The minute you have two mobile homes, you are a mobile home park manager/operator and all the rules apply under the RLTA (if you rent out the mobiles) or the MHLTA (if you sell the mobiles and manage the park). The State rules apply and govern your relationship with tenants; but as to the local rules regarding this set-up, you need to discuss this with your municipality. If you are in Seattle, that is the City; if you are outside the corporate boundaries then you want to discuss this with the County. It turns on exactly where the parcel of land is located upon which you want to place the mobile homes. You could, arguably, even have a homeowner's association that wants notice and has jursidiction to comment on your plan. The Building Department will have rules and a permitting process you'll have to follow. If you fail to follow ALL applicable rules, and get planning permission, it could be very expensive. I'd suggest starting with the on-line information for Snohomish or King County.
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I agree with Ms. Powell and and check the CC&Rs and with any committees of the home owners association if there is one. The fact you are looking at multiple units is something that will influence where you can set them. Some parcels will be fine other you may have challenges that will more expensive than site built homes, so make certian your plans and permits are in order and you should be fine.
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