real estate

It depends on the easement, what it entails and if the fence would infringe on it. You need to have an attorney go over it.

I don't practice law in Washington, and I recommend that you consult an experienced local property law attorney to examine the deed that grants the easement. Generally speaking, interfering with an easement over your property requires the permission of the holder of the easement.

I would only add that before you hire an attorney you might want to simply ask your neighbor if they would mind this 1 foot wall. If they have no objection, you can accomplish your goal without involving legal fees.

If the owner of the easement is not your neighbor, or if you build the wall without permission and nobody challenges you for 10 years, you will be in the clear under the doctrine of adverse possession. There is more to that last statement and if you seriously intend to build without permission with an ultimate goal of acquiring the land under adverse possession, I would strongly suggest you consult an attorney first.