Cost to Build ADU Orange County

Please see our updated post on ADU Cost Here.

The cost to build ADU Orange County varies greatly.  Many people want to know the cost per square foot to build an ADU in Orange County, CA, and that varies also.  It’s important to be educated about the different expenses that go into building an ADU and make sure that everything is accounted for.  Oftentimes, our clients are quoted a price by a contractor to build them an ADU, but they’re not told about all of the additional expenses that the client will incur.  The goal of this article will be to help you identify the different expenses that go into building an ADU, as well as some average costs for each component.


Whether you are building an ADU on-site or planning to install a prefab/modular unit, you will need plans to present to the city.  The plans must meet all of the city’s requirements in order to be approved.  Each city has slightly different building codes and ADU regulations that you must adhere to in order to build.  This can make prefab or modular ADUs difficult to use, because what one city allows, another city denies.  When purchasing ADU plans, be sure that whoever you hire checks with the city before you sign a contract with them.  You don’t want to have to pay for plans only to find out later that the city won’t allow an ADU on your property because of something like public sewer restrictions.

ADU Plans vary in price based on the size of the unit, the number of stories, and whether it’s attached, detached, or within an existing structure.  Junior ADU plans can start as low as a few thousand dollars, while large, highly custom, luxury ADU plans can cost upwards of $40,000.


An engineer is almost always needed in order to complete the architectural plans that the city requires.  An electrical engineer will be needed to do the title 24 calculations, while a structural engineer will be needed to complete certain calculations and documentation to ensure structural integrity.  Depending on the size of the project, engineering for a Junior ADU in an existing structure can be as low as a few hundred dollars, while engineering for a larger, new structure could cost upwards of several thousand dollars.

Permits, Impact Fees, & Utilities

The cost of permits and impact fees vary per city.  You can find out that information by calling your local planning department.  The cost for the utilities is also site specific and can be found out by calling your local utility company.  These costs typically range from a few thousand to several thousand dollars.


The is the cost to actually build an ADU.  Many times, this is the only cost that homeowners take into account, but you cannot get to this step without going through the prior three. 

The cost of the construction will vary based on a few factors. 

  • First, is the ADU going to be attached, detached, within an existing structure, or over an existing structure?
    • Within an existing structure is usually the cheapest, followed by a stand-alone detached ADU or attached ADU.  Over an existing structure, like a detached garage, is usually the most expensive.
  • What kind of finishes do you want?
    • Do you want builder-grade finishes or Italian marble?  Do you want vinyl windows or floor to ceiling glass?
  • Where are the utilities located?
  • Is the lot flat or will trees need to be removed?
  • Does your ADU require solar to be installed?

These are just a few of many different factors that go into the construction cost of an ADU or Junior ADU.  Whenever you are getting a quote, make sure that all of these items are taken into account by the contractor.  If a contractor gives you a cost estimate or price per square foot without asking these questions, be very cautious of the accuracy.  Depending on the project, the construction component could range from $25,000 to well over half a million dollars for a large, luxury unit.

Interior Design

While you can let a contractor decide and be in charge of what type of flooring, fixtures, countertops, and cabinetry go into the ADU, it’s always smart to have an interior designer on the team to help you with these choices.  Fees will vary from a few thousand dollars all the way up to 25% of the project cost.

Project Management

While you can oversee and manage the construction of the ADU yourself, it is always nice to have a professional involved to manage any disputes that may arise.  A project manager would help ensure that your ADU is built quickly, efficiently, and safely.  Project management fees vary from a few thousand dollars to upwards of 5-7% of the project cost.


Hopefully, you can tell by now that the cost to build an ADU varies greatly.  While you can hire each specific professional yourself, it is often better to find an ADU company that handles the entire process for you.  Our team at Superior ADUs consists of real estate experts and brokers, architects, engineers, designers, draftsmen, contractors, and project managers and coordinators.  We all work together to offer a one-stop ADU service to homeowners in Orange County, CA and the surrounding areas.  We handle the entire process, from design to communication with your city to construction and project management.  We offer different finish packages to account for differing budgets and effectively help you quickly and cost-effectively build an ADU on your property.

To schedule a personal ADU Consultation with Superior ADUs, please contact us by phone, email, or web form.

Phone: 949 – 216 – 0686


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