When I was designing our 200 sq foot cabin, one of the design elements was a sleeping loft. This concept is pretty common in the fundamental plans of many "tiny house" designs. The fact that a double to king sized bed eats a lot of space is obvious. Some of the smaller floor plans are greatly compromised by the 35 to 40 sq feet of space needed for a full sized bed. Moving this part time necessity up and out of the way saves room for other more practical items. In my cabin the space below the loft is a bit of a dining nook. Its primary use is used at a sitting level where the reduced headspace does not pose much inconvenience.
Free-Standing vs. Built-In Bed Lofts
In my cabin and in practically every tiny house plan I have ever seen. These lofted bed areas are built in as part of the structure. The bed itself is normally just a mattress or futon-type pad set directly on the floor of this smaller height loft. Using a bed frame of conventional height is not practical due to limited vertical clearances. The advantages of a "built-in" include stability and robustness. The framing of the platform is just as sturdy as the rest of the structure. In many designs this loft area even improves the rigidity of the entire structure, as collar ties and ceiling joists do in larger buildings.
Freestanding bed lofts have the advantage of being portable, temporary and often "multipurpose". I have seen some versions that allowed dual "bunk" type configurations or included desks or sitting accommodations below. Back in my ancient college days I had a nice lofted bed in my dorm room. Here free-standing was the rule as any type of "attachments" to the walls or ceiling was strictly forbidden. In those days there were some good designs as well as some pretty scary ones. Having a safely built and sturdy unit requires some common sense as well as some structural necessities. Common in the poorly built designs of that time were simple "long legged" bed frames. These were often made from basic 2 x 4 lumber and lacked the needed diagonal or cross bracing needed to prevent "racking". I knew some that were braced by wedges or large shims installed between the "lofted bed" and the cinderblocks of the walls. This was a quick and cheap fix to a safer and more robust design.
One of the main drivers back then were cost savings, cheap was good and quality often suffered...after all tuition, books, pizza and beer required financing as well. My particular loft was built from rough sawn timbers of 2 x 6 and 2 x 8 lumber. Diagonal cross braces were used and the entire structure was "bolted" rather than simply nailed together. Although now I am an experienced woodworker, the design would not change much. Prior to me building mine I had the opportunity to view a good number of designs. My analytical mind quickly differentiated those that "worked" from those that were just thrown together. Mine may have cost a bit more but I was able to sell my loft and thereby recover some of my initial expenses. Many of the lesser designs ended up in Dumpsters or as firewood.
Modern Versions of Bed Lofts
Today it is pretty easy to find plans online, kits and even completed outfits. Their quality usually correllates to their price. Simple steel units of imported origins occupy the lower end of the market, while solid hardwood and more complex designs fill the upper tiers. Units that include integrated desks are still my favorite as they provide two functions...the desk...but they also provide lower support and added structural elements. The most common users of lofted beds are typically young. These can be grade school children who share a room with siblings and simply need the added space for a desk or relaxation area, to college aged individuals and users of dorms and basic economy apartments that are short on space.
As an older adult I can still climb the ladder to use the loft at our cabin, but most of the time it is reserved for my teen and pre teen daughters. The dining space below converts to a full sized bed as well...and from there it is much easier to stoke the wood stove in the middle of the night.
Related: Guidelines for Building Hanging Beds