Related Clues:

• Building wing, e.g.
• Extra, and this puzzle's title
• Add, as territory
• Include as a bonus
• Colonize, more politely
• Build an annex
• Enlargement
• Extension to main building
• Add to the end
• Optional item as supplement or increment
• Something attached
• An addition to a main building
• Build a wing
• Proposal for business expansion
• Extra commercial to put on