‘We have a drain-field and reserve drain-field easement on our property, which is partially in an area we want to build. We have talked to our neighbors and redesigned their reserve drain-field to be out of the way, but we need to move the easement line over about 10 feet. How do we legally/officially modify the easement line? I live in King County, WA.’
I can’t give a definitive answer for Washington state, but most likely you’ll need to have an attorney draft a document similar to the one that created the easement. You’ll probably also need a surveyor to create a proper description of where the new easement is.