Cutting Madrona Tree Roots for Retaining Wall??

[Quilbilly]

In need of some advice as to whether or not I can get away with cutting some roots close to the base of a Madrona Tree in order to build a retaining wall. The section where the roots lie is less than 4ft away from the trunk. The tree looks to be in good condition and don't really want to kill the tree but need some advice.

Thanks