How to build an ADU
Now that you have your building permits, you can start construction of your ADU. The builder you hire will lead this step. During construction, your ADU will be inspected multiple times to ensure it is being built according to the permitted plans.
Step 1: Bring on the Builders
There are four key aspects to preparing for construction if you haven’t hired a Design Firm or ADU Specialist to manage this for you:
Project Manager
The first step is hiring the person who will oversee your project. This includes hiring subcontractors for electrical and plumbing, coordinating construction, managing schedules, making payments, working with the city on permits, and more.
Getting Bids
To get bids, you will use the construction drawings your designer completed. You’ll want to be specific about what the bid should include, and be clear on which costs are covered and which are not. If you did not have complete construction drawings prepared (just drawings for planning), it is even more important to know what assumptions the contractors are making.
Selecting a Contractor
When you have at least three bids, you can begin the selection process. First, make sure you understand all the details of the bids and, if necessary, ask the contractors any clarifying questions. Generally, there is a trade-off between experience and cost.
Building Cost
Construction costs for your second unit will vary significantly depending on personal preferences, site conditions, location, and other factors. The cost to develop a second unit typically ranges from $30,000 for a simple interior conversion to $500,000+ for a large unit with high-end finishes on a hillside lot. Generally, most projects are between $250–$500 per square foot, and $400 per square foot is a good starting estimate for an average project.
Step 2: Manage Construction
During construction, you will check in regularly. The checklist below walks you through what to expect during this step.
Communicate - Keep in touch with your contractor and ideally set up a schedule for checking in.
Monitor - Regularly walk through the construction area to monitor the quality of the work and to make sure the work is progressing the way you expect.
Decide - Be prepared to make decisions about the details—light fixtures, appliances, and other materials—in a timely manner so your contractor can stay on schedule.
Adjust - Follow the contract you agreed to, including any changes as described specifically in a change order form.
Inspect - Although your contractor will usually arrange the required city or utility inspections, it is your responsibility as the property owner to make sure that the inspections are conducted as required.
Step 3: Get Inspections
You will need to complete all inspections required by your city.
During construction, your ADU will be inspected multiple times to ensure it is being built according to the permitted plans. When all the work is complete, a building inspector will conduct a final inspection. Upon approval, you will receive a Certificate of Occupancy and possibly a registration letter for your new ADU.
Please note: Your design team or ADU specialist will handle this step. We strongly recommend homeowners work with a professional throughout this and the entire process.
About Casita Coalition
Casita Coalition is the only statewide, multi-sector organization that brings together all key players to remove policy barriers, making a big impact through small homes. Working through our Board, Advisory Committee, and Working Groups, we draw on the diversity of expertise and experience of our members to make it easier to build small homes in all neighborhoods across California. Visit www.casitacoalition.org to learn more.