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How Much Does a Prefab ADU Cost?

Did you know that there are more than 13 million ADUs? With so many homes having an accessory dwelling unit, you might be considering adding an ADU in your own home. If you are wondering how much does a prefab ADU cost, you are in the right place.

Keep reading to learn more about prefab ADUs to see if they are worth it for you.

How Much Does a Prefab ADU Cost?

The truth is that the price of a prefab ADU varies. The price can go up and down based on how many square feet it is and the materials. Just like a regular house the more square feet, the more it will cost.

The average cost per square foot ranges between $345 to $405 but some companies can charge as little as $150 per square foot.

The cost will vary by ADU builders because some use extremely expensive materials while others use cheaper things to build with.

Another cost factor is whether you choose a detached ADU or an attached ADU. Detached construction will cost you more because you are completely starting from scratch. An attached unit such as a basement or above a garage will already have a foundation.


One of the other costs that you might have to take into account are local permits. Some counties and cities will require you to first get permission to add an ADU to your house and they might charge a fee for the permits to move forward with construction.

Sometimes subcontractors have to pay permit fees as well for electrical, plumbing, and mechanical permits. The price for these permits can range between $1,000-$2,000. When you hire a professional to take care of your build then you won’t have to worry about these permits because they will know exactly what they need.

Every area is different, take the time to research before you start construction because the last thing you want to deal with are fines.

Other Fees

Some states also have sewer, water, and park fees that they charge for construction. There are cities like Portland that are encouraging ADU builds and are currently waiving any city infrastructure for construction.

Just like permits you want to make sure that you have every fee paid for before you start construction. Some municipalities are so strict that they might make you take down anything you put up if you did not have the fees paid ahead of time.

Ready to Add an ADU to Your Home?

Now that you have a better idea of what to expect when you have a prefab ADU added to your home, you can make an informed decision if you still want to move forward with this addition to your house. Adding an ADU for your mother in law, elderly parent, or as an investment might be totally worth it for you.

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