Designing My Tiny House

I have been working on a design for my tiny house since I started reading about them last summer. I assumed I would be building on a trailer and need to follow the size constraints of doing so; 8 1/2 feet wide, 13 1/2 feet tall (from the ground) and 40 feet long. I knew I did not need (and did not want) a house more than half that length, but would it be 20 feet or 18 or 16, or possibly even 14 feet? I have looked at hundreds of photos of tiny houses and have toured several of them. It has made me realize that good design is of greater importance than size. Three of the houses at Caravan Tiny House Hotel were each 16 feet long and the same width yet one felt more cramped and one more spacious than the others. This was due to a combination of layout, materials and components (although many basic componenets were similar in all three). I started off with a 20 foot long model during the Tiny House Design/Build Workshop at Yestermorrow last October because it seemed like a pretty common length and it worked well with my floor plan. During the course, I questioned that length, cut off one of my model's ends and readjusted things to fit  at 18 and 16 feet. 16 feet looked too cramped but 18 feet fit everything I wanted well.
Things I want in my house:
Sleeping Loft - I want to have the opportunity to be at a different level than the rest of my space. I am going to have a daybed below for reading, lounging and for when I don't want to climb up to sleep at night.
Galley Style Kitchen - I feel that corner space is often wasted in kitchens so I am planning to have two 4 foot parallel sections which will include a sink, 2 door mini fridge, enclosed cabinet storage below and open storage above. I also want a full range with oven and cooktop. While I don't bake everyday, I do enjoy baking often enough that I would like a convenient opportunity to do so. I am currently living with just a toaster oven and grill outside which are fine and I have had friends offer their ovens for me to use for which I have been grateful.
Bathroom - I have been going back and forth and in circles about this one. I started out with having a shower and toilet in the same room, and possibly a faucet. Then I separated the shower and toilet into side by side stalls with a sliding door that would cover whichever one was in use. Then I put them back in the same room. Then I decided I didn't need to have a shower because I don't shower every day, didn't want the extra moisture build up in a small space, and could shower at a gym or yoga studio. I have recently put the shower back in (with a bath fan for ventilation) but in the form of a wet bath with the toilet and a 3'x3' footprint. I figure I can try out showering other places and see how that works and if not, then I have mine for backup. For the toilet, I may decide to go compost style but am leaning more toward a flush style with blackwater tank that gets cleaned out once a month. After all, I only flush once or twice a day and don't use all that much water. Plus, in a city and in the mild, wet NW climate, I have my doubts about the success of humanure systems unless they are very carefully monitored.
Water Heater - I have lived in several houses with tankless water heaters and have loved the instant hot water. They are more energy efficient than tank water heaters due to storage losses but also more expensive and I haven't found a manufacturer of ones that will work year round and long term for homes that are sometimes mobile. I have heard that 10 gallon storage tank water heaters will give you 10 minutes of hot water which may be what I decide to go with but I have more research to do.
Space Heating - I haven't decided on anything yet. I'm trying to design my space small and tight enough that I won't have much need for heat. I have been using a portable oil filled radiator in my room to supplement the radiant floor heat system which has been struggling when temps have dropped into the 20s and below. It heats the space very well and the bills weren't that much higher during the last cold snap so that's an option. Or I may get an Envi heater since many others in tiny homes are using and like them. Heating blankets work well too although I've heard they can be fire hazards, again more research is needed.
Storage - I'm planning a closet across from the bathroom for clothes, storage under the daybed for suitcases, tools, bike stuff, outdoor gear, and crafts.
Materials - Reclaimed wood for siding, hardwood floors, cabinets, trim
Wood double pane windows, metal roof, foam insulation

Things I do not want in my house:
Materials - OSB, fiberglass, drywall, Hardiplank siding, shingles, vinyl windows
Appliances - Dishwasher, washer, dryer
Solar - Not this time. I am planning on living in Portland where it is easy to connect to the grid and options exist to buy power from wind farms and solar communities. I plan to have all LED lights which are very energy efficient and will have few other plug loads that the solar would otherwise power. I might look into solar water heating though.

Most recently, I have been debating whether or not to build on a trailer or to build on skids. Designing for a trailer has been somewhat limiting because you have to plan for the wheel wells, weight distribution, front door placement, etc. Without a trailer, I have more freedom with my floor plan. While it will likely cost more to move my house on skids than to rent a truck to haul a trailer, I don't envision moving it often or for long distances. I have some cost comparisons to do there. Without a trailer, I would have to double check the permit situation as from what I know, in Portland, as long as you build under 200 sq.ft. you don't need a permit. I'm not against the permit process as it could be helpful for getting insurance, I just don't want it to be cost prohibitive.

Another thing I have been thinking about is building with SIPS: Structurally Insulated Panels. My biggest hesitation is that I have no experience building with them and I have many questions about their connections, tying into interior components, space for running electrical and plumbing, and future modifications to name a few. They cost more than conventional framing but could be worth it for a tight shell with no or few thermal breaks. The other idea is to frame the house and then wrap the exterior with 2" rigid insulation and leave the studs exposed which would lead to some fun design problems inside. My current design shape is a simple shed roof which would make it ideal for SIPS so we'll see.
I really like homes with two shed roofs offset by clerestory windows which is what I had designed during the Yestermorrow course. However, I decided that 8 feet is really too narrow of a space for both. So my plan is to have my main living space under a shed roof and then build a detachable or temporary lighter shed roof frame with clear roofing above my front door which will shelter the bike parking, entrance porch, bench and hammock lounge area. Maybe even a greenhouse if it is able to be oriented south which is how I have designed it. Together, these structures will have the look of a singular clerestory dwelling.