When we adopted our bunny trio last week (has it only been a week and a day? seems like they've been here forever!), we had planned on adding an addiitonal 4x4ft exercise pen to our existing one. However, we've decided to section off an area of our basement walkout for them. Their cage is down there anyway, this saves us money and gives them a bigger area! It's just under 13 ft across and 9 ft deep at the narrow end. It's attached to the only wood down there, the framing for the bathroom door (has two doors, so we can easily access their area without climbing over the 36inch pen!). The exercise pen has a gate, so we can easily let them into the rest of the walkout.
This is a work in progress and I'll add more pictures to this as we change things. I'm ordering another litter box (one is not enough, even though they cram into it to sleep), and will be looking to add a bunny safe couch/futon and tv. The outlets are in the ceiling, so no worries there.