anyone build cabins?

I've done some looking into a very similar plan. What I've found may or may not apply in your area.

Accessory living units can typically be built as a second home on an existing property for part time use, or use for family members who need care. Details vary depending on location. Here, they have a max size OR not to exceed a percentage of the main home's floor plan.

Tying another living unit to a septic is often not possible. Depends on the size of the tank and local regs, so ymmv. Here, the health dept permits septic and its sized based on number of bedrooms. Similar for wells and power. My power is a single 400A service feeding 3 panels and I wish I had more power for the shop.

Our plan changed to basically build out a reasonably size house so that should it come to pass, we could divide the properties and sell two homes in acreage. The cost different once we pay the labor for another elec service and septic narrows quite a bit vs am accessory building.

As far as building a building. Its as much about the work flow and process as it is the actual steps and result. People who have never picked up a hammer can absolutely build a house, but it can cost lots of mistakes and redos when they don't know the details or even how to setup a site so they don't have to rearrange everything for each new wall or step. 12x24 should be a very reasonable size for a 2-3 man build.

For a fullsize house, the excavation and foundation can dominate costs (until the elec-chickens show up), but for under 300sq ft you could probably get away with post and beam on deep footings. Good underfloor insulation would be cheaper for that small a space vs a real foundation.

With a small size, the amount of surface area of the outside will be large compared to the volume inside and so getting it well insulated would be my goal. Reason is, the large surface area (relative the interior space) means it will radiate more heat per cubit foot of interior space and heat up and cool off more quickly than a larger space. I would shoot for something like double your code minimums for the area. I would also avoid banking too heavily on radiant barriers. They defitely help, but they work best in areas where UV exposure actually makes radiant gains and losses a concern. You would also easily get away with 24" stud spacing if
using staggered studs or 2x8 framing, which can keep the lumber costs pretty nominal and support lots more insulation and less bridging. The cost difference once finished will be small but heating and cooling should be stupidly easy (cheap), and make for a great little getaway if your current house ever experiences temperature extremes.

The improved insulation isn't just about heating and cooling efficiency either. Part time buildings can suffer moisture damage from condensation, and better insulation usually also comes with a tighter building envelope which can prevent bug and rodent damage.

My office is about half the size you're looking at and it's one of two rooms heated by a Mitsubishi hyper heat minisplit. Works great down to 10F, but then it quits.

That layout looks decent. I might suggest thinking about what you'd do if it could be 2-3 larger and see if there's a layout that would support future additions. May not be worth the consideration, but it doesn't hurt.

I like the loft idea, but being 6'3", I'd highly recommend making it as tall as possible. Add knee walls or something to help with vertical height. With a loft, the ceiling will be vaulted and roof insulation becomes more involved than blow-in cellulose. But spray foam or foam panels can work exceedingly well. Keeping the exterior wall a stock size (8,10,12 ft) is a very smart idea. Same inside to make finishing easier, but it's even better to have a few more tape joints and an extra foot of insulation IMO though.

I know I'm hammering on the insulation a lot, but after doing an interior remodel last year on a 900sq ft property in our family, it's become very clear how valuable it is. The place I've been working on isn't real fancy, but the additions where they skimped on insulation are real obvious when the temps are below freezing.

Hopefully some of this is helpful. Otherwise you can ignore my rant ;)