Hello,
I have built a few of my own hutches and it's not always as easy as it looks! But it's fun learning, and I can now wield a power tool as well as any bloke *grin*
I don't know on what kind of bunny you're thinking of having, but the measurements seem a little small, particularly if you might want to have another one for company later on.
A nice big run would be something like 48" x 36", but make it quite tall, most runs aren't tall enough, and a bunny likes to sit up with its ears fully up. Unless you are thinking in terms of Netherlands or Lionheads, an average size bunny "periscoping" bun with its ears erect can reach over 18" so it's nice to give them 2ft of height - this allows for scampering high jinks and silly jumps in the air (as bunnies do when happy).
They will also need mesh on the top, and please put mesh on the bottom as well. This not only stops them digging out, but it has been known (depending on where you live) for wild foxes or stray dogs to tip over the run and have a gourmet dinner.
I know lots of people keep their bunnies out in runs all year round, but in the UK it gets cold and damp in the winter, so a normal type hutch is a good idea as well, you can have the bun/s there raised above damp levels, snug and dry, and put them out in their run during the day. Unless the shelter in the run is raised off the ground, some damp & cold might still get in. If you get snow where you live, a normal hutch is almost a necessity, as you don't want them to get buried in the depths of winter.
The best thing - if you can work out how to make it - is a raised big hutch (4ft x 2ft) and a ramp to a big run below. That's the best of both worlds. If you have two bunnies in a run like the one you depicted, the shelter box may not be big enough as they may like to have their own space inside it when the weather is bad. You'll also be able to shut them in at night, raised well out of the way of cats and marauding foxes and dampness, with enough space to keep them in for a few days if necessary.
Sorry if this isn't much help but I've not managed to find many plans online for nicely designed hutches. Take a look at some of the better hutch design manufacturers - try M&D at
http://www.animalhousing.biz You may not be able to afford their products but the design is great, I have seen all their runs & hutches in real life, and as they own rabbits themselves, they are made with all mod cons in mind, in all sizes from dwarf rabbits to giants.