(cooking) appetizer, starter, hors d'oeuvre (small, light, and usually savory first course in a meal) / (architecture) extension, appurtenance (room of a building that is not part of the main structure) / (literature) digression (act of straying from the main subject that could be removed from the text without detriment to the main plot) / (figurative) addendum, extraneity, superfluity, accessory, hors-d'oeuvre (something additional or secondary to the main thing)

(architecture) stand-alone (standing apart from the main body of a building) / (literature) off-topic, digressive (not relating directly to the subject at hand)