Hello, This depends on your house construction and would be a better question for your contractor. Most homes have ceiling joists that are either 16 or 24 on center which leaves a very small area for the coffer squares ~but if you have open trusses, I would imagine you can work with the space as you see fit as long as there is room for any mechanicals/ insulation/ etc.

Thank you for the quick response. My joists are 24" apart and are centered in the room. I have a steep pitch so there is a lot of space in the attic. Could you suggest a good depth for the individual boxes. Thank you again.

Just a comment; on 24" cc joists; you will only have 22.5" between them. Please be careful that the squares are not too small~ they will look like a checkerboard rather than the treatment above. The photo shows 36x42" rectangles. If you do it at your width; the shallower the better; 4" or less.

I was wondering the same thing. Thank you for answering this question.