Q:

Young adult or children's book about creatures from a parallel dimension? 

I've have been racking my brain for a long time now about a book which is centered on a house as a gateway for these creatures from a parallel dimension.  A young boy stops them by solving a riddle and finding a box/key/something in the garden.  I understand it's a very vague description.  I read it about 14 years ago. 

A:

Actually, I was thinking of The Wastelands by Stephen King from the Dark Tower series.  It wasn't completely centered on this, but the character Jake was trying to transport himself from his world to Roland's world via a house with a transdimensional creature as a guardian.  He also collects a key from a garden, and a rose.  This probably isn't the book your looking for, but that was what popped to mind with your description.