Do you mean folded up and one above the other? My two folders are stored this way, though indoors, I bought a secondhand office cabinet, the sort of thing that's £hundred's new and hard to give away secondhand. It's solid, on a steel frame, two of us struggled to carry it and that's with the shelves out and doors off! Not intended to be weatherproof, though with some door seals and outdoor paint I'd be happy to leave it outside.
(https://live.staticflickr.com/65535/51146600083_0776050b88_z.jpg) (https://flic.kr/p/2kVDGht)P1100888 (https://flic.kr/p/2kVDGht) by Paul (https://www.flickr.com/photos/phbike/), on Flickr
If that's not your thing, I'm sure I've seen some garden tool sheds of about the same size and with shelves. I've also built a few things in box steel with plastic joints, a couple of work benches and racked out a mobile workshop, it;s pretty straightforward then just need cladding. I've never done anything with a door, though you get articulating connectors so it is possible
https://www.bpfonline.co.uk/search.asp?catid=10687
The big concern for me would be security, but I don't know the circumstance, maybe the garden is already secure? I just know more people who've had bikes stolen from sheds and garages than whilst out and about, though the latter gets the most discussed.
My goodness, Paul that is the niftiest, tidiest and most efficient bike storage solution I've ever seen.
That's very kind of you to say so Dan, it's in contrast to the usual comments about being in denial over how many bikes I have hidden stored in my small flat :o
Bikes? What bikes?
(https://live.staticflickr.com/65535/51145699267_506341cce6_z.jpg) (https://flic.kr/p/2kVz5va)Cupboard (https://flic.kr/p/2kVz5va) by Paul (https://www.flickr.com/photos/phbike/), on Flickr