The term 'BUILT ON TO' can be summarized by the crossword solution 'ADDED'. This means that something was constructed or attached to an existing structure. This could refer to a physical structure, such as a building, or it could refer to a concept, such as a new idea or technology. In either case, the term 'ADDED' implies that something was ADDED to an existing structure in order to make it bigger, better, or more complete.