Under such an agreement, the parties acknowledge the true boundary line between the properties, and the encroaching owner releases any claim to the strip of land encroached upon. In return, the “encroached upon” owner allows the encroachment to continue for so long as he/she is not adversely affected.
In Washington State, boundary trees are considered common property of the adjoining landowners. Any decisions regarding the removal or substantial trimming of these trees must be agreed upon by both parties. Unauthorized tree work can lead to legal action and potential compensation claims.
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.
You and your neighbour can create a 'boundary agreement' to record: the boundary between 2 properties. who's responsible for maintaining a hedge, wall, tree or fence between 2 properties.
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.
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.