I recently adopted a 2 year old Dutch Dwarf, Oreo, from my local SPCA. As a temporary solution, I had him in an old rabbit cage, but it was pretty obvious that it wasn't large enough. Hay was frequently getting dumped in the water bowl, and Oreo was constantly moving things around so that he could stretch out at the perfect angle.

Here's the wooden, bi-level rabbit condo I made him instead. It's large enough for Oreo to stretch out in whatever direction he wants, and it's still light enough for me to carry by myself. It took me a full Saturday afternoon to build, but I like to putter, and was using a hand saw for most of the cuts, so your build time may be quite a bit less.

Overall, the interior dimensions are 24"x30", and each level is 18" high. The recommendations that I found are 1sq ft / pound of rabbit, so this cage exceeds that for mine.

Step 1: Materials & Tools

Materials:
1/2" plywood, for the floors and top (cut at the store into three 24"x30" pieces)
3/8" plywood, for the sides (cut at the store into two 24"x36" pieces and one 30"x36" piece)
1/2"x1-1/2" plain wood, enough to be the vertical support in each corner, approx. 12'
1/2"x2-1/2" plain wood, enough to make a kick plate on each level and the perimeter of your doors, approx. 21'
Piece of wood for the ramp, approx 8" wide, at least 24" long
Hardware cloth (rabbit fencing) with wires spaced 1" apart or less, approx 8 sq ft
Some sort of furniture feet
Door latches
Door hinges

Tools:
Saw
Drill & assorted screws
Staple gun & staples
Clamps
Wood glue
Measuring tape
Some sort of non-toxic wood finish (I used mineral oil and beeswax)

A suggestion: pre-drill all your holes. The wood I was using had a tendency to split if I didn't.

Step 2: The Base

The easiest way to build this is from the bottom up.
To start, attach feet to the 1/2 plywood that will be the floor by drilling down through the floor and ensuring the screws were flush with the plywood.

Flip it over and screw up through the floor to attach 18" high 1/2"x1-1/2" pieces to each corner.

Step 3: The Middle

To make sure that my rabbit could get from one floor to the other, I cut out a section of the plywood intended to be the second floor. In my case, 6"x6" is plenty large, but a larger rabbit would require a larger hole.

Screw down through the second floor into the top of each corner support. It this point, your structure should be very sturdy, since it's going to be the main support for the rest of the cage.

Step 4: The Sides

Turn everything on its side and clamp one of the 24"x36" pieces of plywood in place. Screw the plywood to the corner supports in at least three places per support.

Repeat for the other two sides.

Step 5: The Top

Attach the second set of corner supports by screwing through the plywood.

Once everything is sturdy, attach the top by drilling down into each corner support.

Step 6: Kick Plates

The kick plates serve two purposes: keep hay and whatnot inside the cage, and prevent the rabbit from pushing out the bottom of the door and getting pinned.

I used 1/2"x2-1/2" boards, with a screw through the plywood and corner support into each end. A smaller board would have worked well, too.

Step 7: The Doors

You can get more fancy than I did for the doors, but these ones look fine.

Cut two vertical pieces and two horizontal pieces for each door, sized to fit within the plywood perimeters. Line everything up square and put three staples in each joint. Flip over carefully and repeat on the other side. Then put a staple on the top or bottom edges of the joints. If you give the frame a shake and it seems sturdy enough, great!. Otherwise, add a few more staples, or use longer staples.

Cut out the fencing to fit within the doors, and staple it to one side.

Attach your doors with the hinges and install the latches.

Step 8: The Ramp

Attach the ramp however you please. I ended up using a leftover piece of wood cut at 45° to keep everything stable, and another piece as a rest for the ramp.

If your plywood is fairly smooth, you may have to attach some sort of grip. I used corrugated cardboard with one side ripped off and glued to the ramp. Works wonderfully. Some leftover carpet scraps or ribbed plastic would also work well.

Step 9: Finished!

You're done! At this point, you can finish the interior wood with something non-toxic, or insert plastic liners if your rabbit isn't quite litter trained.

I wiped the interior with mineral oil, and then ironed on beeswax (like waxing skis) for the floors. And, I line the floors with newspaper. Oreo is very good at using the litter box, so I'm not too concerned about pee stains.

However, you could also staple or glue down some heavy plastic, like is used to protect carpets in office buildings. You can usually find that sold by the foot at hardware stores.

Of course, you can also paint the outside, trim the corners and edges, and generally make it pretty. I'm leaving that for when it warms up outside.