Wanted to add that this tree is not Western Red Cedar, but some kind of Juneper tree.

Your city has the ordinance that describes what you can do to the roots on your property. I also suspect you'll have to split the cost of removal with your neighbor. That's how friendly negotiations go.

Dan

I'd refer you to my post on this same question in the Landscape Design forum. But I'd want to question the ID of the tree first before making any further suggestions. A lot of conifers are referred to as "cedars" that have no relationship to the genus Cedrus or true cedars. Other than eastern red cedar, Juniperus virginiana, junipers are not usually referred to as 'cedars'. And J. virginiana is distinctly uncommon in this area. And while juniper roots can be persistant, they tend not be unfriendly to pipes, plumbing or foundations.

Can you post a photo of the tree in question so identity can be confirmed?

I practiced planning in WA state and going off of memory and don't feel like looking it up, but I suspect that WA is not like OR, and leaves common tree typical disputes like this up to the municipality. That usually means working it out with the neighbors. That means that there may be a town that allows you to sever half the roots of a tree and kill it and get away with it, but the chances are slim if you live west of the Cascades.

It should be the simplest thing to go to the city.gov and search on the ordinances. Or call the forestry dept front desk. Since the neighbor seems to have enough sense to want to see claims in writing, I suggest getting the facts in order, and either learning to deal with it or start a savings account .to pay for cost of removal and loss of property value for the neighbor.

Dan

It's true that you can prune/cut parts that encroach on your property.

But - and this is a big but -- if doing so damages or kills the tree, you are liable.