Is that tree too close to your house? How to tell

How big it will get

The tree you planted, or inherited, is rarely done growing. Heights below are the mature figures in Oregon State University’s horticulture plant database, which lists height for every species and width only for a few, so the crown column gives OSU’s description of habit where a number is not published [1 to 12].

Two numbers to set against that table. For fire, OSU Extension’s PNW 590 asks for at least 15 ft between any branch and the house [14]. For permits, Portland regulates private trees from 12 in DBH [15] and Seattle from 6 in [16], so most of the trees above become regulated long before they are mature.

The drop zone

A tree can reach anything within one tree-height of its base. Pace the distance from trunk to the nearest wall, then estimate height: stand where a straight stick held at arm’s length covers the trunk from ground to top, and the distance from you to the tree is roughly its height. If the house is inside that radius, you are in the drop zone. Most of us are; a 120 ft Douglas fir 60 ft from the back door still lands on the roof.

Being in the drop zone is the normal condition in Portland and Seattle, not an emergency. It becomes a decision when the tree has a defect, when the defect faces the house, and when the ground or wind history gives it a reason to fail. The ISA’s homeowner guide puts it as risk depending on the size of the dead part and its distance from potential targets [17], which is why an arborist’s report starts with what the tree would hit, not with the tree.

How likely is a hit? A Connecticut study of roadside trees published by the USDA Forest Service (Stormwise, 2016) gives the geometry for a tree that falls in a random direction across a long, straight target such as a road or a power line: the chance is arccos(D/H) divided by pi, where H is the tree’s height and D its distance from the target [24]. It is a general principle, not a Northwest measurement; it assumes level ground and no lean, and for a house, which is far shorter than a road, read the figures as an upper bound:

Two adjustments from the same study matter more than the arithmetic. A trunk that leans toward the target, or a crown weighted to that side, raises the odds sharply; the study counted a lean of more than 7.5 degrees toward the infrastructure as tipping the center of gravity that way. And it measured reach only to where the stem narrows to about 4 inches, because the thin top does little damage [24]. A Mississippi State University Extension worksheet scores the target side the same way as a general rule: homeowner yards and trees close to houses or garages sit in its high target class [25].

Roots and foundations

Closeness to the house is as much a root question as a crown question. Norway maple and sweetgum are the two on the table that OSU flags for surface roots lifting sidewalks [9][10], and in Portland that bill lands on you: PCC 17.28.020 makes the abutting owner responsible for the sidewalk, and for the curb when tree roots damaged it, with 60 days to repair after a city notice [18]. Foundations are a different story, roots rarely break them, and the sewer lateral is where the real money goes; both are covered in tree root damage on your property.

Root zones and planting distances

Roots reach farther than most people picture. ISA’s brochure on utility conflicts says roots commonly spread many times the width of the crown and can extend farther than the tree is tall [26]. The Mississippi State worksheet gives a working number for the part that holds the tree up: a critical root area with a radius of 1.5 ft for every inch of trunk diameter at 4.5 ft, which usually reaches past the dripline, and it rates damage to more than 40 percent of that area as a high risk of failure [25]. For a 24-inch fir that radius is 36 ft, so a new foundation, driveway cut or sewer trench 15 ft from the trunk is inside it.

For the next tree you plant, ISA’s zones around overhead distribution lines double as a sensible spacing guide [26]:

The same brochure notes that gas operators often keep 10 ft on each side of a pipeline free of trees, with height or diameter limits up to 25 ft beyond that, and that you should call the utility locator before digging a planting hole [26]. Set against the OSU heights above, a Douglas fir belongs only in the tall zone; on a typical city lot the trees that fit within 15 ft of the house or the service drop are the 20-ft class. For an existing tree, ISA’s mature-tree guidance is mulch 2 to 4 inches deep out to the dripline, not touching the trunk, which keeps mowers and foot traffic off the shallow roots [27].

Signs the tree is failing

Walk around the tree after the first big fall wind and look for these, in roughly the order a risk assessor would weigh them.

• New lean, or soil movement. A tree that has leaned for decades has adapted; a lean that changed this winter has not. Cracks or separations in the soil on the side away from the lean mean the root plate is lifting, which the ISA guide lists as a sign of soil heaving [17].
• Cracks and splits in the trunk, a major limb, or where two stems meet; and cavities or rotten wood along the trunk [17].
• Conks and mushrooms at the base or on the roots, the surface sign of decay below [17]. In Douglas fir, laminated root rot is the one to fear: WSU says infected trees are considered hazardous and prone to windthrow before any crown symptom shows, and the first visible signs are a thinning, yellowing crown and a heavy crop of small distress cones [19]. The fungus persists in stumps and roots and spreads by root contact, so neighbors of a confirmed tree are suspects too (laminated root rot).
• Dead branches in the canopy, which the ISA calls the most obvious hazards; the larger the branch and the closer the target, the higher the risk [17].
• Dieback and thinning from the top. In western redcedar, thinning crowns, topkill and flagging across a pocket of mature trees is the dieback pattern the state agencies ask you to report [20] (western redcedar dieback).
• Construction damage. Wounds, root loss and compaction from a recent driveway, addition or trench can show up as decline years later [17].

None of these is a verdict on its own. The ISA guide says evaluating how serious a defect is “is best done by a professional arborist” [17]; what you are buying is a tree risk assessment, a written rating of the likelihood of failure, the likelihood of hitting the house, and the consequence, from someone with the ISA TRAQ qualification.

When it is a permit matter

In Portland, a private tree 12 in DBH or larger needs a permit to remove, but the no-development permit has a fast lane: Type A, with no public notice, covers dead, dying or dangerous trees and any tree whose trunk is within 10 ft of a structure, and the permit has been free since July 1, 2025 [15]. Heritage trees are the exception; they go through the Urban Forestry Commission.

In Seattle, trees are tiered by trunk diameter: Tier 4 from 6 to under 12 in, Tier 3 from 12 to under 24 in, Tier 2 at 24 in and up, Tier 1 for heritage trees. On a developed lot you may remove a limited number of Tier 4 trees without review, but a hazardous Tier 1 to 3 tree is removable only with replacement, the hazard call needs a TRAQ assessment, and the work must be done by an SDCI-registered Tree Service Provider that posts public notice 6 business days before a removal [16]. Bellevue still requires its clearing and grading permit, but a tree that a TRAQ-qualified tree professional rates hazardous (high or extreme risk) does not count toward its annual removal limit or need replanting [21]; Tumwater exempts trees within 10 ft of a dwelling shown on a building-permit plot plan [22]. Every other city is in Oregon tree removal permits and Washington tree removal permits.

What it costs to fix

Three outcomes, in rising order. A crown reduction or thinning to pull the canopy off the roof is 3 to 16 crew-hours depending on size (crown thinning and reduction). Cabling a split union is arborist work priced per install. Removal is priced by height, and a tree within 10 ft of a structure carries a 1.25 multiplier in our model for the rigging it takes to lower every piece. Crew cost basis: BLS mean wage for tree trimmers and pruners, $35.40 in Oregon and $32.20 in Washington (May 2025), times payroll and equipment, rounded to $225 and $210 per crew-hour [23]. Modeled estimate, not quotes (method).

Modeled estimate, not quotes. A crane day adds $1,500 to $3,500 and is common for the over-80 ft class on a city lot; backyard carry-out with no chipper access adds 30 percent. Base ranges without the structure multiplier are on tree removal cost; the decision itself is laid out on tree removal.