ADU Cost Orange County

ADU Cost Orange County CA

ADU Cost Orange County, CA

Welcome to our comprehensive article on ADU costs in Orange County, CA. With the increasing demand for affordable housing, homeowners are turning to the option of building Accessory Dwelling Units (ADUs) on their properties. In this article, we will explore the various factors that affect the cost of ADU construction, as well as other costs that must be considered. We will also provide budgeting tips and discuss financing options.

Key Takeaways

  • Average costs of ADUs in Orange County
  • Factors that affect the cost of an ADU include site conditions, structure size, type of build, and chosen finishes.
  • Budget Considerations for building an ADU
  • Financing options, such as home equity lines of credit or construction loans are available for ADU projects.

Average Cost of ADU Construction in Orange County, CA

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.

The most important site conditions include accessibility and ease of the location. Most sites are flat and have access for construction vehicles along the side of the property. This allows contractors to bring in small machinery to help with hauling of heavy materials and assist with trenching for utilities and digging the foundation. This reduces labor costs immensely as compared to sites that have poor accessibility. Sites that are on private streets, slopes, or have side yards that are less than 3 feet in width, are considered more difficult and labor-intensive. The construction on these sites is often $50,000 to $100,000 higher depending on a number of factors.

The location of the ADU is another site condition that affects cost, particularly in relation to distance from the front of the house where the sewer cleanout is generally located. Long distances of trenching to utilities, which include sewer, water, electrical, and gas, increase costs. In some cities, the fire department may require fire sprinklers if the ADU is too far from the street. Cities require properties that are on slopes and hills to get soils reports, and often they are required to install deeper foundations, caissons, or retaining walls.

Structure size is another factor that determines cost. Though price always varies depending on location, city, finishes, and all the factors mentioned, this is a general guideline for construction only in Orange County: *

ADU Specification Price Range
500 SF detached ADU with 1 bed & 1 bath $200,000 - $225,000
650 SF detached ADU with 2 beds & 1 bath $220,000 - $250,000
850 SF detached ADU with 2 beds & 2 baths $240,000 - $270,000
1000 SF detached ADU with 3 beds & 2 baths $290,000 - $310,000

Type of build also affects cost. An attached ADU costs approximately $10,000 more than a detached ADU because of the complexities of tying into the roof structure of the existing home. ADU design options, such as layout and architectural features, can also influence the final price. Detached ADUs will be required to add solar, which increases the cost by $8,000 to $10,000. Attached ADUs and garage conversions are generally exempt from solar.

Additionally, the choice of ADU construction materials and finish materials will impact costs, with higher quality materials often resulting in higher expenses. Finish materials include flooring, lighting, countertops, cabinets, and plumbing fixtures. The difference between lower-end finishes and higher-end finishes can range between $25,000 to $75,000.

Additional Costs to Consider

Though construction costs make up the bulk of the overall ADU cost, there are other fees that must be considered. These include fees for architectural plans and engineering, permit, and other city and utility fees. Though engineers and draftsmen can develop plans for less money, we recommend using an architect. Their insurance and license cover more liability, and we’ve found that they move the projects through the cities quicker and with fewer corrections. Here are some possible extra costs:

Service Price Range
Architects & Engineering (structural & electrical) $8,000 - $25,000
Designers, Landscapers $2,500 - $8,000
Civil Engineers (utility requirements for water, sewer) $2,500 - $5,000
Plan-check and Permit Fees $5,000 - $10,000
School Fees (all ADUs over 500 SF) $1,000 - $3,000
Impact Fees (all ADUs over 750 SF) $3,000 - $10,000
Solar (Most detached ADUs) $8,500 - $10,000

Budgeting for ADU Development in Orange County CA

Interestingly, budgeting for ADU development in Orange County CA requires careful consideration of financial resources and strategic planning. As you can see, the costs can vary greatly depending on the location of the site, the city, and the size of the ADU. It’s important to do as much due diligence during the planning stage as possible to get an overall estimate of the cost. Even then, it’s extremely important to build in contingency, as sometimes unexpected issues arise. Building in at least a 10% contingency and being prepared for this, is highly recommended.

Payment Options for ADU Projects in Orange County CA

Multiple payment options, including home equity lines of credit and construction loans, are available for homeowners in Orange County CA to finance their ADU projects. ADU financing can be a significant consideration when planning to build an accessory dwelling unit. Government grants are also available for those who qualify.

Homeowners can explore various financing options to choose the one that best suits their needs and financial situation. Home equity lines of credit allow homeowners to borrow against the equity in their homes, while construction loans provide funds specifically for construction projects.

These payment plans provide flexibility and convenience for homeowners to start their ADU projects without the need for upfront cash payments. By considering these financing options, homeowners can make their ADU dreams a reality, while managing their budget effectively.


Ultimately, understanding the costs and factors associated with building an ADU in Orange County, CA is essential for homeowners considering this option. Estimating costs of the construction is complicated and dependent on a number of factors, such as site conditions, build-type, structure size, and chosen finishes that can greatly impact the overall cost. Proper budgeting, including architect or designer fees, is crucial for a successful project. Homeowners should also explore available financing options.

To schedule a free consultation and rough estimate, please reach out to Superior ADUs. Enter your information in the web form, give us a call, or send us an email anytime.

Phone: 949-216-0686


*The prices and costs that are included in this article are estimates for construction only as each ADU is site-specific. Costs vary depending on location and many other factors mentioned in this article. The costs cited are to be used as a reference only, and they do not include the costs of permits, plans, engineering, design, or other city/utility fees.

ADU Cost Orange County

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