In the Brady Bunch episode "Greg in the Attic" Greg and Marsha fight over moving into the attic.  But I remember when early on Greg the Groovy Guy wanted his old room and they were forced to use the den because there "wasn't" an attic, or it was uninhabitable.  Remember that?
 

From the Brady Bunch FAQ:
"[5.5]  Why did Mike say:
       "The attic? That'd be great if Greg were 2 ½ feet tall"?

Mike uttered this line in "Our Son, the Man" in reference to a separate bedroom for Greg.  Only problem is, Greg did in fact move to the attic 2 years later in "A Room at the Top".  There are several possible explanations for this continuity error:

1) Schwartz decided that the idea of a bachelor pad for Greg outweighed an
insignificant story constraint.

2) Mike knew that the attic was big enough, but had collectibles or adult
magazines stashed up there that he didn't want Greg to get his mitts on.

3) After some storm damage or a tree fell on the house, the roof was
partially rebuilt, raising the clearance of the rafters in the process.

4) All the junk stored up in the attic (which was cleaned out the day
Greg moved in) made it impractical for a taller person to live up there.

Other theories state that the house was actually built by M.C. Escher
(thanks, Chris Mulvihill) or Greg shrunk to a height of 2 ½ feet in the
interim."