Select the Right Property for Your ADU

During any construction project — no matter what the size — there are a mountain of decisions to make. Perhaps the most important decision in building an accessory dwelling unit (ADU) is selecting the primary house and lot. In hindsight, I wish I had given this decision more consideration because choosing the right property can not only save money on construction and landscaping costs, but help you maximize the potential for your ADU.

Learn from my mistakes as I share some tips for selecting the best property for adding an ADU. And if you plan to build an ADU on your existing property, these tips can help you address some of the challenges you may experience.

A large enough lot

Many cities put a limit on the size of the ADU based on both the size of the primary house and the size of the lot. Check with your city planning and permit office see what the size to lot limitations are. In Portland, Oregon your ADU can only cover 15 percent of your lot. So when I constructed my one-story ADU on a 5,000 square foot lot, I wasn’t able to build to the maximum of 800 square feet, but was limited to 750 square feet. And any covered exterior area, like my front porch, has to be part of that square footage, which is why my house is 700 square feet. And these are exterior measurements.

In Portland the size of your ADU is also limited to 75 percent of the primary structure. Basements converted to an ADU can be larger.

A level lot

A level lot will save you a lot of money both in construction and landscaping costs. My ADU was built at the back of my property which is a few feet lower than the primary house and the street level. The property also slopes in the other direction. If the slope had been any lower, it would have required a pump to bring sewage up to the street level. Sloped lots can be more challenging for erosion control and water mitigation. All water runoff for my ADU runs into an underground barrel.

At the end of any new construction, you’ll be left with a muddy yard requiring landscaping for erosion control, aesthetic purposes and outdoor enjoyment. Building on a sloped lot, as I did, increases landscaping costs. My property required a four-foot high by 40 foot-long retaining wall plus a four-step stairs down to the ADU. Another retaining wall was built between my property and the house next door which is on a higher lot. A six-foot high fence was built over it, which hid the neighbor’s chain link fence from my view (a real plus!). The third retaining wall was a small one by the edge of the patio a foot away from the back fence. Plus, I had an 18-inch-high retaining wall install by my driveway.

No large trees nearby

Bordering the back of my property live a beautiful 100-year-old California Bay Laurel (or what Oregonians call an Oregon Myrtle), as well as an equally large Cedar. These trees are not on my property but were still a major consideration in building my ADU.

Before designing the ADU, I met with three certified master arborists who inspected the trees and recommended that the ADU be built a specific distance from the trees to avoid any impact to their roots. The root protection zone blocked off this area to all construction activities including the storage of materials.

A report from the arborist was also required by the city. If any large roots were discovered during the ground excavation, the arborist would need to return to trim the roots and address the impact to the trees. Fortunately, no large roots were found.

I certainly didn’t want either of these 150-foot trees to fall on my home, so I followed the arborist’s recommendations to a T. To address the many large branches hanging over the construction site, with my neighbor’s permission, I had these branches trimmed back to the cost of over $2,500.

A corner lot

I’m rather fond of corner lots, at least in cities. ADU's built on corner lots generally face the side street giving the ADU more light and privacy from neighbors and the primary house. And facing the side street creates a feeling of a true second home. Additionally, the ADU dweller’s access is directly off the street and doesn’t require hauling groceries to the back of the property. On the other hand, living in my ADU at the back of my property, I do enjoy the lack of street noise.

Good access to your construction site

Sometimes the position of the primary house makes it difficult to access the ADU building site. A construction project requires large trucks and equipment. The further they have to go to bring in concrete and other materials, the more labor is involved and naturally the higher the construction cost. Fortunately, there was a huge gate for easy access to the back yard via the driveway and several additional feet on the side of the primary house. This is another advantage of a corner lot, which provides easy access from the street.

Good neighbors

This is so important no matter what kind of house you are buying. When you build an ADU, especially one where you and your family members are going to live full time, you have to realize that you are going to be in much closer proximity to your neighbors, especially if the ADU is in the back of your property. You’re basically in the neighbors’ backyards. Walk around the block see what's going on, especially during the evenings and weekends. Do the neighbors’ barbeque often, and will that smoke pour directly into your future ADU? Do they have a firepit? Are they chain smokers, pot growers, composters? Do their children play outside and scream at the top of their lungs all day?

Another thing to check is the neighbors’ tolerance for infill housing. Are there other ADUs on the block? How will your ADU impact your neighbors, especially if you are building a two-story structure? You might even want to have a conversation with the neighbors before putting in an offer.

A primary dwelling with a newer two-car garage or large basement

Building a detached ADU from scratch is the most expensive way to go, unless you are constructing a prefabricated home. There are more affordable options, all of which start with a solid foundation, a roof and four walls.

If you want a detached ADU and to save money, converting an existing garage can be a big money saver. But it needs to be a newer garage with a solid foundation and one that is large enough for your needs.

An even less expensive option is to do a carve out from the primary house. Say you have a three-bedroom, two-bath house with a living room and a family room. You could convert one bedroom, bathroom and family room into an ADU by adding a kitchen and building a firewall.

Basement conversions are another option if you have adequate ceiling height and it’s watertight. A house with a daylight basement would be best as it requires fewer egress windows. Personally, I’ve never been able to live with people above me due to the noise factor. If you go this route, seek professional help with soundproofing.

Build a new house with an ADU

If you plan to build a new house, you can save money by incorporating an ADU into the plan and building both structures at the same time. You might want an attached ADU if it’s for a family member. Or you could design a detached ADU with a separate patio and yard and privacy fencing. Or you might want to build a traditional carriage house with an apartment over a double-car garage.

I wish you good luck in finding the perfect property for your future ADU.

Take a look at the photo gallery of the construction of my ADU. And if you have any questions, comment below.