Used to be 8.5m but I've heard it is now higher. I recall when we thought about putting a second level at the back of our house, we were told that because our house is two levels at the front and one level at the back due to the slope of our block from front to back, that the height is measured from "ground level", ie. we could have built up to 8.5m at the back but only at the back. We couldn't have build higher over the whole of our house or the front would be three levels, and higher than 8.5m.
I also recall being told that because we had cut into the ground under our house to create living areas, if we wished to add another level at the back, we would measure from the ground level that was original, and not from the lower cut that we made ourselves.
Your best bet is to call BCC or ask RPI on this forum. He knows all this backwards.