For a project like this a 3D printed scale model is very useful, so I suggest you to make one if you have a 3D printer. You will find attached 3D drawing files in last step.
Size of the bunk bed is of course personalized, but the height is quite standard and you can easily stretch the stair to determine exact dimensions for your own bunk bed or loft. As for my last projects I didn't had time to make the wood-working, but I hope that this time we can obtain in a short time developing process pictures.
Step 1: General Views
Here you can see some views of the stair in open position. As you notice this is not a classical ladder, indeed as requested it has wide and comfortable steps, to reach the top level with less efforts. All steps have same width and there is no danger to take a false step descending the stair.
Step 2: Some Details
In a closer view you can recognize the two halves of the stair. Each one has four steps, you can obtain all detailed dimensions at the end of this instructable.
Step 3: Closed View and Animation
You can see that the stair can be easily folded inside the bed frame. An animation is much more effective to show the opening and the mutual movement of the parts.
For now I din't considered necessary a locking mechanism for when the stair is closed. Of course it depends by the force made by the person which climb it, and if person pushes horizontally with his feet there is danger that the stair moves. A try is indispensable to decide how to proceed.
Step 4: Unit 1 Dimensions
With these three blueprints you can determine all measures to build yourself the first half of the stair.
The slim oval you see on the first step plant is a groove on the bottom face which acts as handle and helps you to raise the stair. The carved corners will receive the top-body legs in open position, and avoid the stair closes with the body's weight.
Step 5: Unit 2 Dimensions
...and here is the top body of the foldable stair.
Step 6: Bed Frame
The bed frame is usually existing, but if you need to make it is not difficult. Just add some strong joint between legs and horizontal boards, so to stiffen the structure.
In the image you can see the dimensions needed to fit with the foldable stair. All the pivots can be made with steel bolts and nuts. Just be sure that bolt heads don't interfere with structures movement, you can drill a cavity at one end of the holes, so to include bolt heads or nuts.
Step 7: Family Is Growing
The family of foldable wood furniture is growing, and you can see here the bed and his fellow the table.
With the movie of the folding stair this instructable should be all you need to make one yourself. Let us know if you succeed in that.