Zoning issues Bellingham, WA

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I am in the process of selling a duplex I purchased in '06. During a recent appraisal they mentioned that they couldn't find if it could be replaced if destroyed; turns out the zoning was changed in '08 and the city doesn't have record it was legally done. The house was first mentioned as a duplex while a person owned it from 1945-1960  in the historical survey done but it doesn't specifically mention what year it was done (if I can find record it was done prior to 1947 I'd be home free). Apparently there was a building permit in 1980-81 mentioned by a prior owner on microfiche; the city has no record of this. 

Does anyone have any good ideas of how this can be remedied or how much it may cost to bring in an attorney? I currently have calls into a few land use attorneys but so far only one call back who doesn't do free consultations, I'd at least like to know what I'm getting into before I crack open my wallet....