Rabbits need space and they love to jump. Therefore the cage follows the garden duplex principle, that is living space upstairs and access to the grass downstairs. The height of the ground floor is 60cm (~24"), plenty for serious jumping!
One special element is the care taker's access to the top floor. The roof is actually quite heavy, so in order for my daughter to be able to clean out "the appartment", I have mounted the roof on rails used for large drawers. So rather than lifting the roof, she just has to push a bit open (up against gravity, note the roof is slanted) with relatively little force.
One very succesful feature is the "appartment floor". It has an angle of 10 degrees such that urin flows towards the back and out through a slit in the back wall. While rabbits eventually learn to make their business in a litterbox, it takes a while until you get the box and their preferences aligned. Until then the slanted floor makes the cage much easier to clean and keeps your new friend's feet dry.
Some changes from the drawing: The side door on the ground floor was added to allow the bunnies to take a stroll through the garden. The lower frame touching the grass was added once I realized that I need to cover the floor also with mesh, so the rabbits can not digg their way out - and cats, foxes what have you their way in... Also during assembly I notice that my daughter (10 yrs) can not reach all corners of the appartement even with the roof fully opened - her arms are just too short, so I reduced the depth of the whole cage to 90cm. As mentioned above, the roof is not mounted on hinges as suggested in the drawing, but on two drawer rails.
I used green glass fibre tiles to cover the roof, but in sunny weather the temperature of the roof and hence the "appartement" just gets too high for comfort. Rabbits gets into real trouble above 28C (82F). After noticing their distress I painted the roof white with a special polymer roof sealer, which reduced the mid day temperature inside by some 10C. Additionally we put a 1.5l bottle with frozen water into the cage in the morning. If it gets too hot the bunnies just rest against the bottle and keep cool this way. The bottle is still cold when we get back from work.
The CAD drawings were made with DeltaCad, a very simple 2D CAD program available in the Apple App Store. If you know how to make a technical drawing on paper, you'll learn this software in 20 minutes. This was my first DeltaCad project and I am very pleased with the result. Please note the dimensions are metric.