Tommy Bolin

John F Dean wrote: I, and many others, hold that if a small supplier is making is quality part critical to making my product, it is in the best long term interest of my organization to help him stay open and to continue to produce that product.

As a homesteader, I find myself having to make similar decisions.  While I will not roll over and be taken advantage of, I do often make the decision to purchase at my small town hardware store rather than buy on the net or wait for my next trip to the big city.  It is in my long term best interest to keep that little hardware store open….even if I pay a few Pennie’s more.

100%

While I must allow there is an opportunity cost to the amount of money I 'save' buying online, no one seems to acknowledge the redundant carbon footprint of all those one off Amazon deliveries of items available locally.
No one acknowledges the lost local opportunity costs of incrementally adding to the wealth of the world's richest people, rather than help fund the necessary economy of your local area. Same wealthy folks whose wealth and spending power will never add anything to your local community.
Same wealthy folks who have by direct action suppressed the profits of suppliers, driven sources overseas, narrowed choice.

Erika House wrote:Two things: 1. Not having adequate floor support for the weight (and not easily having a way to fix that). 2. Really loving the view of a fire in my fireplace and not yet being willing to sacrifice that for the efficiency of a RMH.

Everything is a compromise. Espousing a 'permaculture' ethic while demanding the redundant carbon footprint of the immediate delivery/instant gratification of everything Amazon, is a compromise.
I've never owned a home without a fireplace or woodstove. Wet fall/winter heat supplement and a small joy. Even with inserts, my fireplaces were a somewhat indulgent, not too efficient pleasure.  
Our Fisher woodstove is an excellent and life critical heat source/cooktop, but no dancing flames, and I burn a bit of wood. One of the compromises for a quiet, off grid, (more) self-sufficient life.

Live the best you can, leave the scorekeeping to the zealots.

The roquetinho.eu site features a very efficient woodstove, with a lighter mass, and a glass firebox door.
From the proboards site there is a section on the Vortex stove. Efficient, lower mass, potential cooking use, also fitted with glass.
Neither of these are as grassroots, DIY friendly, as the Ianto derived cob stoves, but all do a fine job of clean burning with varying levels of heat extraction and storage, speed and density of heat. They have been refined and tested so that anyone mechanical skill and motivation could build one.
Aesthetics, skill, willingness to work or compromise, access to materials or labor, end use, environment, cost, all have a place in your calculation.

John C Daley wrote:In Australia I am held back by lack of concrete information.
I habe been advised by members on this site about what type and size I need to build.
But detailed information about build plans are very hard to find.
I have ascertained from batchheater.eu what dimensions I need but that is all.
I would love detais for a 125mm [ 5 inch] batch rocket  unit.

Have you considered the following resources?
The designs, builds and dimensions are well spelled out. In the case of roquetihno, mass and size scaled down it appears for more temperate climates, build well documented. Not all stoves are rockets, not all heaters have huge mass. In the case of the 'proboards' more math, figures, analysis than most folks will digest.

http://www.roquetinho.eu/en/Welcome.html
https://donkey32.proboards.com/

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.