Design
Phase Four
- Identify what you want in a design
- Understand the design process and professionals involved
- Hire a professional team to create your design
Project Timeline
The Design phase typically takes 1-6 months. Most ADU projects take 12-18 months to complete, but some extend to 24 months or more.
Step-by-Step
Hire experienced professionals
Design Step 1
You can build an ADU as an owner builder, but bringing on a professional early in the process is often key to getting your ADU approved quickly, managed efficiently, and built cost-effectively. Relevant experience and fit are critical, and it’s important to look at their past work and check references.
Your team may include one or more of the following:
- Licensed architect or designer to design your ADU and potentially see you through permitting and construction
- Contractor to build your ADU
- Design/build company that designs and builds your ADU
- Modular/prefab company who sells preset designs for modular/prefab homes
- Septic/wastewater consultant to help you plan early on, if relevant. See the County’s lists of Septic Engineers and Septic Contractors.
Our ADU Exercises include a list of questions to consider when hiring a team, and our Workbook provides an overview of who you may need to hire based on your project. Visit the Marin Builders Association to find a contractor or confirm if someone you are considering hiring is licensed.
Create initial design
Design step 2
Once you have your team in place, you will work with them to design your ADU. Together you will consider size, use, layout, specific project needs (storage, laundry room, etc.), architectural style, and privacy.
The design portion of an ADU project is driven by budget and choice. One of the first choices is size – the square footage of your ADU has ramifications on permit costs, total project costs, and of course, available living space and features. Size and features (such as a second bedroom, or washer/dryer combo) can often make a slightly larger ADU more valuable both to property values and potential rental income, but can also incur additional fees (500 square feet and 750 square feet are common cutoffs for certain fees).
You may also think about “Universal Design,” the idea of making a space usable for the greatest number of people regardless of age, ability, or stature. This includes such things as putting light switches lower than standard to allow easier use from a wheelchair, lever handles for doors instead of knobs, and clearance in hallways and doorways to allow for ease of wheelchair or walker usage. If you plan to use your ADU for aging-in-place or senior family members, these decisions are crucial.
It can be a good idea to involve a general contractor during this stage, since they can bring in up-to-date expertise in controlling construction costs. For example, a general contractor may suggest a simpler roof design to ensure that factory-built trusses can be used instead of a design plan that requires hand-framing the roof on site.
Once you have an initial design, it’s a good idea to discuss it with local Planning staff so they can point out any issues before submitting your permit application. Your design team can attend this meeting to clarify drawings and help you understand requirements.
Key Resources
Finalize design
Design step 2
After you’ve made decisions about your initial designs and received feedback from local staff, your design team will work through any required changes and prepare the permit application (see Permitting).
At this point you can also get construction drawings from your designer that you could use to get construction bids from potential contractors.
FAQs
Find answers to the most frequently asked questions about design.
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.
How do I find an architect, designer, or prefab company?
Most homeowners choose to work with some type of design professional to plan their ADU and help throughout the process. Bringing on a professional early in the process is often key to getting your ADU approved quickly, managed efficiently, and built cost-effectively. Relevant experience and fit will be critical.
There are a variety of types of designer, and they may be an architect, builder, “designer,” design/build, or a modular/prefab company. If you’re hiring a local individual or team, they’ll likely start the process by visiting your home and talking to you about your ideas and goals. If it seems like a good match, they will prepare a proposal detailing their services and fee. Professionals typically charge for an initial consultation or proposal.
See our Exercises for a list of questions to ask a potential architect or designer, our Glossary to be clear on terms, and our Workbook for more details.
When do I show local staff my designs?
Once you have a design established with your architect/designer, it’s a great idea to discuss it with Planning staff so they can point out any issues before you prepare your application.
For contact information, see Local ADU Rules.
This is also a good time to reach out to utility agencies (water, sewer, gas, etc.) to inquire about their infrastructural requirements and confirm connection and service fees.
How much does it cost to design an ADU?
Design costs cover the work of your architect or designer and may or may not include engineering, surveying, and other professional services necessary to prepare for permitting. Together, these services may total around 10% of your total project budget – the exact amount depends on the details of your property and project. See the Budgeting and Finance page for more details.
Do you have any resources on eco-friendly ADU design?
Yes! Check out this blog post on Green Building Strategies for ADUs in California.
