Protected trees vs. heritage trees
Austin regulates trees by trunk diameter, measured 4.5 feet above the ground. On residential lots, any tree 19 inches or larger in diameter is a protected tree, regardless of species, and you need a permit to remove it or to impact it during construction. Removing more than 25 percent of a regulated tree's canopy also requires a permit (City of Austin City Arborist).
A heritage tree is a tree 24 inches or larger in diameter that is one of these species (City Arborist):
- All oaks (live oak, post oak, Shumard, and others)
- Pecan
- Cedar elm and American elm
- Texas ash
- Bald cypress
- Bigtooth maple
- Texas madrone
- Arizona walnut and eastern black walnut
Ashe juniper (cedar) isn't on the heritage list, and palm trees aren't regulated at all (Austin's Tree Smart guide). Removing a heritage tree is prohibited unless a variance is granted (City Arborist).
Commercial and multifamily sites are stricter
On projects that need a site plan, Austin regulates trees down to 8 inches in diameter. A permit is required to remove one, prune more than 25 percent of its canopy, or disturb its critical root zone. The City offers a pre-plan consultation when a project may affect a heritage tree, and it's worth doing before the layout is fixed (City of Austin commercial and multifamily tree rules).
The critical root zone is the real constraint
Most tree conflicts aren't about the trunk. They're about the roots. Austin's Environmental Criteria Manual defines the critical root zone (CRZ) as a circle whose radius in feet equals the trunk diameter in inches, so a 24-inch oak has a 24-foot CRZ (Environmental Criteria Manual). Inside that circle (ECM Section 3):
- At least 50 percent of the full CRZ has to stay at natural grade with natural ground cover.
- No cut or fill over 4 inches is allowed within the half-CRZ, the inner half of the radius.
- No cut or fill at all is allowed within the quarter-CRZ.
- For sensitive species such as post oak, staff can require protection out to 1.25 to 1.5 times the CRZ.
The City Arborist's office has noted that a 24-inch tree's roots can span about 20 feet from the trunk (Austin Monitor). For a builder, that means footings, retaining walls, utility trenches, driveways, and even the staging area all have to be planned around the CRZ. On sloped lots, we often use pier-and-beam sections or bridged grade beams near a tree instead of a full slab.
How heritage tree variances work
The tree ordinance sets out a few paths (Austin heritage tree ordinance; Urban Forestry Commission briefing):
- Dead, diseased, or imminent hazard (any size). An administrative variance under Section 25-8-642, with no fee and no mitigation required.
- Heritage trees under 30 inches. An administrative variance is possible when the tree prevents reasonable use or reasonable access to the property. You have to show you sought other variances first, such as setback relief, that the need isn't caused by the development method you chose, and you have to provide mitigation.
- Heritage trees 30 inches and larger. These go to a Land Use Commission under Section 25-8-643, which is a public process (City Arborist).
When tree review happens
On residential projects, tree review runs at the same time as building plan review. If you want to remove a tree without a development project, you file a Tree Ordinance Review Application instead (City Arborist). Austin's residential permit application requires a tree survey whenever trees 19 inches or larger are on or next to the lot (residential application). We cover the rest of that review in our guide to the extra reviews on a new-home application.
What we recommend
Get a tree survey before you buy or design, draw the CRZs on the site plan, and design the footprint around the heritage trees from the start. Redesigning after a rejection costs more than designing around the tree the first time. Tree rules change, so confirm current requirements with the City Arborist before you commit to a layout.