I know the feeling when everything starts spinning from so much info and ideas. I have to tell you, though, that I like threads like this, even if they sometimes get a bit bumpy. I am convinced that the vast majority here want to support your project and are therefore also highly persuasive. I don’t think you’re the kind of person to be blown away by the first gust of wind. Often this is seen as proselytizing, but I believe there is a crucial difference. Everyone here has made good and bad decisions, and each affects people differently. You should/can discuss freely before construction, but once it’s built, negative comments should be avoided—I would find that malicious.
My approach is actually that you (one) should really build what you like, especially if you live, like you (and me), in advanced age. Therefore, your project cannot be compared to the 85% others here, which are built by young families with completely different needs and experiences. In the end, I (and others) honestly don’t care what you build, but such discussions at least always help me find new ideas, possibilities, and also limits.
Right now, I am buying a new espresso machine and have been going endlessly in circles for weeks from cheap to really expensive, from necessary to nonsense to nice. Today I canceled my current reservation for an expensive machine still in development and ordered a completely different type, which has a parameter that I (we) initially never wanted; well, after all the extra rounds, it now fits us perfectly and will also cost only half as much with at least the same satisfaction for me. In my opinion, you should allow the same for your house construction.
So I would only ponder/feel the following things:
Your teenager will be at least 18 years old upon completion and probably leaving soon—so far, I do not think this circumstance has been considered enough. He/she could easily live in a separate wooden annex (-house), which could later serve as a guest apartment, similar to what described. As a woodworker, you can well design this yourself and only equip it with basic systems or simply buy a prefabricated wooden house. For toilets, etc., there are functional solutions here in Saxony and also Brandenburg. Ask and excite him/her and show some pictures—that’s amazing for a young person, their own place but still close to parents if needed.
Next to it, there is a smaller, individual house for you both that exactly fits an aging couple.
Also, if I were you, I would insulate maximally (better than KfW40 if possible), which as a woodworker you can easily implement (e.g., I chose a second, cheaper insulation option), maybe also a simple and affordable facade insulation with wooden cladding, suitable for a wood craftsman.
This high insulation standard would basically be the defining element of your project because you no longer need an extremely expensive heating system. An air conditioning system is a given anyway (we also have that here), and with such a system, heating is basically solved. If you set up your desired fireplace nicely, you are absolutely flexible and also inexpensive. Depending on needs, you install one or another nice infrared glass module in the ceiling, and for your wife, a thin net beneath the bathroom’s tile floor for warm feet. We have it set up something like that here.
For example, we really wanted these 3 cm thick solid wood planks and no sluggish underfloor heating beneath, that’s how I had it for 30 years in the old house and always liked it, especially because it allows temperatures to be adjusted very flexibly and even on short notice.
With the air conditioning/air-to-air heat pump, you are sufficiently supplied with good insulation and could relatively easily attach the annex, which would also always be comfortably warm/cool.
I have made some mistakes here as well but always boldly implemented my ideas; I live with the small disadvantages because it feels like our special home, which might not please someone else because something might be missing (e.g., underfloor heating), but it suits us.
I would really recommend you think more and more outside the box and especially imagine your life “without” the teenager because soon that will be the case. Perhaps an unpleasant difficult son-/daughter-in-law or something else awaits you; then it’s nice if the visitors can be there but live in their own area. I find that nice and would probably implement it that way next time.
Fireplace is great, but why then all the expensive and maintenance-intensive other technology if you already have a fireplace + air-to-air heat pump in the house? We need very little wood or energy and have the small amount of wood delivered—that’s no problem in Brandenburg.
What integrated, not what.
For example, in the sense of quickly controllable heating/cooling, possibly even room-wise.
As I said: air-to-air heat pump climate is a given, plus stove and infrared or partial electric underfloor heating if needed, done.
The kitchen stays (almost) the same size, it corresponds to our way of life.
But the dining area is really… modest. Something must be done there.
For example, with us, the living room on the ground floor wouldn’t be needed for the two of us at all; it could be eliminated altogether. We have a “normal kitchen size” and next to it a dining area with a super comfortable dining bench that is used permanently like a living room sofa. In the next house, the living room would be removed entirely. Upstairs, I have an area that I enclosed with half-high shelves as a workspace-TV-and-something-else area with a large window for a view and even a balcony where I sometimes sit (not absolutely necessary). My wife and I like being together in the house but often sit in different areas also because of different interests. This is what I mean by privacy, even between us as a couple; we like that. Everyone has their nice corner, and partly everyone uses their own bathroom; all these peculiarities can be implemented perfectly for two, so I would give the young adult their own place, which they could then enjoy and also help design.
We (unfortunately) also have a fairly large plot, so my tip is to use it also for your buildings; otherwise, it just hangs there like a “towel.” I used to know someone who had a beautiful wooden house in the garden; the daughter lived in it except in winter; it stood at the other end of the property and was used in many ways after she moved out—simply great.
Uffff…. quite long, sorry. I’m getting the urge to build again.