In the City of Austin, there are protected tree ordinances to preserve and promote the growth of healthy trees. If you are interested in developing a lot, remodeling a home, or adding an accessory dwelling unit to your property, it’s helpful to understand these tree regulations.
The City of Austin determines which trees are protected by looking at the size of the tree, type of tree, heath of tree, and the site. Trees are measured at the diameter, at 4.5 feet from the ground. A tree with a diameter of 19 inches or greater is protected by the City of Austin’s tree ordinance.
If you find a tree with a diameter that exceeds 19 inches on the subject lot, that does not mean all your development plans come to a shattering halt. It does mean, that your process is going to be a bit more difficult. You will need to submit a Tree Ordinance Review Application at the same time you submit your residential building plan. The application will be reviewed by a City Arborist, and depending on your plans you may also need to obtain a tree survey and or a tree inspection.
The Critical Root Zone requirements of the Tree Ordinance most limit the development potential of a lot with one or more trees with a 19 inch or greater diameter. Proposed development plans must demonstrate that trees will be preserved. The mechanism the city uses to illustrate root system preservation is known as the Critical Root Zone Area (CRZ). The CRZ is based on the diameter of the tree. The formula to calculate CRZ is tree diameter (in inches) x 2, then convert to feet. For example, for a tree with a 30 inch diameter, the CRZ= 60 ft. CRZ can be visualized as 3 circles that surround the base of the tree. The outermost circle is the Critical Root Zone where development is most lenient, here you can impact up to 50% with a driveway, porch, dwelling etc. In the next smaller circle, the ½ CRZ, you could add a ribbon driveway, etc. as long as it is no more than 4” below grade so usually any structure wound need pries with footing no more than 4” below grade. In the innermost circle, the ¼ CRZ, there must be no impact.
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