This little house is drawn without a space for utilities marked. Small water heaters could be installed under a cabinet in the kitchen. Electrical panels could easily be in the bedroom closet, or virtually anywhere. Perhaps it could be hidden behind a painting like a safe. A woodstove would work well here, at the end of the closet in the living/kitchen, or just use electric heat for ease of installation.
I have left a large gap in the walls through the center of the house, indicated by dotted lines. I would either install a laminated beam on this line to support the second floor, or use trusses that will span the entire house without support. Leaving this house without ceilings, exposing the roof to the interior would be quite possible with a good truss system, or rather large roof beams. You would have to use tie beams or iron rods to span the house from exterior supporting wall to opposite exterior wall. A simple roof of rafters would push the walls outward(especially with a snow load) if you did not have something to hold them in. You could just run the interior walls up to 8 feet and stop, or run the partition walls right up to the roof. This would certainly make it feel light and spacious.
In the 19th and early 20th century, it was common in areas where there were large ranches and farms to have a tiny residence in town for use on the weekend or just for Sunday services and shopping trips. Generally the house was for the parent couple or a single person. Children or guests would access the loft by a ladder, mounted on the outside of the house. One could easily put a ladder anywhere inside to go up to the loft.
I have indicated a couple of sets of French doors and a front door, but they could be placed almost anywhere. I have not indicated the windows. They too, could be placed almost anywhere. It is a good idea to avoid putting openings on the north side. Think carefully about placement of windows so the facades of the house appear balanced.
As with other houses I have written about, a pergola or roofed patio would shade French doors. A solid roof or deciduous vines on a beamed structure extending out in front of large expanses of glass would make good shade to keep the inside temperature down. See other posts for suggestions on this.
I have drawn this almost to scale assuming 2x4 construction. The interior dimensions will be a bit smaller when using 2x6 wall studs. Always use 2x4 for interior partitions or search my posts about space saving partition walls in this blog. 2x4 exterior walls could be supplemented with rigid foam insulation on the outside of the house, under the siding.
The house is basically 20x20 feet., meaning that it is only 20 sheets of OSB all the way around. Not expensive to build at all. the exterior sheathing of the walls could be around $500.00 if a bargain is found. Possibly less if you use Board and Batten sheathing (Board and batten needs horizontal supports between the studs for nailing, or an inexpensive sheathing under the siding)
As this cottage is square, there could be a hip roof with four equal sides, but the loft would be more spacious with a simple gabled roof.
It is not usually a good idea to do a hipped roof, especially if the house is small. The space is much smaller, and the framing is more difficult for a novice. Too bad, I really like the hipped roof. Perhaps you could consider a Jerkin Head roof, also a bit of a nightmare to frame, but saving plenty of space.
Thank you Wikipedia for the photo above of a Jerkin Head(or half hipped) roof and the hip roof above that..