The Boundary Line Agreement (BL AGR) provides a procedure for County review whenever a point or line determining the boundary between two or more parcels of real property cannot be identified from the existing public record, monuments, and landmarks or is in dispute. RCW 58.04.
If the property line goes through the middle of the tree trunk, the tree is jointly owned by you and your neighbor. Since you jointly own such boundary trees, you are allowed to trim them. But if your trimming kills the tree or you cut down the tree entirely, you could be liable for timber trespass.
A boundary line agreement is a legally binding document that sets clear boundaries between neighboring properties, providing certainty and preventing conflicts.
A boundary line agreement is a written document between abutting landowners which first identifies their common boundary as being “ambiguous”, “lost” or “uncertain” and secondly describes an agreed to boundary line that is certain.