The rafters are going to go one way or the other. In one direction your mounting brackets will presumably be parallel and right under a rafter, so every screw will go into that rafter. If you mount perpendicular to the rafters, you will have to attach to several of them. And you will have to make sure the holes in the mounting brackets line up with your rafters so you hit them every time. Do you know which way the rafters go, and how far apart the predrilled holes are in the brackets?

At least 2x3. You never know how it may be used in the future. What is the spacing between the vertical members?

Thanks everyone for your comments. I met with the foreman earlier in the week. Yep, not a lot of storage. When I asked if there was any easy way to do this, he said he could build the steps as usual and then let me look at them to see if it would be helpful to have just the middle step tread hinge. Because the steps will be 45” wide, there will need to be support in the middle, so it will be two compartments...maybe even less useful than I’d imagined. So...I think you are all right... Thank you! BTW, in terms of style, I talked to my architect. He said that yes, people most commonly people do concrete, poured when we pour the foundation and garage floor. That didn’t happen. Option 2: redwood. Option 3: something nicer, with painted risers (painted-grade plywood?). Then nice treads. what do you think of this: https://www.homedepot.com/p/Stairtek-1-in-x-11-5-in-x-48-in-Unfinished-White-Oak-Tread-BTWO1148/202019913 FWIW, my home has new white oak flooring. Directly inside this garage door is the little hallway/mudroom leading to the adjoining powder room. Both of those are tile (a 12x12, sandcolor tile). For consistency, my architect said that same tile could be used and obviously would wear well. I’m a little hestitant since it seems like so much work and is so permananet. So I am leaning towards to the wood treads. What do you think? Thanks for your help!

I should think you could hang (very securely) some long 2x4s or even metal standards. Only trouble is they are going to wiggle back and forth so the entire unit may not seem terribly secure and solid. What bothers you about drilling the concrete? A hammer drill and some lead anchors would make short work of it.