Serving San Diego, Greater LA, SF Bay, & Seattle
Watch our free ADU webinar
All ADU Resources
Financing & Pricing

What does it cost to build an ADU? (Part 2)

Homeowners often come to us asking how much it will cost to build their ADU. After covering the soft costs in our previous post, we break down the nuts and bolts of your project: the construction hard costs.
Updated May 08, 2021
Check out our previous post here for Part 1 of the costs of building an ADU.
From talking to countless homeowners, we have learned that it's surprisingly hard to figure out what an ADU will cost before you’ve already committed to a project, paid thousands for designs from your architect and permitting fees, and pulled your ADU permit.
We’ve created this guide to transparently share all the components of costs that go into building an ADU, whether you're creating a new construction, a garage conversion, an in-law suite, or a basement apartment. Costs can be divided into hard costs (labor and materials related to construction) and soft costs (all of the other expenses related to the construction project). In our previous post, we covered the soft costs of building your accessory dwelling unit from the ground up. The second part of the cost of building an ADU are the hard construction costs for your ADU—we cover those in the rest of this post.
Cottage's process is set up to estimate the costs for your project before you commit a penny to building your ADU. Click the "Get Started" button at the top right hand corner for a free consultation and estimate — otherwise, read on to learn more about the hard costs for your ADU.

The Hard Costs of Building An ADU

Site Work & Preparation

Before your detached ADU or attached ADU goes up, there typically is work to be done down in the ground. To start, your intended ADU site may need Grading to provide a flat, level area for the ADU to be built and foundation to be poured. If your lot is sloped or needs a retaining wall to hold back or hold in soil to maintain the flat area, that may incur additional site work fees in the thousands to tens of thousands of dollars.
Once the intended accessory dwelling unit area is ready, the concrete slab on grade Foundation is ready to be poured. This foundation will support the house up to code and provides a firm footing for your unit. In situations where the ADU is being built on a slope, a pier-and-beam or a raised perimeter foundation may be required, which also adds additional cost to the backyard home.
Site preparation work will also involve Utility Trenching to prepare for sewer and water pipes to reach the ADU, as well as electrical and gas lines if necessary.
Cottage includes all of these site-specific costs into your tailored cost estimate. Our recommendation when working with other contractors is to confirm all these easy-to-overlook details are included - we have seen some contractors hide these costs in up-front construction cost estimates.

Building Structure

Once the ADU site has been graded and foundation has been poured, your custom ADU project is ready to move on to having the Framing and Drywall go up for the unit. In addition to these key structural parts of your ADU, the Roofing and Insulation will be installed at around this time.
Towards the end of your ADU journey, builders will typically install the Exterior Finish for your backyard home. Typical finishes include stucco, horizontal wood siding, and vertical siding, though we at Cottage have worked with custom options for different types of ADUs to meet homeowner needs and match existing homes.


Your existing house has all the utilities necessary to be able to call it home, and your ADU is no different. From Rough and Finished Plumbing to gutters to Electrical and Gas (if desired and allowed in your city or county) to Water and Sewer to Heating, Ventilation, and Air Conditioning (HVAC), your ADU will require all the necessary utilities to support one or more bedroom, bathroom, and kitchen areas in your ADU.
Cottage makes sure to include these utility hookups and details in our tailored cost estimates for your turnkey, custom ADU. Keep in mind that the key foundation, structural, and utilities steps to this point account for the majority of the cost of your ADU, rather than the fixtures and finishes to follow. This is why larger changes to the floor plan, square footage, shape, ceiling height, and roof shape have an outsized impact on the cost estimate for your ADU—they contribute the most to your ADU's cost!

Doors & Windows

No accessory dwelling unit or backyard home is complete without an entryway swing Door(s) and Windows to allow ample light to enter your ADU space. From double swing French doors to Marvin windows to skylights, there are many different custom options for homeowners to choose from for the perfect ADU.
Cottage's design team helps homeowners like you design the ADU that's right for you, including the right assortment of doors, windows, skylights, and other entryway features. We help guide you through deciding the cost-benefit tradeoff of simple versus luxury windows and glass for your project.

Fixtures and Finishes

Once the base shell of the ADU is complete, that's when the interior decorating fun comes in for many homeowners. Paint and Flooring choices are often at the forefront of homeowner's minds, while for others, the Baseboard, Crown Moulding, Tile Countertops, ​​​​and/or Custom Millwork bring out the HGTV excitement.
The finishing decisions extend throughout the ADU, Cabinets and Light Fixtures are staples in every ADU, while Appliances like a dishwasher and Sinks & Faucets are especially important for a chef-inspired ADU kitchen space. Bathrooms introduce additional finishing choices to the mix: Showers & Bathtubs, Vanities & Toilet, and Faucets & Fixtures provide opportunities to add in another custom flair to your ADU space.
Cottage's custom ADU design process allows homeowners to pick and choose the level of fixtures and finishes they like, while also offering off the shelf options for homeowners who want a streamlined, no-hassle design for their custom ADU.

Contingency & Construction Management

Most general contractors will include a contingency in their budget estimate to account for any homeowner changes to the scope of the project between the time of the estimate and the date of the actual work being completed. In addition, most will charge a fee or profit margin to account for the coordination, sequencing, and management of subcontractors and/or skilled tradesmen/tradeswomen completing your project.
Cottage makes sure to provide contingencies and fees where needed as up-front components of an estimate as part of our transparent, A-to-Z custom ADU process.

So What's Next for My ADU Project?

Once you've received an estimate for your project, you're ready to proceed with actually building the new unit. Typically in the industry, you would have to face many or all of the up front soft costs before seeing a cost estimate from a general contractor. And once you did finish paying all the soft costs, you would be faced with waiting for the city permit to come back before you could use the stamped plan set to bid out your project. If you plan to build rentals, there may be even more permits necessary to legally rent space in what was formerly a single-family home.
At this point, however, you would have little way of knowing that your design was the most cost effective or that the general contractor(s) giving you an estimate were charging a fair price—let alone the best price. We have heard of many blown budgets at this point when the plans are hard to adjust without re-designing and re-permitting your granny flat.
At Cottage, we believe in providing you with a tailored, up-front cost estimate before you financially commit to your ADU project, so you know what type of mortgage, line of equity, or renovation loan you'll need to make this construction affordable.
Now that you know what goes into the hard costs and soft costs for your California ADU, click the "Get Started" button below to get a Free Consultation and Estimate Today!

Get your free consultation and estimate today!