Use tiny house as an extension or affordable extension options?

And what about heating? It should not be unheated, right? Or what room usage is intended there?

Ah, I knew I was missing something just now – I assume that I actually include these costs in the 40,000 EUR for the conservatory; unfortunately, I don't have my planning with me right now. But I'll use the comment to address the question about room usage – we would like to use the additional space for a master bedroom + 1-2 home office rooms.

As a layperson, it seems a bit strange to me when someone wants to enlarge the house by almost half. The location must be really good for that. Keep in mind that extensions look nice and practical on paper, but in reality, they can quickly become awkward or dark when you extend or widen existing living space. I live in a very densely built-up area and I often see that such an enlargement is less impressive in reality than people probably expected. Even with skylights in the roof and stuff like that. With poor planning, you end up with a lot of traffic area and little light in the middle part. I would rather plan "straight" and very well than build too generously.

Depending on how big your family is, I would see if it’s possible to manage with less extra space. That would also be cheaper. Or just bite the expensive bullet and make a proper extension. The 100,000 euros are a lot, but in my opinion, not excessive for fully usable living space.

We would like to use the additional space for a master bedroom + 1-2 home office rooms.

hm, isn't it a bit impractical if the bathroom is only accessible via the conservatory in the "main house", especially during winter? I'm mainly thinking about nighttime. (And the conservatory will then also be heated?, that also sounds somewhat tricky) Or is the whole thing well protected from being overlooked on your property anyway? I myself am not really one to get up at night to use the bathroom, but my better half is (and I've been told that changes over the course of life, for example due to pregnancy or "aging"). Then having to stumble through the glass conservatory (cold or not) doesn't sound very optimal. Maybe posting floor plans including the plot wouldn't be a bad idea. (Plus maybe also what changes were requested?) And your space requirements. So far it's somehow all just implicitly "small family" and "2-3 extra rooms" (master bedroom/office), but I think to get somewhat more helpful suggestions here, information is actually missing. Maybe something can be restructured inside so that it better meets your needs or something like that, or the room program could be implemented with a smaller bay window/fewer interventions.

What always comes to my mind first with such matters is that it does make a difference whether it’s a "Gartenhaus" or permanent living. That means building application, structural engineering, building energy law, and so on and so forth. And fooling the building authority is absolutely not an option; sooner or later a neighbor will want the same and will point the finger at you, and then you’re in trouble.

And what about heating? It shouldn’t be unheated, right? Or what kind of room usage is intended there?

They mostly operate via electric/ventilation.

We are currently calculating about 50,000 EUR for a tiny house from Pineca

These are _garden houses_ that are insulated, but they do not replace a home according to the Renewable Energy Act.

Transition middle winter garden

This must also be included in the thermal envelope. Everything must meet current standards, as far as it concerns "residential construction".

I’ll quote myself

It also has to be part of the thermal envelope. Everything must meet the current standards, as far as “residential construction” is concerned.

The Building Energy Act applies to permanent living spaces. I see a problem in that you are basically splitting it up: first a conservatory, then a tiny house/mobile home. There is a lot of potential for water ingress or thermal bridges.
You need an energy certificate from 50 sqm onwards.
You need about “18cm of insulation on exterior walls, roof and floor” (that’s what Google says). “With heat pumps, pellets and/or solar, insulation can be reduced by a few centimeters.”