The word "pre-fab" is short for "pre-fabricated", which refers to a building or structure that is made in parts, usually in a factory, and then assembled on site. Antonyms for "pre-fab" would be words that describe structures that are built on site, without any pre-built or pre-manufactured components. Some possible antonyms for "pre-fab" include "custom-built", "site-built", "hand-built", "artisanal", and "bespoke". These words suggest a personalized and unique approach to construction, as opposed to the mass-produced and standardized approach of pre-fab structures. While both types of building methods have their advantages and disadvantages, choosing between them ultimately comes down to individual needs and preferences.