ADDON

• Accessory

• Affix

• After-market item

• After-market purchase

• Ancillary item

• Annex

• Append

• Architectural afterthought

• Attach

• Attach an ell

• Attachment

• Auto option, e.g.

• Build a new room

• Build an annex

• Build another room

• Building contractor's job

• Building wing

• Building wing, e.g.

• CD burner, e.g.

• Computer accessory

• Computer peripheral, e.g.

• Cramped handyman's project

• Enhancement

• Enlarge a house

• Enlarge as a room

• Enlarge the house

• Enlarge the house, say

• Enlargement

• Expand

• Extension

• Extra

• Extra charge

• Extra device

• Extra-cost item

• Extra, and this puzzle's title

• Final buy-in during a tournament*

• Firefox extension, e.g.

• Garage, perhaps

• Give the house a new room: 2 wds.

• Home builder's afterthought

• House extension

• Increase

• Increase stadium seating capacity

• Last tourney buy-in*

• New room

• New wing

• Not standard

• Option

• Pepperoni or sprinkles, e.g.

• Peripheral

• Residence afterthought

• Rider

• Rider, e.g.

• Scanner, maybe

• Something attached

• Something extra

• Subjoin

• Supplement

• Supplemental component

• Supplementary feature

• Surcharge, e.g.

• Webcam, for example

• Wing

• Wing, perhaps

• Write a codicil