All ADU Resources

Where Can I Build My Accessory Dwelling Unit?

We've seen hundreds of Bay Area properties. We can show you what makes for a good ADU site candidate, and how to determine if your ADU project is feasible.

April 23, 2021

With accessory dwelling units coming in many different types, shapes, and sizes, how do you know whether your property is a good candidate for an ADU? It helps to break things down step-by-step to determine exactly what is feasible for your backyard home.

Where can I build my ADU?

The simplest question to start with is to look out into your backyard, side yard, or garage space to see where you might be able to fit your ADU. Cottage has built attached ADUs, detached ADUs, and garage conversions on lots both small and large, working within the design parameters and space limitations that the potential accessory dwelling unit lot has to provide.

Beyond the physical limitations of your backyard or garage space, there are city, county, and/or state-level regulations that your second unit must comply with in order to be permitted as an ADU. The California statewide exemption ADU allows for all second units of up to 800 square feet, 16 feet in height, and with 4 feet of side and rear yard setbacks to be built without lot coverage, floor area ratio, open space, or minimum lot size requirements.

However, cities and counties vary in their regulations for ADUs beyond the California statewide exemption ADU; for example, minimum and maximum ADU sizes can be established by local governments by ordinance under certain conditions. Because of these variations in accessory dwelling unit regulations from city-to-city and county-to-county, we recommend starting with our Municipal ADU regulations page for more details for your area, or contacting us today by clicking on the "Get Started" button at the top of this page and we can chat about our experience on properties in your city or town.

What kind of ADU do you want or need?

As we covered in our last post, there are different types of ADUs: detached ADUs, attached ADUs, and garage conversion ADUs. If you are looking for additional space while maintaining privacy in your second unit from the existing home, then a detached ADU may be the right choice for you. But you have limited space or want to maintain your existing backyard, you may want to consider an attached ADU. And if you've been staring at your underused garage for years wishing you could use it for more than old bicycle storage, then a garage conversion ADU or even expansion beyond the existing garage might be the answer for you.

Often times we meet with homeowners that have plenty of open space for their backyard cottage, but aren't sure what direction to go in. In cases like these, checking out featured projects can provide a great source of inspiration for your own ADU housing, and talking to an ADU expert can help refine your decision. Read on to learn more about how specific site conditions at your property can impact the size and placement of your backyard ADU, granny flat, or in-law unit.

Does my property have site conditions that limit my ADU?

When we visit backyards and lots across the San Francisco Bay area, we look for a few key things about the property that can have an impact on the location, cost, and orientation of backyard homes:

Slope & Grade: From the rolling Los Altos Hills to the flatlands of Fremont, property lots across the Bay Area can vary greatly. Typically, the flatter the intended site for the accessory dwelling unit, the better. The flatter the area is to start with, the easier it is for a construction crew to level the ground to pour the foundation for the backyard building.

While we have worked with homeowners with less than ideal slope conditions for their ADU, it is helpful to keep in mind that these conditions can have a significant impact on your granny flat budget. A significant slope can have a tens of thousands of dollar impact on the site preparation costs, while building on a hillside could require significant site work, drastic erosion control, and different foundation type than the typical slab-on-grade poured concrete foundation that is typically used.

While almost anything is possible with our custom approach to stick-built ADU construction, these conditions can add anywhere from ten to a hundred-plus thousand dollar cost to your project, regardless of the ADU company or general contractor you choose to work with.

Trees & Landscaping: While that large oak tree in your backyard great for a treehouse or shade from the summer sun, it can get in the way of your ADU plans. Cities vary in their treatment of protected trees, but common species that cities try to protect include California Redwood, Coastal Live Oak, Sycamore, and Elm, and others go by trunk diameter at chest-height for defining their protected trees. Removing or building around larger trees can vary widely; however, building near or on the site of existing trees often will require an arborist report and city approval of a tree protection plan. This can add some cost and complexity to your project, and Cottage is here to help guide you through that process for your ADU.

Structures & Easements: Two common things we see in folks' backyards are existing buildings and overhead power lines. Structures such as garages, sheds, storage units, and even outdated accessory dwelling units typically have to be removed for the construction of your new backyard ADU. Our general contractor partners help remove these obstacles - but it does help to know how large a structure needs to be demolished or removed, as well as whether the structure sits on old concrete that would need to be excavated.

While structures merely impact the scope of your ADU construction, easements such as overhead power lines can throw a wrench into your ADU plans. An easement is a right to use and/or enter your property without explicit ownership, such as the power company needing access to an electrical pole or water company needing access to a storm drain. You can check for easements in your property's title report or deed, and also with the local city or county building and planning department of residence.

In the case of ADUs, an easement prevents you from constructing an accessory dwelling unit within the designated easement area on your property. The most common example we see is above-ground power lines at the front or rear of your property. If you see power lines and/or electrical poles in your back yard, there may be a utility easement. Having overhead power lines in your front yard or sidewalk is often an immediate blocker for any prefab ADU company that is hoping to crane the completed structure over your existing house; city and county utility companies won't allow the risk.

Prefab ADU companies across the Bay Area have advertised quick construction times and lower sticker costs for their prefab homes and modular houses. We believe stick-built construction can be delivered at the same time and cost as prefab while opening up lots that wouldn't work for prefab due to power lines or difficult craning situations. In our next post, we comprehensively cover all of the pros and cons of each and why Cottage builds custom, stick-built ADUs from the ground up.