Feasibility of a DIY house using timber frame construction

but rather something in the direction of a Tiny House/DIY and should be actively lived in for the next 7-8 years.

The question is whether the project is not unnecessarily expensive as a temporary solution. Is the DIY implementation a realistic undertaking for you? Like preparing a slab, possibly kit assembly, electrical, sanitary, heating, possibly tiles and so on. Also with your sewage treatment plant idea. 2x50sqm is not that little, so you can't just squeeze it in on an afternoon. For me, a chair sitter with little manual experience, that would be an effort I wouldn't be able to manage on the side. And as has already been said, you mainly save on small projects if you can take on a whole trade yourself, which you would outsource for something bigger. Is that realistic for you?

Good evening, first of all, thank you very much for the input. So, if it is not building land, for example, there remains the possibility to register a "Biohof mit Hundezucht" and try it that way. Do you consider this practically feasible? One more question for understanding: With pellet heating and hot water via photovoltaic plus central exhaust, I come to KW55, right? In general, the project makes no sense as it is, the topic of tiny houses would then be my next consideration. I will read up on that, but as far as I understand, that would be our best option if no building land is available. Does the type of property play a role here (e.g., Streuobstwiese)? Regards

I think you are imagining it too simply. If it is in an outdoor area – which I assume from your description (getting wood at the edge of the forest) – then you will most likely not get anything approved that has to do with agriculture. Anyone could come along and register something and build something. The building authority is currently trying to prevent that (urban sprawl). Most likely, farmers who have been doing this successfully for years and want to expand have a chance. I also see dog breeding as more critical here, and growing fruit/vegetables for yourself or for a few euros a month (too little space) is absolutely not a justification in this case. Just as an example: even shepherds have huge problems just to put up small shelters for their sheep. They then attach wheels to them. You’re welcome to ask the building authority, but then you should have a first-class idea and not just a quick thought to somehow get something habitable approved. Building new in the outdoor area is even stricter than renovating or expanding existing buildings (farm, yard).

So if it is not building land, for example, the option remains to register a "organic farm with dog breeding" and try it that way.

These are the options available to you:

Of course, you can start an organic farm with dog breeding. But then we are talking about a farm with everything that goes with it. Agriculture, etc.
Can you plausibly present that to the building authority on the only 1,000 sqm mini plot? Along with the business registration and operation?
That has nothing to do with one or two tiny houses. The potentially expensive development is also necessary here.
And personally, I think that dream and reality are hard to reconcile here...
So

Do you consider the whole thing practically feasible?

I consider that utopian.

One more question for understanding: With pellet heating and hot water via photovoltaics plus central exhaust, I come to KW55, right?

Clear answer as in university: it depends.
At least your budget of €80k has just shrunk by at least half with this combination, leaving you that much less for house construction.

the topic tiny house would then be my next consideration.

I would first try to clarify what the property actually is before you dive too deep into ideas. Whether there is, for example, a development plan for an area can often be found online. Unfortunately, there are about 20 portals with some kind of map information (at least here in NRW, it feels like every city has at least one, rather two, and for statewide there are also about half a dozen different ones), but by searching for city or municipality name + development plan you can usually find the development plans, and otherwise you can see if you can gather more information by googling various terms related to cadastre or geodata/geoportal etc.

It is amazing how much data is generally publicly available, but sometimes you really have to look for it.

Another question for clarification: With pellet heating and hot water via photovoltaics plus central exhaust, I come to KW55, right?

The question that arises, however, is: where should pellets be stored on 50sqm and where to get the roof area for photovoltaics? The technology listed alone consumes money and square meters.