Floor plan of bungalow with hip/gable roof, 130 sqm

For health reasons, one will not be able to use stairs later. Of course, everything is still in the future, but I do not intend to build a second time.

If you are sure that you will not be able to use stairs later, then plan your bungalow accordingly. Turning circles for walking aids, shower without the partition wall, threshold-free door

Small thresholds or unevenness are actually more difficult to walk on than stairs.

Because of a possible later illness, which is still uncertain, and considering your budget, I would not rule out a small gable roof house.
Stairlifts are already affordable today

Stairlifts are already affordable today

usually only the main bathroom and bedroom are upstairs. additionally, 2 vacant teenage rooms. Prefer a two-story house over a bungalow for that?

Due to illness, one will later not be able to use stairs.

Those who know this at such a young age usually have an illness that does not only become noticeable as a limitation in "old age" and remain as fit as a fiddle until then. That is why I asked: rheumatism, gout, and the like can also suggest enlarged ranges of motion / turning circles (or conversely: more narrowly grouped work areas) that do not only come into play "later."

but I do not intend to build a second time

The mindset "you only build once" comes from the era of marriages "until death do us part."

Or stairlift

Then better to build new: with pedestrians in the household, stairlifts are not a nice solution if the stairway is not "two-lane"—and with that, we are back to an amount of extra effort that makes rebuilding more attractive.

A staircase equipped with a lift can be easily used. (Exception: spiral stairs or narrow townhouse stairs, as installation is not always possible there) Not two lanes wide and what extra effort. Yes, the wall must be stable enough and an electrical connection is needed. You can even rent these things – the effort is no comparison to building. It only gets tight when the lift is moving and you try to walk beside it. Even with two pedestrians, a staircase becomes tight. Whether due to illness or age. A bungalow is by no means barrier-free or comfortable in old age. It's just missing the stairs, and how often do you use them in everyday life? How about the toilet, shower, space next to the bed, trip hazards from the terrace to the mailbox? Can the washing machine, dryer, dishwasher be installed higher, etc.

we for health reasons

If you already know that limitations will restrict later life, you should build something more accessible. Bungalow does not automatically mean barrier-free, and although this should be the focus, I don’t see it being feasible within the budget and with 130 sqm. The utility room definitely won’t work then, and the access to the bedroom would probably also need to be somewhat wider. Overall, a little more width should be allowed everywhere than a normal person needs, in case you need crutches or a wheelchair, for example. Corners should be avoided.

Have you ever raised this problem with the architect? Problem implementation, budget