It is unclear if have tenants or trespassers, either way written notice will be the first step. If you have to evict you will need a lawyer to save time and money. If they refuse to leave with notice and you have title an unlawful detainer action will be necessary.
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If you are the landlord, this is indeed your responsibility. I am glad to see that you recognize that a basement lacking egress and fire safety is not a suitable residence. It is entirely possible that you (or your predecessors in interest) are going to have to pay to relocate this person residing in the basement. You may want to discuss this with your insurance company, as it is likely that they have never heard of the SMC code regarding relocation. The state law section is at RCW 59 18 085. Elizabeth Powell
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