Phase 4
Design
- Identify what you want in a design
- Understand the design process and professionals involved
- Hire a professional team to create your design
Once you have a sense of what you want to build, know what you can build, and have estimated your budget, you can start designing your ADU, which begins with design inspiration and ends with permit-ready plans.
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.
Key Resources
for Design
Frequently Asked Questions
Here are a few of the most frequently asked questions about ADU Design. See the content below and your County’s ADU Guidebook for more guidance, resources, and tips for all steps of the process.
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.
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.
Note that if you’re not using a licensed architect to design your ADU, your plans may need to be stamped by a licensed engineer. Check with your staff early on.
See the Exercises for a list of questions to ask a potential architect or designer, the Glossary to be clear on terms, and your County Guidebook for more details.
Once you have a design established with your architect/designer, it’s a great idea to discuss it with County staff so they can point out any issues before you prepare your application.
You may be able to schedule an appointment to speak with a planner or walk in to the Planning Counter or Permit Center. For contact information, see Contact page.
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.
The Mother Lode ADU Plans Gallery provides property owners interested in building an ADU with an easy way to compare and select from dozens of plans – including modular/prefab and site built – saving you both time and money. Like many online shopping experiences, you can filter by the kind of ADU you want (number of bedrooms, square footage, features) then view photos, floorplans, and details of all the designs you like. You can then connect directly with the designer or company.
To see translated Spanish FAQs, head to All FAQs.
Design, Step-by-Step
Before You Begin: It is helpful to have a clear sense of what you want early in the process. An architect or designer can help you brainstorm, but they cannot start designing until you’ve made basic decisions like the type of ADU you want (see Getting Started) and how many bedrooms it will have. Our Exercises can help you think about these questions.
Design Step 1
consider Different Kinds of Plans
Consider a Prefab ADU Plan
See our Prefab ADUs Showcase to browse some examples.
Consider a Design from the ADU Plans Gallery
Jump start your design by visiting our Plans Gallery to view and compare dozens of pre-existing designs and connect directly with the designer or prefab vendor. Choosing any one of these plans will save you time and money. While you will most likely need to hire an architect to customize any plans you choose based on local rules and your property, you will be saving a lot on developing an initial design, and whoever you hire can usually help you through the application and permit process. The Gallery also includes the prefab plans featured on this website.
Helpful Tools
Types of ADU Construction
There are two types of construction to consider for your project. Both have pros and cons, so it can help to think about what you want, like the level of customization you want and how the ADU would work on your specific property (like slopes and obstacles for installation). For more information on Prefabricated ADUs, see our ADU Guidebook.
Site-Built is a traditionally constructed ADU designed to your preferences and property and built from scratch on site (AKA “stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefab / Manufactured / Panelized are built in a factory, then shipped to your site to be assembled or placed on a foundation. Some companies will provide a “turnkey” service that includes help with permitting and all on-site construction, from laying the foundation to utility hookups. If not, you’ll need to hire additional professionals to help.
design Step 2
Hire Your Team
Although you can build an ADU as an owner builder, we recommend hiring a licensed architect or designer and a licensed contractor, or a design/build team, 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.
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
- Licensed 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
Our ADU Exercises include a list of questions to consider when hiring a team, and your County Guidebook provides an overview on design contracts and costs.
design Step 3
create initial design
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. If you decide to go with a Pre-Reviewed Plan, you will work to customize the plan to your property.
Once you have an initial design, it’s a good idea to discuss it with County 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 Contact page for all relevant contact information.
