Discuss BILLY: stacking extensions on IKEAFANS.com. We're Personalizing the IKEA Experience. BILLY: stacking extensions - Please limit the posts in this section to questions and answers about assembly and installation issues only..
I've done both. You can easily stack extensions because they have holes and pins on top and are made to stack. You do end up with fixed height shelves however. The more versatile (and less expensive) alternative if you're going up a substantial height is to buy one of the low Billy units. Basically you need to duplicate the structure of the BIlly height extension. If the sides will be visible then you should trim off the bottom 10 cm to get rid of the skirting board cutout at the back. Since this cut down unit has no back brace I repurposed the narrow kick plate piece from the front and screwed it to the inside back along the bottom. I also cut down the backing board to fit the new height. I drilled holes on the bottom of the cut down unit to line up with the holes on top of Billy and then used wooden pegs to align and attach them. Most importantly I secured the Billy unit AND my hacked extension to the wall with L-brackets so neither can move.
Hope that helps.
The sides won't be visible, but still cutting of the bottom part would look nicer, though I could also hide the horizontal line by a long piece of painted wood... The whole thing will stand against a 3,30m x 4,50m (10.8'x 14.8')wall. I plan on using 5 wide and one narrow Billy bookcase.