First, contact your local building department to see if an out house is acceptable within the local building codes. Besides local restrictions, if you are in or near a forest, federal jurisdiction may come in to play as well. Make sure you check with all government agencies who have regulating authority. If an out house doesn't work for them, have them consider a port-o-potty with a makeshift shower as another option.
If you don't get a permit the government can claim environmental harm and impose substantial fines and onerous mitigation measures.
Unfortunately in most areas of Washington a pit toilet is not permitted for residential use - only in wilderness camping areas. To know your exact rights I suggest contacting the Pierce County Health District concerning their regulations. If you plan to go ahead and do this regardless of the health regulations, then you need to be cautious about sharing too much specific information about the property location with the County when you contact them.
I agree with the previous answers and there are composting toilets and on-site sanitary disposal systems. but what you described will not be acceptable. However you rent a port-a-potty or an old trailer with systems and use that while you are getting settled.
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