Semi-detached house - Which one is the best? Who has experience with that?

Firstly, this is (more or less) possible without problems and secondly, buying a used property consumes much less energy than a new build.

If you are eco-friendly and want to think about the environment, you must not build new under any circumstances. You cannot save as much energy as you waste during construction and the environment gets destroyed

But it is still nice to live in a new house...

If you all write this, there must be something to it, good thing I didn't just go ahead but asked first! Can the differences somehow be quantified? And how old / how up-to-date should an existing building be at most to make sense?

New builds are certainly nice but not a disqualifying factor! I want to think about the environment, but not only that, rather in a reasonable framework ... if it becomes a renovation, that applies as well ...

To my knowledge, it is not possible to achieve a proper energy consumption in existing buildings with reasonable effort ...

There are two vague definitions here.

1. Existing building stock:
Houses for which the building application was submitted and approved after November 1, 1977, i.e. after the first thermal protection ordinance came into force, have a certain construction standard that fundamentally takes thermal protection into account. As a result, all subsequent investments to improve insulation of walls, roofs, basements, floors, etc. are less "sensible" than in older buildings with lower energy standards. Moreover, thicker constructions are already present, which make reinforcement difficult or even impossible or very expensive.
So anyone who wants to dramatically reduce consumption should buy an older house or pay attention to retrofitting possibilities in newer ones. With a 40cm wall containing 5 cm insulation, there isn’t much more that can be done. But with a 24 cm wall, you can still add 15 cm - as an example.
It is true that it’s not always possible; the substance has to allow it, but then it is cheaper than building new.

2. "Proper" energy consumption
Where does "proper" start and where is it already "very proper" or when is it too much effort for too little benefit? Most existing buildings can be brought to a "proper" heat demand level of 100-120 kWh/m² per year with manageable effort, i.e. without wizardry.
Compared to the average demand of just under 200 kWh/m²a, that is only half. Compared to new buildings, it is sometimes twice as much. What is proper?
Anything below that requires suitable substance, as mentioned above. There is no limit then (passive house possible).

The big advantage in renovating existing buildings for your objective is that you are not bound by the Renewable Energy Heat Act or the new building requirements of the energy saving ordinance regarding the primary energy factor. Thus, you can carry out what really saves energy and omit what makes it expensive (residential ventilation, etc.).

What is expensive in renovation is converting existing structures, i.e. changing floor plans and extensions as well as dormers, etc. However, wall breakthroughs are the most cost-effective measure here.

No; it doesn't work like that. Either you are ideologically committed to it and then you follow it through fully.

Especially if it is supposed to be sustainable. And bioconstructively meets your own standards. But anyone planning something like that can hardly make any compromises.

Or you look at how much house you can get for your budget; realize that it's not that much; cut out one or the other goodie (in the sense of expensive; meaning the house will be more square; does it really have to be like that etc.

Sometimes, for the green conscience, it goes towards more insulation, thicker walls or a photovoltaic system; but that is close to the limit of greenwashing. How the pipes, the mortar etc. came about then becomes more than secondary.

And 600K; with 200K for the land is also not really generous; but good standard

As you yourself already write, I won’t be able to follow it through fully.

But it should at least go in the right direction ...

There are two vague definitions involved.

1. Existing stock:
Houses for which the building application was submitted and approved after November 1, 1977, i.e., after the first Thermal Insulation Ordinance came into force, have a certain building standard that fundamentally takes thermal protection into account. As a result, all subsequent investments in increasing insulation of the walls, roof, basement, floor, etc., are less "meaningful" than in older buildings with a lower energy standard. Moreover, thicker constructions are already present, which make reinforcement difficult or even impossible or very expensive.
So if you want to go really low, you should buy an older house or pay attention to retrofit options with newer ones. For a 40 cm wall that contains 5 cm of insulation, I can’t do much more. For a 24 cm wall, however, I can easily add another 15 cm—as an example.
It is true that it is not always possible; the building fabric must allow it, but then it is also cheaper than building new.

2. "Proper" energy consumption
Where does "proper" begin and when is it already "very proper" or when is the effort too great for too little benefit? Most existing buildings can be brought to a "proper" annual heating demand level of 100-120 kWh/m² with manageable effort, i.e., without any wizardry.
Compared to the average demand of just under 200 kWh/m²a, that is only half. Compared to new builds, it is sometimes double. What is proper?
Anything below that requires suitable fabric, as mentioned above. Then there is no limit (Passive House possible).

The big advantage of renovating existing buildings for your goal is that you are not bound by the Renewable Energy Heat Act or the new building requirements of the Energy Saving Ordinance regarding the primary energy factor. This allows you to do what really saves energy and leave out what makes it expensive (residential ventilation, etc.).

What is expensive about renovation is changing existing structures, i.e., altering the floor plan and adding extensions as well as dormers, etc. However, wall openings are the cheapest measure in this regard.

Accordingly, would it be best to look for something very old where the fabric is in good condition, pay as little as possible for it, and then spend as much as possible on the sensible changes?

Accordingly, it would be best to look for something very old where the condition is okay, pay as little as possible for it, and then spend as much as possible on reasonable improvements?

I would basically agree with you, but at the moment the market is somewhat out of balance. I don't know exactly where the thread starter lives, but here old properties go for so much that it's better to build new. Provided you can somehow get a plot of land...

Accordingly, it would be best to look for something very old where the substance is in good condition, pay as little as possible for it, and then spend as much as possible on the meaningful changes?

"Very old" is tricky again. It is always a case-by-case decision. There won’t be much to do with 50 cm sandstone walls in an Art Nouveau villa with monument protection.
But if the wall consists of 24 cm solid brick, 15 cm air, and 11 cm plastered brick (as with an acquaintance of mine on my street), you can get the thing up to new-build level with little material and money investment.

Post-war buildings up to the mid-60s are the easiest to renovate energetically. Ideally 1955-1966. This excludes the worst poor constructions directly after the war, money was abundant thanks to the economic upswing, open architectures with lots of light already existed, and energy saving was irrelevant, so there are no botched 3 cm insulations behind clinker facades.
But as I said: case-by-case decision.