noun which may take the genitive case particle 'no', na-adjective, noun
unexpected; unscheduled; unplanned
Describes something not part of the original plan or schedule. Often used for events, expenses, or visits.
Unexpected expenses piled up.
His visit was completely unscheduled.
突然 is an adverb meaning 'suddenly', describing the manner of an action, whereas 予定外 is an adjective/noun describing the nature of an event as unplanned.
予定通り means things happened as scheduled, whereas 予定外 means something happened outside or beyond what was scheduled.
Compound of 予定 (plan, schedule) and 外 (outside). The meaning is transparent from its components.