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.
予定外の出費がかさんでしまった。
Unexpected expenses piled up.
彼の訪問はまったく予定外だった。
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.
Compound of 予定 (plan, schedule) and 外 (outside). The meaning is transparent from its components.