Construction
Phase Six
- Get bids
- Hire a contractor
- Understand everyone's roles during construction
Project Timeline
The Construction phase typically takes 6-12 months (or more). Most ADU projects take 12-18 months to complete, but some extend to 24 months or more.
Step-by-Step
Hire your contractor
Construction step 1
If you are not using a design/build firm, you will need to hire a contractor for the construction phase of your ADU. You will use drawings from your designer to solicit bids from contractors and make sure you are specific about what you want each bid to include (license details, insurance information, examples of past work, etc.). Your design team may be able to help you with this.
Different contractors will provide different bids, perhaps even different kinds of bids. The two most common are the fixed-price “not to exceed” bids, and the “cost-plus” bids. A “not to exceed” bid is a bid that has a total price determined upfront. There are often many caveats and assumptions built into that price, and changes that occur after the bid is produced are added to the total price. In the current building environment, “not to exceed” bids are not as common as they once were. Labor and materials prices continue to rise, so many contractors are unwilling to provide a “not to exceed” bid. Many choose to use a “cost-plus” bid instead. This is a bid that gives an estimate before construction based on assumed costs based on the design and level of finish desired plus some profit for the builder. Should those costs change either before or during construction, the end cost will reflect that.
It’s a good idea to select between at least three bids, and it’s okay to ask contractors clarifying questions about their bid. Your designer may be able to help you compare the bids. It’s a good idea to reach out to references and to consider your personal interactions with the contractor – you’ll want to feel like you work well together and that they understand your goals.
See our Workbook for many more details on getting bids and hiring a contractor and our ADU Exercises for a list of questions to consider when hiring a team. Visit the Marin Builders Association to find a contractor or confirm if someone you are considering hiring is licensed.
Monitor construction
Construction step 2
Once you have building permits, the contractor you hire will lead the construction of your ADU. Be sure you have all funding in place before you start construction.
Communicate frequently with your contractor to make decisions about fixtures and finishes as needed and verify progress before making payments. Timelines for construction vary, but 6-12 months is fairly common. See more details about managing construction in our Workbook.
During construction there are often many questions and decisions that need to be made as the ADU comes together. This is normal, and one reason it’s important to have a good relationship with your General Contractor. They can help you understand the importance and ramifications of the decisions you’ll be making.
Key Resources
Get inspections
Construction step 3
During construction, your ADU will be inspected multiple times to ensure it is being built according to the permitted plans (typical inspections include foundations/footings, framing, electrical/plumbing, exterior finishes, etc.). See our blog post about sample building inspections.
Once construction is complete and receives the coveted Final Inspection, then the ADU is ready to be lived in. Congratulations!
Related Blog posts
FAQs
Find answers to the most frequently asked questions about construction.
What is the difference between a site-built and a prefabricated or manufactured ADU?
Site-built/Traditional: A traditionally constructed ADU is designed and built specifically to your preferences and property and built on site (“stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefabricated/panelized/modular: These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.
What will it cost to build an ADU?
Our ADU Calculator is a great place to start when developing a budget. It provides a rough estimate of costs and income and will help you understand how choices can impact your budget over time. In general, it is helpful to avoid having a fixed budget total in your head as you explore your options.The cost to build an ADU typically ranges from $30,000 for a simple interior conversion JADU, to $400,000+ for a large detached ADU with high-end finishes on a hillside lot. Cost per square foot is a good way to estimate, though this too can range — a very rough placeholder for you to use is $450 per square foot for construction and other costs (design, fees, etc.), depending on your design and the materials you chose.
See more details about costs – including design, permitting, and construction – in our ADU Workbook. Also see our blog post on 5 hidden costs.
How do I find a contractor?
If you are not using a design/build firm, you will need to find a contractor to take over for the construction phase of your ADU.
First, you’ll solicit bids. See our Workbook for more details on what you want to see in a bid, what other documentation to collect from potential contractors, and what to look for in your bidding candidates. You will want to get at least three bids for comparison. Typical construction costs for new, detached ADUs are between $400-$600 per square-foot as of May 2023.
When you have bids, you can begin selecting your contractor. See our Workbook for more details on how to compare bids and choose the best option for you.
Before you hire a contractor, make sure to check their license and insurance and when they present you with a contract, review everything carefully. See our Guidebook (LINK) for more details.
How can I keep construction costs down?
Construction costs for your second unit will vary significantly depending on personal preferences, site conditions, location, and many other factors.
Size: Despite what many think, smaller ADUs may cost almost the same as larger ones. Many costs like foundation, kitchen and bathroom work only increase slightly for larger ADUs. Kitchen costs will range from $25,000–$50,000 with each bathroom ranging from $15,000–$25,000.
Type: New construction, both detached and attached, tend to be the most expensive. Garage conversions are not much cheaper than new construction if at all. Conversions of interior space (basement or otherwise) are often the cheapest.
Other factors that affect cost:
- Quality of interior finish work and amenities
- Architectural form and details
- Extent of utility, structural, mechanical, electrical, and plumbing upgrades required
- Required site upgrades (sidewalks, sewer and water)
- Whether sprinklers are required
- Lot complexity (slope, trees, fault lines, etc.)
How long does construction take?
Traditional construction will take 6-12 months, though this will vary depending on the specifics of the project. Stages of construction include:
- Site preparation: 1-2 months
- Foundation: 1 month
- Walls, roof, doors: 1-2 months
- Plumbing & electrical: 1-2 months
- Insulation & drywall: ½-1 month
- Fixtures & finishes: 1-2 months
- Final touches: ½-2 months
What are my responsibilities during construction?
While your contractor will lead the construction process, you will have the following responsibilities:
- Keep in touch with your contractor and set up a schedule for checking in.
- Regularly walk through the construction area to monitor the quality of the work and make sure the work is progressing the way you expect.
- 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.
- Follow the contract you agreed to, including any changes as described specifically in a change order form.
- 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.
