Related Clues:

• Write a codicil
• Supplementary component
• House extension
• Enlarge a house
• Not standard
• Computer peripheral, e.g.
• Supplemental component
• Home builder's afterthought
• Criminal's helper
• Expansion
• Garage, perhaps
• Building wing, e.g.
• Extra, and this puzzle's title
• Include as a bonus
• Build an annex