In unincorporated Washington County
No city tree code applies here, even with a Beaverton or Portland mailing address. The county generally leaves trees on a developed home lot to the owner, with two big exceptions: mapped Significant Natural Resource areas, and the vegetated corridors along creeks and wetlands that Clean Water Services regulates. Cutting inside either can need review, even for a dead tree.
Trees along the road are generally the adjoining owner's responsibility, and a county right-of-way permit is needed when planting or removal affects traffic. Many subdivisions add a layer of their own, with covenants and common-area trees that need the homeowners association's approval. For neighbor disagreements over trees, the county points residents to free community mediation.