Development Costs
How Much Does an ADU Cost?
This is a deceptively simple question, with a complicated answer.
A decent ballpark figure is $300,000, but several variables can make things as low as $80,000 or as expensive as $880,000. That’s a rather wide range and admitedly not very helpful.
The final cost (design, permits, & construction) will depend on the existing circumstances of the site, how large it will be, as well as the level of finishes and detail desired.
Below we’ll break down some of the contributing factors to better understand where things might be for your project. Some of these will be general to all ADUs, and some will be specific to each site and set of circumstances.
It’s All Relative
Compared to a typical home remodel, the cost per square foot for an ADU can be quite expensive. But you have to keep in mind you are not just adding a bedroom, bathroom, or dream kitchen remodel. Building an ADU is building a small house.
With that in mind, total development costs for ADUs are much lower than typical homes in part because of the size restrictions placed on them, but also because there is no land to purchase, and the utilities are largely existing and only need modifying. This makes ADUs a much more affordable housing type.
And let’s not forget, an ADU can generate rental income while that new bathroom or kitchen remodel isn’t likely to start paying you back any time soon.
Proportion and Ratios Matter
Typical
House
25%
ADU
40%
Kitchen & Bath Percentage of House
*Kitchen & bath vs. house percentages based on a number of factors and based on averages.
ADUs are limited in size, but still need a kitchen and bathroom which are the most expensive rooms to build. Making these rooms smaller saves money, but they can only be so small and there are certain features and appliances that they must have.
Because of these realities, the area dedicated to the kitchen and bathroom(s) is a much larger percentage of the ADU than for a typical home. This increases the average cost per square foot for ADUs when compared to typical homes with similar finishes.
Additionally, there are basic overhead costs (ex: permit fees) that come with every project no matter how big or small they are.
This can be discouraging if that is your only focus, but remember, you are building a small house and the overall cost of this ADU is going to be much lower than that of a typical house.
Existing Building Conditions
VS.
This is a detail sketch of a typical turn down style footing found on many older garages. This type of foundation may not be sufficient to support a second floor.
A spread footing with stemwall, like the example shown above, will be more likely to support a second floor, but still needs to be verified for depth and thickness.
If you are building an attached ADU by building out, building up, or reusing an existing space with a conversion, the existing construction is important.
Assuming the framing and other materials are in good shape, the existing structure may still pose issues. For example:
If the walls are 2x4 framing instead of 2x6, they will need additional insulation to meet current code requirements. Also, if you want to build above that 2x4 wall, it may need to be reinforced, or potentially removed and built back with 2x6s.
If the foundation is small, or too shallow, it will need to be modified or replaced. This can be determined with a bit of excavation, but it’s important to know how and where to do this.
None of these issues are insurmountable, and we can help navigate these issues.
Existing Site Conditions
A
B
Adding an ADU to a lot with an existing house, means also navigating the existing site features and infrastructure. Trees, neighbors, views, and other site conditions will dictate where the best location is, but this is not the only thing to consider.
You also need to get water to the ADU as well as connect the ADU to the sewer lateral for the main house.
In the example images here, situation ‘B’ is almost certainly more expensive than ‘A’. The only difference is the location of the existing sewer lateral. Now you don’t make all your decisions based on just this, but it’s important to know as much as you can going in so you are prepared.
