My daughter has a bunk bed with that sort of design. It is smaller -- double on the bottom, single on top, but the same idea. I was quite impressed with how well it was built. (It is commercially built, of course.) I will post a picture from the web site below.
The picture is not quite the same as the one my grandkids sleep in. They have one with a set of stairs that is available in the stores but not on the web site. Strange. I can get my daughter to take a photo. If you want we can measure and draw up some of the details, but you would have to adapt them to the larger dimensions of your bed.
Web site: http://www.morfurniture.com/detail.aspx?ID=722307627&CatID=134&Group=19
EDIT -- Sorry, the link didn't come out 'clickable' but you can copy and paste.
I should add that the 'stairs' (it has been a while since I've seen it so that might not be the correct word) do add to the overall length but can be arranged in a variety of ways. I think they add a lot to the usefulness of the bed.