Phase Four
Design
- 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
Consider pre-designed plans
Design Step 1
For detached ADUs, using an existing design can save you time and money. You can also look for pre-approved plans, which could make you eligible for faster permitting.
Please note: all existing ADU designs – even pre-approved plans – require planning for your property and other permit applications. You most likely need to hire a professional to complete this work. Not all designs work on all properties, so ask a professional before purchasing or using a design.
Check our Plans page for sources of pre-designed plans.
ADU Plans Galleries
Many communities in San Mateo County have a local ADU Plans Gallery, launched now or coming soon. These galleries feature site-built and prefabricated plans selected by local Planning and Building departments, including any locally pre-approved plans.
Hire experienced professionals
Design Step 2
Although you can build an ADU yourself (as an owner builder), we strongly recommend hiring a licensed architect or designer and a licensed contractor, and most homeowners do. 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.
Based on the type of ADU and details of your project, 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
- Engineers and specialty consultants (examples: septic consultant, landscape architect)
- Licensed contractor to build your ADU
- Design/build company to design and build your ADU (instead of a licensed architect/designer and contractor)
- Modular/prefab company to manufacture and install a move-in ready ADU (designs are generally not customizable)
See our free ADU Guidebook for more details on who you might need to hire. Our ADU Exercises include a list of questions to consider when hiring a team.
Key Resources
Create Initial Design
Design Step 3
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. First, your designer will take measurements of your property (and/or ask you to hire a surveyor to complete detailed, accurate technical drawings of your property) and create conceptual drawings including the potential ADU and its relationship to the main house and other features. They will then help you to decide on a design concept (size, orientation, etc.) and start making more detailed plans. Check out our free ADU Guidebook for more design tips including green features, privacy and security, small-scale living, and more.
If you decide to go with a pre-designed plan, you will work with your team to customize the plan to your property.
Once you have an initial design, it’s a good idea to discuss it with local 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.
If you haven’t already, this is also a good time to reach out to utility service providers (water, sewer, gas, etc.) to confirm your design meets their requirements. See Local ADU Rules for all relevant contact information.
Browse real-world floorplans
Check out our Floorplans Inspiration page for dozens of real-world California ADU floorplans. Keep an eye out for innovative uses of space and combined use rooms.
Finalize Plan
Design Step 4
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.
Frequently Asked Questions
Here are a few of the most frequently asked questions about design. See the content above for more guidance, resources, and tips for all steps of the process.
How long does it take to build an ADU?
Building an ADU is an investment of time as well as money. Most projects take one to two years to complete. Typically, it takes homeowners one to three months to get started and assemble their team, then one to six months to develop plans, meet with the city, and submit the application. Depending on what permits are required, how many rounds of review are required and how quickly a homeowner and their project team can respond to comments, it will take one to six months to get permits. Construction usually takes six to twelve months.
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.
Can I put an ADU in my garage?
Homeowners can convert legally built structures (garage, barn, art studio) into an ADU. JADUs can be converted from an attached garage (but not detached).
If you demolish your garage or other enclosed structure and build an ADU in its place, the ADU can be in the same footprint if it’s the same size and height of the structure it’s replacing.
You do not need to replace the parking space if converting the garage to an ADU, but you may need to replace it if converting to a JADU. Demolition permits for an existing detached garage can be processed at the same time as the ADU permits. Note that garage conversion ADUs may require significant moisture barriers and other design elements in order to meet building codes.
How do I find an architect or designer?
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 designers, 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. Check with local staff for local requirements.
See our Exercises for a list of questions to ask a potential architect or designer and our Glossary to be clear on terms.
When do I show the City/County my design?
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.
Some utilities will be coordinated by your local government and others by outside agencies. You can find contact information for your local Planning Department on our Contact page, and local staff can direct you to your area’s services providers.
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.
Does San Mateo County have pre-approved ADU plans?
Check our Plans page for more details, including these two sources:
- Some communities in San Mateo County have an ADU Plans Gallery that shows locally pre-approved plans, plus prefabricated options (which are considered pre-approved at the state or federal level).
- HEART of San Mateo County offers free ADU plans that can be customized for your property. Explore plans online. Depending on where you live, some Planning Departments offer a quicker application process for these plans. To see if you’re eligible for expedited review, check the GLADUR Pre-Review webpage and talk to local staff.
ADU Layouts that fit your needs
Browse real-world ADU floorplans
