ADU Size: 500 sq. ft.

Construction Type

Lot Slope



Utilities & Finish


Please note that this calculator provides an estimate to familiarize you with ADU construction costs. It is not meant to be entirely accurate and should not be interpreted as a final quote.

California ADU Construction Cost Calculator

An ADU in California costs approximately $125 to $300 per square foot, but some can reach as high as $600 per square foot. Your costs will vary depending on unit size, location, lot conditions, building and safety regulations, zoning requirements, and personal preferences like fit and finish.

The main factors that impact ADU construction costs are:

  1. ADU Size
  2. Construction Type
  3. Lot Slope
  4. Utilities & Finishing
  5. Soft Costs
  6. Contingency

ADU Size

Larger ADUs cost more to build but have a lower cost per square foot than smaller ones. This is because larger ADUs spread the total construction cost over more area. To reflect this, our calculator projects higher costs for the initial 500 square feet and less for additional footage.

ADU Size (sq ft)Average Low PriceAverage High Price
300 sq ft$37,500$90,000
500 sq ft$62,500$150,000
800 sq ft$100,000$240,000
1000 sq ft$125,000$300,000
1200 sq ft$150,000$360,000

Construction Type

Converting existing space (like a guest house, attic, basement, or garage) is typically less expensive than new construction. For such conversion projects, construction, labor, and materials costs can range from $125-350/sqft. New construction project costs may range from $250-400/sqft.

ADU conversion may be less expensive since it reuses the existing foundation, walls, and roof rather than building an entirely new structure. New construction ADUs may also be subject to additional requirements, like installing fire sprinklers and solar panels. For this reason, we estimate a +/-15% cost range for new construction projects.

Also, consider that the cost of a conversion depends on the foundation's quality. Old or cracked foundations will need reinforcement. We account for this by calculating a +/-20% cost range between the Low, Medium, and High estimates.

Construction TypeAverage Cost / sq ft
Conversion (attic, basement, garage, guest house)$125 - $200+
Attached ADU$150 - $250+
Detached ADU$180 - $300+
Building an ADU over existing structure$200 - $350+

Lot Slope

Building an ADU on a flat lot is more effortless and less expensive. Sloped lots may require grading, soil compaction, or a retaining wall to support the foundation and prevent drainage. Also, bringing equipment and materials to a property on a slope can be challenging if the site lacks an accessible driveway or side yard for easy access. Thus, we assume that design and construction costs will be 25% higher for an ADU on a sloped lot.

Utilities & Interior Finish

Utilities and interior finish significantly impact the final cost of ADU construction. These include electrical, plumbing, and HVAC systems, appliances, windows and doors, flooring, cabinets, countertops, tiles, fixtures, etc.

Finish LevelAverage Cost / sq ft
Entry-level utilities and appliances, inexpensive fixtures, carpet flooring, vinyl windows, etc.
$10 - $20
High-quality finishing, stainless steel appliances, granite countertops, hardwood flooring, etc.
$20 - $40
Best possible finishing, top-of-the-line appliances, solid wood cabinets, ceramic tile floors, etc.

Soft Costs

Soft costs relate to project planning and typically include:

  1. A boundary survey outlines the borders of the parcel, confirms parcel ownership and proves the property complies with zoning regulations and ordinances.
  2. Architectural and engineering costs associated with designing, drafting, and engineering a complete set of plans required for all ADU projects.
  3. A structural engineering report confirms the building plans meet the state's code and safety regulations.
  4. Title 24 energy calculations estimate the unit's energy usage and confirm the plans meet the state's energy efficiency requirements.
  5. Permitting fees charged by the city, as well as additional surcharges (like school district or energy fees) for new construction projects. These vary greatly by municipality.
ServiceAverage Price
Design$2,000 - $30,000+
Architectural Plans$2,000 - $40,000+
Engineering$0 - $5,000+
Permits & Application Fees$1,500 - $8,000+
Impacts Fees (for ADUs over 750 sq ft)$2,000 - $14,000+
Utility Fees$0 - $20,000+
Project Management$2,000 - $25,000+


Contingency is funds set aside to cover unexpected costs that can arise throughout an ADU project. We recommend budgeting a construction contingency equivalent to at least 10% of projected construction costs. Rising materials costs and supply chain delays are typical these days, and planning for some cost overrun can help mitigate these.