Tommy Bolin

EweTube and the like are 95% self promoting, attention begging, drivel.  Anything with pulsing, garish lettering, bizarre facial expressions on a host carrying a selfie stick, has nothing useful to add to your mindspace.  I've been building houses long enough to realize most of the 'Tube is just  chimps lecturing the undeducated. I've been drunk and hungover while building, (not very often) my opinions might be suspect.
Books are calm, noiseless, free of trolls, and allow you to absorb at your own pace. Does not work as well for those that need 3-D and hands on. Sketching your ideas out if you can a bonus.

I'm little in the dark about how much of this you wish to do yourself. If you are at the mercy of helpers with their own agenda, sorry to hear that. If you are stacking your 'un'happy DIY place with uncooperative 'assistance', this can become shortly very stressful.
You should get used to planning/thinking in metric. Avoids static from uncooperative help and the need to constantly convert.

Wood framing with a metal roof might be the best choice for DIY. Barriers to knowledge entry are low. Can stop at any stage to regroup, mistakes worked around. It is also the most flexible for design. In Canada, 'metric' plywood and lumber just mimic US sizes, there's no trouble crossing over. Find out how lumber, (not timber) is sized in Spain, layout from that.
Real exterior grade plywood, not OSB. Cost difference for a project your size is minimal. I've inhaled enough OSB dust while cutting to know I hate it. Probably toxic while airborne. OSB does not take (waterbased) paint well, it's splinters are miserable and infectious, and it is quickly destroyed by water, swelling and falling apart.

Pictures of Pokey's apartment, no. Two years of foreign exchange in Korea was a serious, anxiety filled part of her life until she settled in. Think of an RV bathroom, RVs in general.

A chicken coop is not usable space covering the lot/earth, but an entirely separate little deck built to support a human filled tub is?

RE: the floor. I imagine insulating between the joists. Cold floors suck and waste heat, although that sounds like lesser priority for your case. Some barrier underneath is necessary to keep vermin, if any out, 'seal' in the dead air space. Anyone recommending bubble wrap has never built anything of substance. Probably a EweTuber. Plywood under or between the joists checks all boxes.

A 'kitchen' island on wheels could be self contained storage/counter space for all things cooking, shove against the wall when open space is needed.  

If you are decided on piers, beams, joists, and sheeting for your floor, you'll need a decent idea of you interior layout, to avoid having to workaround, head out, or move joists. Flat blocks nailed between joists flush to top support partitions/interior walls.

In order of immobility in general are HVAC, plumbing, then electrical. Whichever apply to you.
That is the order in which trades are allowed in after the house is framed, and the order in which a framer prioritizes layout. Framers can fix or accommodate anything they do/need.  Building in your mind from the roof down solves all structural and layout issues and is how good carpenters go about their work.

If I was in your situation, I would first frame the floor, get it sheeted, then layup for a bit. Then frame and stand walls, layup for a bit, then close it in with a roof, being mindful of seasonal precipitation.  This breaks your helper monkey's task oriented mental overload. Go have tapas and wine for a few weeks or whatever. When the deck is framed you can start actually laying out your partitions and fixtures, storage, or whatever. You'll have an excellent visual of your space to be, better reference for doors, windows, etc. Layout snapped by you on the floor makes arbitrary decisions less likely. Involve the helper monkey's input at the design part, help avoid all that pesky, counterproductive, thinking for themselves at future date. Drawing the four exterior elevations and a floor plan starts to get all the wizards into the same plane. Be clear about what is flexible and what is inviolable. Don't start without understandings about time and money. You'll need patience sounds like.

FWIW I've heard that folks with skin issues like 30% hydrogen peroxide or salt for hot tubs in lieu of chlorine. Maybe too expensive for a pool, don't know.

Good luck.

Your deicer salt is likely potassium chloride. Potassium chloride draws moisture from the air, why it always feels damp.
Scrape it up, wash it down a bit with clear water. Soak up the salt saturated water with portland cement, shop floor dry, sawdust, cat litter, depending on availability or aesthetic tolerance.
Good part is salt saturated wood is unlikey to rot. Not something to spend any energy worrying about.

Toilets have a fixed distance from the wall that works, only constraint unless adding grab rails.
Your post foundation allows you place fixtures anywhere in the  future, just crawl under to relocate drains.
A wood fired/RMH hot tub would not count towards your sq. footage, and if part of a 'separate' deck/porch with trellis would be almost like a second room.
My daughter's school mini claustrophobic box apartment in Seoul had a 'bathroom' that was really just a toilet located next to a small shower, with a tile floor and glass partition separating it from the bed and workspace/kitchenette.
Poach storage ideas from the mass of online tiny home builders. Nothing makes a small space smaller as fast as clutter.
Dunno what it's like where you are at, but here, the packrats will destroy and carry away the insulation of your floor. Plywood sheet it for durability and airtightness. "BubbleWrap", "plastic sheets", fully useless.
Have a look at my skid shack thread. An awning, posing as a wall, would allow you some indoor/outdoor space for many seasons. Lightweight rolldown screen, the mosquito plan.
Stack plumbing vents in a common wall, reduce roof penetrations. In wall venting also possible. That would allow you to put a sink in a small island for example.
Casement windows open out and when placed properly cross ventilate a small space well.
Do you have a friend with a small crane? Build your roof on the floor/deck. Nice, flat, level, and square. Slide it aside, frame your walls in place, brace them plumb and SQUARE, pick the roof up and drop it on.  Save a ton of ladder work if you cannot walk wall tops.
In your mind build/plan the house from the roof down. Start at the end and work back. Allows you to foresee problems, not react to them.
Build the hot tub deck first, practice a little power tool usage, layout measuring and placing/fastening.
Cut a few decorative 'milk crates', garden benches, cabinets to store building supplies and tools, whatever. See if this really is something you are comfortable with.
Properly/respectfully handled power tools are neither dangerous nor scary. But inexperience and carelessness are.
Don't let your mind tell you it can't be done.

I looked at the link provided. I love your book.
I have been a framer off and on since early 80's. I like anything to do with historical construction. The carpentry section caught my eye, all the joinery. Section on roundwood framing for secondary structures. Greenhouses, chicken coops, gates, fences, all kinds of stuff.

The painters trade for residential has been reduced to clowns slopping latex bear sourced from an orange box over every unprepared surface in a home.
The old painters made their own paints, specific to the surfaces being treated. The leads were toxic, but not all mixes used them. Preparation was thorough. Quality brush and mix work, getting the paint to layout, a point of pride.
The original trimwork that existed on the never occupied third floor of the 1892 Victorian my folks bought in Helena when I was in high school, looks nothing like the poured on 'paint' finishes of today.

Gas light fixtures and plumbing still on the walls, by the way.
There was also a touching little hand written script in one of the bedrooms, never painted over as the room was never used.
'Happy April Fool's Day
April First, 1892' The two little girls who lived there, along with two of their friends signed their names using their best grade school cursive. One of the girls lived there her entire life, passing away in 1969 or so.

Pretty interesting ideas on paint formulation. Few gallons of blood sourced from a slaughterhouse cauldroned and mixed to make primer for plaster? Paints for various types of floors? Paints for damp surfaces?

I hand file my own saws, there's a section for that. More skill and knowledge than I have. Slate roofing, like wise. Interesting section on heating a home and improving the efficiency of contemporary coal fireplaces, as well as improving air circulation and efficiency  ventilation in the days before electricity.

We have something similar I bought my wife years ago. A sort of "How to Build a House" collection of small texts, from the early 1930's. Mixing your own paint included.

These are the sorts of books I hope I can encourage my grandsons to read and keep. I hope they are interested.
I have a lot of things to teach an open mind and inquisitive little hands.
No phone or internet required/permitted.

Hung a door. Prototype, see how I like it.
Laminated core of 16ga. steel and plywood. Should be fairly  chainsaw resistant. Opens Out, therefore cannot be kicked IN. Heavy commercial hinges, pins cannot be punched out.
Full 2x roughsawn frame. Will have matching rougsawn 1x filler. Outswing saves space inside, leaves another vertical surface for small shelf storage.

The panel under the window is also removable sill to floor for long projects. Gives about 12 ft of covered work area if necessary.  
Window is 60x16 double paned junkyard salvage.
Looking for old school spring assist overhead garage door set to help with the heavy awning. One person can manage it but not super easy.
That hydraulic tiltbox conversion on my bush truck is probably one of the best things I have done yet. That 'table' in front of the shack is actually a prototype dumpbox insert made from plywood. Holds 2000lb of gravel or cow/sheep poo easily. Worked well enough I'll weld one up from steel.  Rear mounted winch as well. Very useful.
General Disarray and Major Chaos are in full command at my place obviously. Have always at least four projects in hand and three more kicking over in my head.

Don't seem to be able to post photos here anymore. My bandwidth out here is pretty poor, or maybe my javascript settings are too restrictive, dunno.
Regardless, got the roofing bent and screwed down, finished the awning as well.
Roofing was a bit of work. Local lumberyard shipped me 26ga. high tensile steel, instead of the 24ga. mild steel I most specifically ordered. Happily, it bent all right, the radius is about 21+-ft. Would not try to bend it much tighter than that.
Cut some curved roughsawn fascia to finish the roof ends nailed to the lookouts, used roughsawn 1x4 for the eave overhangs. I laid the roofing 'backwards' to wind up with smooth steel to start and end the layout.
I used a couple of ratchet straps thrown over each sheet as I was fitting them together and was able to pull them down easily.  But, I give up a tiny bit of durability with thinner steel and looks like I will need to stitch the lower edges down a bit more to smooth the profile out. More than sturdy enough for the 2+ft. of snow it will end up carrying this winter. Should be water tight if I am heating it this winter, used 30lb tarpaper under the roofing.
Don't have anywhere near enough siding/trim milled to finish it and only a temporary door, but it is closed in for the winter and lockably secured.
Can now work on interior fitout, getting the gen set and inverter/solar up and running, little woodstove going.

The reason I showed up here was to explore the idea of masonry/batch/RM heating and the ethics and possibilities of efficient combustion. I found quite a bit more I can relate to in this community.
I appreciate the volume and breadth of ideas available, the efforts of the folks around it, and my mind connects with Mr. VanDenBerg's empirical approach (open source no less) to gas flow/combustion efficiency.

Our home is a two story, 600s.f. main floor, very simple post frame cabin from the early 70's, well off-grid. Wood siding, wood paneled interior, good windows but not a particularly tight building.
Our heat is an 80's Fisher two step MamaBear. We love this stove. Central fixture in the corner of an L-shape kitchen/dining, contained within a 30x60 footprint. Two story chimney through the bedroom above, insulated ceiling up, excellent draw. Burns pretty clean when up to speed. Two different levels of surface temp depending on how it's loaded. Careful not to overfire it, although couple times per winter I will really rip it, clean out the lower stack. Brush the chimney every fall. Creosote is a zero issue except on the cap.
We cook on it good part of the year, and it puts out enough heat my wife can run three canning pots. Walking in from winter work, stripping down and absorbing that massive radiance is luxury. Takes two days of pretty good running to force the heat back into these walls when we leave the house empty for a month in the winter.
We have about a section and a half of timber. The idea of running out of wood is ludicrous. Costs me very little to stock a seasons' wood. My only concerns are the ethics of cleaner burning and getting too old to enjoy cutting as much firewood as I do now. Nothing else about this stove I'd give up.

I've seen and been around a couple Scandanavian type mass wood heaters over the years and always liked them. The idea of getting under my house to reinforce the old floor framing from within a 16inch crawlspace is likely a non-starter. Finding a design that fits this space that we can appreciate is another.
I was force fed the 'smooth plaster' fake adobe/'cob' SantaFe/Territorial style of building in Scottsdale doing custom homes when that style was in vogue early 90's. Pretty much detest it.  The idea of some red brick pile of fec' in my house is also going nowhere. I like linear, I like steel, I like finished stone, and I like wood.

We are going to build an addition to the lakeside of our house. Sun room, maybe screenporch, accessed through french doors. I'm certain an efficient mass heater designed from ground up to fit this space could take care of at least three seasons of heating, help provide hot water, use up some of the rems coming off of the sawmill, make me feel better about burning wood in general. It will be up to me to find a design I can appreciate and live with.

If your question is 'what's holding you back', then maybe it is worthwhile to examine the message/dissemination.
I admire anyone who is self made or gets things done, and I like the idea of useful repurpose.
But, if brick cannot be laid perfectly square/straight/level without mud spilled all over the surface, if steel cannot be cut and welded and venting built professionally, if the unit looks like it was for the most part collected from behind a barn, if the background of the photos shows slovenly disarray and poor craftsmanship of other projects, then the message will be lost on a huge number of readers and the authority of the author justifiably questioned. Work needs to be clean, in all it's meanings  for many people to imagine putting what you ask in their homes.
The messenger is the first message people receive.
An idea can only take hold if the receiver has not subconsciously shut the door in the first few seconds.
My feelings on the subject.

An outdoor survival class I took in the 70's suggested that for treating diarrhea, which is life threatening in a remote survival situation, that a handful of ash from your campfire, mixed with water and consumed was a remedy.
My friend from high school was a Pararescue Specialist in the AirForce. His survival training taught him the same.

Deters slugs, place around edge of garden.

Finns clean the glass of their woodstoves with ash. According to my Finnish mother.

My dogs sleep in dry ash in the Fall. More to do with the heat stored in the ground and being very dry. Likely. Does seem to help with black flies/mosquitoes in the early Summer a bit if they let me put it on their faces. Anectdotal only. Your experience may differ.

SIPS are a decent building idea. As a framer I don't care much for their inflexibility of design. Big rigid rectangles, sized like plywood. Foam core bonded permanently to OSB, chipboard type ply.
Not nearly as eco/permie friendly as locally produced wattle and cob/daub. Some consideration must be given to off gassing of manufactured components, although that has been mitigated in recent years by production changes.
They spline together with standard framing lumber and 8d nails. Very fast. Great insulating value. Air tight. Shear rigidity, built in.
If you don't plan on lighting your house on fire, then don't confine your mind to the terror of toxic gasses. Make sure your electrical is up to snuff. Have enough water to handle your own situation until help arrives. If your house is burning, you have bigger problems. Stand up wind to watch the flames.
Like wise, don't confine your mind to the idea of needing a crane. 4x8 ft SIPS 6 in thick weighs less than 100lbs. That's a lot of wall. I've handled SIPS, alone, almost 12 ft long.
DryVit was a light, semi-flexible, stucco like plaster with some insulating qualities we used in Montana back in the 80's. Had some perlite in it, believe. The old stagecoach inn in West Yellowstone I helped build as a framer '88 or so has a bunch on the outside. Still there. The idea has been there forever, I'm sure you could do just as well now on your own jobsite. But you will give up some of the more permie aspects of cob, my guess.
Prefinishing stand alone panels would work, but you'd have a permanent joint and it's possible drawbacks to deal with. The panels referenced above are more likely to be infill in a timber frame type setup.