Architectural design - planning crisis!

Yes!! Because you have to pay the invoices caused by the architect. The architect does not spend his own money.

Uihhh! We had specifically said the 400 is the maximum turnkey price. As a lawyer, I would have some problems if the architect then makes a proposal that is not feasible in this form at all and then completely escapes liability.

Evolith, that seems to me to be the heart of the matter overall and also corresponds to my initial feeling. If the living space can be created quite easily, then why should one embark on the adventure of a doubtful extension on the slope? The children are not supposed to take care of us either, we wouldn’t want that at all. But an easy-going multi-generational living is not bad either. And if not, then not. In case of doubt, I would then found a Methuselah shared flat or just completely get rid of it and spend the retirement at an attractive beach.

First of all, I believe that it is difficult to map several living scenarios at once. The idea that one might eventually move into the smaller living unit at some later time sounds sensible to me, whether initially the youngest child (can she already walk or is she already driving a convertible?) or the parents or whoever else might live there, I would classify as "secondary," since these are rather uncertain/unplannable options. What might please the youngest child could be a nightmare for the parent or vice versa. One possibility could also be to design one or more apartments (at least separable as an option), because as you write, you might then need WohnungEN, so plural. Then better 2-3 small ones than one big one.

In this respect, I (or better, the architect) would devote more thought to a sensible and flexible interior design than to finding an eye-catching facade that then consumes the capital that I lack for the interior fittings. An extension can also be stylish if it is kept simple; it does not necessarily have to be a stag’s antlers with carvings on it.

If my husband were to fall off the ladder or elope with the neighbor

An old well-known woman’s dream... from a certain point on the guys don’t run away anymore unless the food no longer tastes good at all or the beer brought to the sofa is constantly warm.

I would not want to live alone in 170 sqm, detached and opposite a forest.

... that, in turn, could then be changed depending on interests o_O

But if you have set a cost framework, do you then have to think about the feasibility within this framework yourself?

As I said, I would invest significantly more in the interior; possibly in this case or for these intended options an interior designer would be the appropriate contact person for flexible living spaces and situations. By the way, in most cases they can also plan the entire extension project.

Actually, I had no idea at all about this variant,

... exactly this should also come from the other side.
From my point of view, the project still feels overloaded with usage variants, plus the "special" design of the architect, which will surely strain your wallet. So far it sounds like the jack of all trades and I would be concerned that the actual motive or the common thread would be lost in the process.

Ufo and insertion requirement regularly clash ;-)

My basic idea was to create a usable attic by raising the knee wall. I also had various ideas for this and still think that you can design a rather narrow, tall house wonderfully. The architect more or less drastically rejected that and preferred an extension solution.

A gable-side extension continuing the existing roof or an eaves-side extension with a shed roof over the existing roof interfere least with the existing building. However, if the eaves side on the hillside already "stands on the edge," a gable-side extension is more appropriate. Raising the knee wall is the most invasive.
The integration of the youngest child into the family house—which is pedagogically usually not advisable anyway—can easily mean significant additional costs compared to having the youngest child move out.

I would first make an inventory—with a classification of walls into load-bearing, only bracing, and neither load-bearing nor bracing. Then I would assign all existing rooms an occupancy, whereby walls of the second category can be shifted and walls of the third category can be removed. I would not be surprised if the calculation already worked out under the assumption of the youngest child moving out, without building an extension or otherwise expanding. I could even imagine that a starter apartment in the unraised attic would suffice for the youngest child. After this analysis step, the rooms that exist only in the demand plan but not in the actual building would be identified specifically. And again, I would not be surprised if this could be satisfied from reserves in the basement. Pareto is the enemy of all unnecessary expenditures ;-)
A designer architect, of course, does not do it this way.

Nesthäkchen (can she already walk or is she already driving a convertible?)

cool ;) Nesthäkchen is a newly qualified clinical psychologist and is currently taking the bus. I think we urgently need to keep her close to us during the project duration!

Nesthäkchen is a newly graduated clinical psychologist and is currently driving a bus.

Hopefully not coffee tours. If a school bus, she will soon become independent.