $250 to $450 per square foot, depending on several factors, such as site conditions, structure size, type of build, and chosen finishes. The final construction cost for a detached or attached ADU generally falls between $200,000 and $400,000.

Though it seems obvious that larger ADUs cost more than smaller ones, you’d be surprised to learn that price per square foot is usually lower for larger ADUs. The highest costs when building an ADU is related to the infrastructure (utilities & sewer), the foundation, plumbing, and electrical. Since small ADUs still need kitchens and bathrooms, which are the most expensive part of the build, the cost per square foot for a 500 square-foot ADU versus a 1000 square-foot ADU will be higher. Bedrooms cost the least, so adding a third bedroom will cost less than adding a second bathroom.

Garage conversion construction costs range between $200 and $300 a square foot or $90,000 to $120,000 for a typical two-car garage. Attached garages cost less than detached garages, as the utilities are generally more accessible, and the garages are newer. Detached garages cost more, as they’re usually set back from the main house, which requires more trenching to the utilities. Detached garages are also older and often need new roofs and replaced lumber and foundations.

Factors Affecting ADU Costs in Orange County CA

Factors such as site conditions, structure size, type of build, and chosen finishes significantly impact ADU costs in Orange County CA.

How Much Is My ADU Worth?

An ADU or in-law or granny flat or outside cottage (or whatever you want to call it) is a habitable unit that is separate from the primary dwelling.

And its value depends entirely on the prevalence of comparable sales in the area with similar ADUs.

One of the many advantages of so many ADUs getting built right now is that it will make it easier to find valid comps that will in turn support higher values.

ADUs are so popular now that many builders are including them within the footprints of primary residences and marketing them as “junior ADUs.”

And finally, because of the overall shortage of affordable housing, many jurisdictions are making the permitting process for new and existing ADU’s much easier – so we should expect the ADU trend to continue.

Scroll to Top