First things first, don't forget that most bulding codes require that such ladder access be able to support a "live" or an "in use" load of a minimum of 300 lbs, so don't go for a cheapo method or ladder that supports only 250 lbs or less.
I would suggest for the cheapest solution would be to acquire a rated and approved access /fire escape exit/egress ladder for a basement window well. However, remember that although some building codes only provide for a 250 lb capacity in such a shorter length basement window well (assuming that anyone heavier would also be tall enough to clear that height without a ladder), portal, scuttle, hatch access to an attic or eave space requires at a minimum the capacity to carry a live load of 300 lbs., and if planing for using for storage, one should take in mind the weight of what is being carried up and down to that storage area as well, so a 500 - 600 lb might be necessary. These can be found in as wide as 28" with a 22+ tread width, and as skinny as 14" wide.
With the height and width restrictions involved, these are easily found throughout the internet and in home improvment stores...the trick is to assure that you acquire one that is providing a tread depth that is user-friendly AND supports a live load of AT LEAST 300 lbs. Found easily here for well under $100 bucks:
Using for attachment the "wide window sill" attachment method mentioned here:
You will also find some basement window-well escape ladders at the fireescapesystems.com site - but I couldn't find weight "live load" specifications there...but I'm sure you could email them or call them for that information.
If you want something retractable within the wall hatch opening, you might consider custom ordering the size you need of this model:
whereby you could mount the glide on one side of the inside of your "knee wall hatch-door opening". That will unfortunately cost you about $500-700 bucks and seems a silly expense for what most homeowners accomplish with a fold-out step-stool (available at Target for example for about $50-$100 bucks and has the advantage of a seat-back.
Another thought is to acquire a 3- step ladder, and mount it to the floor opening with hinges SUFFICIENT to support 600 lbs and carriage bolts, one would require a distance and cross brace support against structure (not just against the wall surface) while in-use, and the safest would be to also have a locking safety chain on each "leg end" that could be slip-locked to the floor (so the ladder floor end doesn't lift up one side or the other, twist, bounce, etc. especially of consideration when decending the ladder with a load (removing storage).
BTW, this area of the "board" is supposed to be for ADA and so-called "handicapped accessibility" issues.