A Homeowner and Arborist Design an AUD Around Big Trees

Building an accessory dwelling unit while preserving nearby large, healthy trees took extra planning and effort. But for this Portland resident and the arborist who helped him, keeping the trees was non-negotiable.

By Kyna Rubin

April 29, 2022

When Chris Dearth decided to build a two-story accessory dwelling unit (ADU) in the backyard of his home in Multnomah Village, he knew he didn’t want to cut down any trees to do it.

Because of that, he worked from the start with an arborist who knows how to build next to large-form trees while minimizing damage to their roots. Damon Schrosk, president and owner of Treecology, handles tree work for eight to ten ADUs or remodels a year. If he has any say in it, all of them result in big-tree preservation.

Schrosk’s approach is to keep trees around “however we can.” He turns down business with people “hell-bent on taking a tree out that doesn’t necessarily need to come out.” Maintaining his principles about tree preservation is important to his reputation, especially in Portland, he says. The homeowners who approach him are somewhat self-selecting due to that reputation.

Dearth wanted the 800-square-foot ADU to go in among two Western redcedars and a few black walnut trees. One of the cedars, which he believes is 100 years old, the age of his house, is likely to soon be designated a Portland Heritage Tree.

“Somebody could have used the ADU as an excuse to cut down those trees,” he says, “but that would have been tragic because you don’t have to, and they add to the beauty and value of our house.”

How They Protected the Trees

Dearth brought Schrosk in early on, before any plans were drawn for the ADU. Building placement and construction techniques are key to designing a structure that won’t disturb nearby trees. The main goal is to minimize digging, especially within a tree’s root zone. Avoiding root disturbance requires deviating from construction norms such as traditional foundations. Instead, Schrosk uses grade beams or pin pilings to minimize digging. A grade beam, he explains, is a reinforced concrete foundation wall that sits above ground on individual pads rather than being set two feet into the ground. In the case of pin piles, which Schrosk used in this project, holes are drilled—at a depth that depends on geology and soil supports—to create concrete tubes 2 to 6 inches in diameter that carry the building’s weight. At ground level these tubes create a footing. Putting the building on discrete points of disturbance, he says, helps avoid tampering with the tree’s structural and feeding roots.

Another precaution Schrosk took was to use a pneumatic (air-powered) soil excavation tool to pre-dig at the pin pilings spots. Workers dug down about two feet, where most of the root mass would be, to make sure they weren’t going to hit any large structural roots. And when it came time for the excavator to drill the full holes in the ground for the pin pilings, Schrosk had him first lay down a bed of wood chips and plywood for his machine to sit on so it wouldn’t compact the tree’s root system. The excavator used the pre-dug holes as reference points for his final drilling.

Getting Everyone On Board From Square One

Building near trees requires upfront, coordinated planning. Schrosk met with his client before the architect came in, to see where Dearth wanted the ADU and how it could be built while preserving all the trees and their long-term health. Schrosk submitted the required tree plan to the City.

The arborist says he often works with designers or architects from the get-go, at times meeting on site or sharing drawings to convey what he’s looking for to minimize tree root disturbance. Occasionally he gets pushback. “It sometimes takes more work than you expect to get the architects and engineers to understand,” he says. In this case, things went smoothly.

The fact that the client was motivated to preserve the trees and had the financial means to do it was important to the project’s success, of course. Schrosk notes that Dearth was really excited about the challenge of building the ADU in the best possible way. But the arborist works with a range of homeowners across different Portland neighborhoods. Sometimes the homeowner decides to rethink, redesign, or reduce the project scope to keep costs down.

Today, four years after the work was completed, Dearth is very pleased with the outcome. He admits that working with the trees was sometimes “a hassle.” His builders weren’t happy about having to build so close to trees. But, he says, the process was worth it. “We have a beautiful project and beautiful trees.” 

Schrosk senses that architects and designers are becoming more familiar with techniques to preserve large trees next to new structures. At the same time, he acknowledges that this may be because he’s working with some of the same people across projects.

“I’d like to think that people are becoming more knowledgeable and maybe doing their homework beforehand on how to build around trees,” says Schrosk.

For now, the homeowner or developer building a new structure pretty much calls the shots on whether or not big trees get preserved. Extra cost can be a consideration. But if homeowners take into account the added worth that preserving a big tree or two brings to their property value in the same way that, say, renovating a bathroom does, they might view a bit of added investment in their trees differently. And higher property value aside, every big tree retained is a tree that is providing a host of proven health and environmental benefits to the residents living on or near that property.

One day, maybe when the City revises its Title 11 tree code in a few years, the code itself will create strong incentives or requirements for homeowners and builders to save large, healthy trees through a bit of extra planning. Meanwhile, for now—and perhaps forever, if Portland’s tree code remains unchanged—new ADUs all too often come at the expense of big trees and human and ecological health. (Read about one especially egregious case here.)

For another positive tree/ADU outcome, this one in Los Angeles, read here about a single mom who built an ADU while preserving a 60-year-old Chinese elm.