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​What Is The Average Cost Of An ADU? Understanding Accessory Dwelling Unit Cost

September 25th, 2020

The cost of an ADU (Accessory Dwelling Unit) can vary quite a bit. But if you’re planning on building an ADU on your property in Santa Barbara, San Luis Obispo, Monterey, or Kern County, you may be wondering if you can get a rough estimate of the cost of your project.

So we’re here to help. In this guide from Mettle Construction Group, we’ll discuss a study that looks at the average cost of ADUs and help you determine if investing in an accessory dwelling unit is a good option for you. Read on to get all of the details.

The Average Cost Of An ADU Is $181,000, But There’s A Lot Of Variation

According to this post from Building an ADU, homeowners who built ADUs spent an average of about $181,000, with an average cost range of between $112,000 and $285,000. The average cost per square foot is about $305.

The cost of an ADU can vary quite a bit based on the type of ADU, as well. The average cost of detached new construction ADUs and basement ADUs was closed around $181,000 – while attached ADUs had a lower average cost of $154,400, and garage conversions had the lowest average cost of $142,000.

On the other hand, the most expensive type of ADU is a detached new construction above a garage, with an average cost of about $218,000. This is the most complex type of ADU to build, so the high accessory dwelling unit cost for this type of ADU is not a surprise.

Each ADU Is Different – And A Lot Of Different Factors Can Affect The Cost!

cost of an ADU

As mentioned in the report linked above, the “average” cost of an ADU may be $181,000, but there can still be a lot of variation. The problem with providing a solid “average” for the cost of an ADU is that there are so many factors that can affect the cost of construction. A few of these include:

  • Geographic location – Because the cost of living is higher in California, it will be more expensive to build an ADU in the San Luis Obispo or Santa Barbara Counties compared to building an ADU in a Midwest state like Ohio or Indiana.
  • Local permit costs – The process of submitting designs, obtaining permits, and working through the paperwork and bureaucracy involved in building an ADU can be quite expensive, both in terms of fees and the time spent on the process.
  • Utility connection costs – Depending on your home and your ADU, you may be able to run your own utility lines to the ADU, or you may need to pay for new utility connections like power, sewer, water, gas, cable, and more.
  • Type of ADU – ADUs built inside your home (such as garage conversion or basement ADUs) tend to be cheaper than standalone ADUs, for obvious reasons. Remodeling an existing structure into an ADU is a simpler process and requires less work and raw materials compared to building a new structure.
  • Square footage – The larger an ADU is, the more it will cost, in most cases. The larger the ADU, the more work it is to remodel or build the accessory dwelling unit, and more raw materials will be required.

To get an accurate estimate of accessory dwelling unit cost in the San Luis Obispo, Santa Barbara, Kern, or Monterey Counties, it’s best to consult with an experienced ADU builder like Mettle Construction Group.

Need An Estimate For Accessory Dwelling Unit Cost? Contact Mettle Construction Group!


As experts in building ADUs in California, Mettle Construction Group is the best resource if you want to build an accessory dwelling unit on your property. An ADU is a great investment, so don’t wait. Contact us online or call us at (805) 361-7403 to speak with an expert – and take the first steps toward getting an estimate for your ADU project.

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