While it is legal to do and does not void your lease, the requirement that you go smoke free within your apartment is likely not enforceable unless the lease specifically states that the landlord can make these sorts of rules and apply them during the term of the lease. The landlord may have to wait until the lease is up for renewal and include the nonsmoking provision in the new lease to make it stick. Of course, if your lease is month to month, all this would mean is that the landlord may have to wait until the current month ends to adopt that change in the lease.