Key ADU Design & Modular Tips from a Sonoma County Designer
Author: Napa Sonoma ADU Guest Blogger Robin Stephani , Founder, 8th Wave & Advisory Committee Member, Napa Sonoma ADU Center
With 24 years of experience working in Design, Architecture and Construction, I founded 8th Wave, a design firm based in Santa Rosa focused on infill housing and development.
Key projects include design/build Accessory Dwelling kits utilizing pre-permitted manufactured and panelized assembly to help lower costs, streamline permitting and minimize build time. My firm 8th Wave has also been working with local nonprofit Homes for Sonoma to develop and build housing for people displaced from the series of recent Northern California Fires.
Here, I will share with the Napa Sonoma ADU Center audience some of my key ADU design tips to keep in mind as you begin to think through your project. To hear me talk through some of these tips plus see images of ADUs that illustrate these points, check out my talk on Napa Sonoma ADU’s on-demand webinar, ADUs 101 for Napa & Sonoma County Homeowners.
With that, let’s dive in! Building an ADU is essentially a jewel box home -- and so good design has a huge impact. As Benjamin Franklin once said, “A place for everything, everything in its place” -- this is my ADU design motto.
“Siting” the ADU on your parcel
Site considerations such as access, solar orientation, trees, parking, and neighboring buildings are key design considerations to keep in mind.
An L shaped-ADU works well if you are tucking your ADU in the rear corner of a property, since it can serve to create privacy from and for neighbors. With a 2 bedroom, the L shape allows for cascading roof lines and works well to allow for a lot of natural light, which I will cover more about shortly.
The right design depends on the particulars of your project and why you are building an ADU. Think about how you want to use the remaining yard and if you have any existing trees that you want to work around. Since we have such lovely weather here most of the time, you will want to consider how to use your yard to extend your living space. This work, called “siting” a building on your parcel, is a really important step, so don’t skip this step.
Open floor plan & vaulted ceiling
An open floor plan and vaulted ceiling makes a small space seem larger. In addition, windows on at least 2 sides of the ADU is crucial. Window layout is a balance between function and views, creating a beautiful exterior, and structural simplicity.
Another design element I use a lot are islands which are great flex areas and can function as dining, office, project space or just to create separation between the kitchen and living area.
Glass upper cabinets help to create more depth, and your furnishings need to be selected carefully so that they do not overwhelm the space but maximize storage. Remember, a place for everything and everything in its place.
Going bigger & thinking creatively with your square footage
You want to think long term about your financial investment in your ADU. What else can this ADU project solve for, beyond your short-term goals? Do you need additional parking or workshop space? Do you want an office or guest room? Are you downsizing in stages and need flexibility over time to do this?
You can build an ADU above a garage or studio space and reverse engineer the ADU ordinance to maximize what can be done without triggering impact fees, which under January 2020 California state laws takes place when your square footage is over 750 square feet.
Designing small requires total commitment to functionality
For some homeowners, aging in place is a huge consideration. You can build rooms in your ADU that can be easily converted from a study to another bedroom, for example if you might need full time care at some point in the future. Consider designing some basic universal design into your project as well so you can keep this option open, such as minimal clearances for doors and backing in the walls for adding grab bars, etc. Adding in elements of universal design is a tough and expensive change after the ADU is built.
Prioritize based on how you live
For example, with one of our ADU projects, the homeowner wanted to build the ADU, move into it and then rent out her 3-bedroom home to a family member so that she could retire early. Her dream was to travel and learn how to cook. So, she prioritized the kitchen and dining area in her ADU design and sacrificed having a large living room. She estimated that thanks to her ADU project, she was able to retire 5 years earlier and have extra money and freedom to travel!
Exterior design elements to keep in mind
When designing your ADU, you want to play off design elements of your existing house such as window types, roof pitch, siding material, and trim details. You do not need to match exactly, but the new building should relate to the main house so that it is in conversation with it.
For smaller ADUs, natural light is really important
Using light-colored paint and a vaulted ceiling opens up the space in your ADU and creates a sense of a larger volume. On one of our ADU projects, our interior designer used art to add depth of field to what is otherwise a narrow space. Do not forget the design criteria I mentioned earlier such as open floor plan, kitchen islands, and windows on at least 2 sides of the building.
Some thoughts on modular ADUs
One of the most popular ADU topics is Modular construction. Modular is built in a factory and brought to the site via truck so its width is constrained to what can drive on the freeway. Because of this transportation constraint larger units come in pieces and are stitched together on-site. You lose a little natural light, and in my opinion, it can feel like fitting a square peg in a round hole -- especially if you are going larger than 500 square feet for your ADU, I recommend that you consider other options.
However, modular is great for smaller ADUs. You pick your design off the shelf and the unit arrives on the back of a truck ready to plug into your utilities and foundation -- which shortens development time considerably and can cost less.
If Modular sounds like an option for you, there are a few things to consider
First, Modular is a good fit for flat sites with easy access so that you can get the unit onto your parcel without high costs and stress.
Second, if you decide to go modular - build for quality residential construction.
When we build an ADU, we want the buildings to be able to stand the test of time. For a slightly higher cost in materials we invested in quality construction, which pays for itself by being a more energy efficient, durable, low maintenance building that is fire resistant.
If not modular -- there are other emerging solutions to consider, such as Insulated Concrete Forms
“ICF” walls go up like large Legos: Rebar is placed in an interior web sandwiched between the foam forms, and then concrete is poured from the top into the cavity. You get an extremely energy efficient, fire resistant building envelope. These structures have incredible seismic performance and utilizing this system with solar can get you close to a net zero ADU, essentially generating the energy you will use.
Wall Panel System are another way to speed up construction time
For the Cecile ADU design produced by our firm 8th Wave, we use a sustainable Wall Panel System to construct the entire shell of the building. The panels are built in a factory and delivered to the site where they are unloaded and installed in wall units.
Wall Panels go up very quickly: It took about 3 weeks to assemble the weather-tight shell of a 1500 SF ADU. For the exterior siding we used a fiber cement product which is more fire resistive, can be painted in the factory, and is extremely durable and low maintenance. We played off the main house in terms of the pattern for the siding, double hung windows and trim style. The goal being to create an interesting visual conversation between the new ADU and the main home.
Hopefully, these design tips sparked your imagination, and you are eager to start designing your ADU project. It is a lot of work to undertake any building project, so pace yourself and relish in investing in your future and your community.
ADU’s are a “triple-win” solution: They help create resilience for the homeowners, the tenant, and the community.
I will close with another Ben Franklin quote to keep in mind during your building adventure: “Energy and persistence conquer all things.”
Good luck and happy building!