Realistic cost estimate: Single-family house with unfavorable development location

I never said that we would necessarily build only once; as I said, maybe we will build again in Australia in 20 years. But you can still think about the future, and the case "oops, now we've been living here for 40 years, too bad the stairs are no longer passable" is not all that uncommon.

I don't want to fully build out 980m², but rather sensibly fit about 100m² and about 70-80m² on it, that should be possible, right, or am I missing something?

I don't want to fully build over 980m², but sensibly fit about 100m² once and about 70-80m² once on it, that should be possible, or am I overlooking something?

To find out the planning intentions of the municipality. §34 does not mean "carte blanche".

But you can still think about the future

I see it the same way. It doesn't have to be one's own future; just as well, the consideration that a senior-friendly house might be easier to sell later (if one then wants to start anew in Australia). It also plays a role what exactly that means. Door 100 instead of 80 probably fits everyone, DIN-compliant maneuvering space for the wheelchair is probably often exaggerated.

Finding out the municipality's planning intentions.

Yes. Nevertheless, it is not a mistake if you can already roughly tell the municipality what you would like. Also here: two or three planning rounds are usually better than one missed opportunity.

Yes. Nevertheless, it is not a mistake if one can roughly tell the municipality what one would like in advance.

If, in an individual case, the local municipality is unsure because there are too few building applications to make fundamental considerations in advance, there are usually common guidelines at the level of the collective municipality or even the district. From these, for example, orientations arise based on the effective floor area ratio of the neighborhood as well as actual building windows. With three-story neighboring houses – building lines like in development plans do not exist in §34 areas – one already has a generous building height framework. However, one is unlikely to have a "free choice of location" otherwise while respecting boundary distances. As mentioned, in small municipalities the mayor more likely calls a colleague in the neighboring town to find out what the current practice is for "where to put the house" there. Or is the municipality so small that the town hall only stamps what was discussed at the shooting festival?

It also matters what exactly that means. A door of 100 instead of 80 probably fits everyone, a DIN-compliant maneuvering space for the wheelchair is often probably exaggerated.

Standard-compliant accessibility is mainly of interest to apartment building developers. In "private" practice, walkers tend to be more critical for maneuvering than (at least manually operated) wheelchairs. In addition, today's grandmas and grandpas suffer considerably more from dementia and balance disorders than from rheumatism, gout, venous diseases, and swollen legs combined. Wide doors do not help against everything.

Something off-topic:

Wide doors do not help against everything.

"Wide doors can help older people stay safer and more independent in their homes. Studies (e.g., Hughes, J. et al., 2002) have shown that increased mobility and independence of older people can lead to a longer stay in their own homes. This can slow the progression of dementia (e.g., Forsell, Y. et al., 1997)."

The entire ensemble of parcels belongs to the two of them and they need a long-term age-appropriate solution. They themselves once planned to build there and we just thought you could do everything "all in one go" since you're already making a mess :D Nothing is to be divided or sold, only the owners of the front house’s property would move into a new house in the back.

Something like this belongs in the initial thread. Just like an open question about what would be best with one or two parties. You can ask the forum that.

At least for me, I keep considering different alternatives.

Yes, that’s also nice. But then showing up with a bunch of T&M floor plans and not leaving any alternatives doesn’t make it easier for the forum or yourself and doesn’t lead to a solution.

I’m writing here precisely because I am open to implementing such proposals :)

Then it’s best to present options openly from the start and ask for them.
There are quite a few here who like to and voluntarily and unsolicitedly improve a floor plan and think a lot about it. Even for hours or weekends.
If the time spent becomes pointless because suddenly completely different prerequisites are created, it could be that the forum’s potential is wasted.