Mini house - possible for 300K €?

Thank you for your answer! That means with €300K it's already very tight but could work. At Town & Contry there is even a bungalow in my desired size, great tip :)

The problem is that if something goes wrong you have no option for additional financing. That's where it ends.

I saw something like that last year in a new development area. A really small house, felt even under 80 sqm. The garden was mostly a utility garden.

For myself, a ground floor condominium with a garden share would make more sense. I really can't imagine that you'd come out cheaper with a new build house :)

I would like to thank you for your input; it helps me make a more realistic assessment.
- Reselling with a very small house is difficult
- Fixed construction costs make very small building uneconomical
- In some areas, minimum plot sizes, resulting in land waste
- No house really ready to move in for 150,000€
- Margin of 300,000€ too small for possible cost increases during construction, refinancing then difficult as a single person
- Possibly poor property appraisal by the bank due to very small size, so loan amount questionable anyway
- Work falls on one person, is that sustainable in the long term
- Life situation could still change

In summary, there are more arguments against than for it.
It would probably only make sense if you find a really small building gap in a rural area, which is well below 100,000€ and well under 500 sqm. And then not 70 sqm but rather >90 sqm. And that is rather unlikely.

With your budget, you will either barely manage to somewhat stick to it with the classic house providers and miss qualitative goals, or completely exceed the budget.
I would therefore approach the goal of living in a detached housing unit on your own land a bit more creatively. Take a look at the Netherlands to see what kinds of chalets are available and get inspired. For special solutions, you also need a special connection to the building authority. I would start building that connection and ask for creative assistance. With a bit of luck, you will find someone who thinks along and supports your cause.

For example, with us there are always older weekend houses with existing use rights and permanent residential rights (also in winter) for rent or sale. Owners renovate them and then 50sqm becomes 150... legally.
No matter...
Maybe you can find something like that for rent where you are?
Camping parks also offer cabins for year-round rental.
What about a leased plot of land?
Otherwise, keep an eye out from time to time: maybe a sublease in a large house is an option for you initially, or someone is doing a project and looking for people to join in.

In summary, there are more arguments against than in favor.

What, in your view, speaks in favor of building a very small single house now in your mid-30s?
Is it already out of the question to build a larger house together with a partner later on?