When can a landlord give a tenant a 7-day notice to quit?

Question Details:

Tenants in my owner-occupied 3-unit farmhouse have allowed conditions to deteriorate to where I have cat urine seeping into my ceiling - the carpeting in the room was brand new over never-before-carpeted wood floor. They originally rented the space on a 6 month lease paying 1st last and one month's security. This lease reverted to a tenancy-at-will at the end of the 6 months but I retained the last month's rent and the security deposit. Since I believe this damage can be classified "serious" and makes the unit unfit am I within my legal rights to give them a 7-day notice to quit?

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