In reply to kenneM:
If you use the height well with climbing right to the very top 10.5m vertically is plenty. A 30deg overhang (besides being pretty bloody steep) is 12m.
There's always potential to be creative and add length with bits of roof, 3d weirdness and a hanging fin in the roof space.
Higher would be better though if you have a choice of buildings.
Will roped climbing attract enough extra custom to justify the additional rent (bigger building) and running costs (staff, training, inspection, equipment etc) over a good dedicated bouldering center?
jk