IKEA billy bookcases extension unit with glass doors for Sale in Bellevue, WA - OfferUp
How I Created My Ikea Billy Bookcase Shelving Hack — MELANIE LISSACK INTERIORS
Why would there be an extension option for the tall BILLY but not the short one? I bought the extension in the clearance section, already assembled, so no hardware came with it.
BILLY Bookcase, white, 311/2x11x931/4" - IKEA
BILLY Bookcase, white, 153/4x11x931/4" - IKEA
From Billy to Built-Ins - Storefront Life
IKEA Billy Bookcase Built-in Hack - arinsolangeathome