@Branden R. I just went through this process on a 4-plex purchase (not in Washington). It was originally a large single family that was converted to a 4-plex and then the zoning changed and would only allow a duplex on that size lot.
Perhaps the road we went down will be similar enough to your situation to be helpful.
In our due diligence we discovered there were no permits for the conversion to a multi-family. We had the sellers obtain a letter from the county building department that changed it from "illegal non-conforming" to "legal non-conforming". The significance of this was that we would legally be allowed to continue to operate it as a 4-plex. Virtually all parties involve, with the exception of us, considered this a valid "burn down" or "build back" letter. I checked with the county to confirm and they said it was not. In the event of a catastrophic loss, we would only be able to rebuild as duplex which would ruin the cash flow.
The sellers then had to go back to the county and petition to obtain a variance. This basically provided a pre-arranged authorization from the city that would allow us to rebuild a 4-plex contrary to the current zoning (but all codes would have to conform to current building codes). The county mailed out notification to people in the area advising them of the sellers intent to obtain a variance. The matter was then presented at the city council, where people could oppose it if they wanted. The city council issued the variance and we moved forward with the deal.
The "legal non-conforming" letter was a fairly quick process. The variance took a few weeks because it had to be scheduled in advance on the council's agenda.
From a buyers prospective, we very much considered it a "go/no-go" issue and would have walked away from the deal had we not obtained the variance.
I hope this helps. Let me know if you have any questions.
