Here is the portion of the lease that talks about written notice:
"No notice to terminate at the end of the fixed term is necessary unless otherwise agreed to in writing. If a tenant remains on the property after expiration of the fixed lease term, with the permission of the landlord, tenant and landlord will continue to be bound by the terms and conditions of this lease on a month-to-month basis/ The lease may them be terminated by either party after giving
30 days(specify number of days -must be at least 21) written notice prior to the end of the rental month."
I "drafted" the lease by using the Boulder Model lease:
https://bouldercolorado.gov/media/735/download?inline
My understanding of the meaning was that any form of written notice would suffice but it was never specifically discussed, when we went over the lease together we just discussed "written notice".