Is building a house feasible with my budget? Or should I give up the dream?

You will get a loan with the data, but you have to check if it will work cost-wise because everything is currently a bit difficult. Depending on the bank, you may also receive a rejection, but with the right advisor, it will work.

There’s certainly no need to make excuses, but one should stay realistic.
100 sqm is nice and great, but if a family is supposed to move in, one has to be aware that it will be cramped.
It does make sense to coordinate such a project with your partner.

If it no longer fits, it will simply be sold. It is and remains a new building. And relatively inexpensive. You will definitely be able to sell it and look for something else if all else fails.
There will surely be a couple looking for a cozy little place at a fair price.

I would aim for a bank appointment: in my opinion, you won’t get one alone, 1% is not possible either, but sometimes you have to create facts.

I’m also financing alone and had no problem with that. Income is somewhat higher, so is the loan, and I even get my girlfriend and child counted in even though my girlfriend also has a job (at least again after parental leave ;) ). Banks don’t calculate nearly as conservatively as you might think. However, you have to be aware that the bank’s calculation neglects something like personal retirement provision. And before anyone brings it up... no, in my opinion a house isn’t sufficient as retirement provision unless you sell it. A new build today will be 40 years old then and require money for maintenance. At the end of September, I still had offers for about 0.85% over 10 years in the open market, promotional loans a bit cheaper. Since then, things have worsened slightly so 1% is certainly a bit optimistic right now. But for me, it was also about simply establishing a reference point. 28 years is quite a long time, the repayment doesn’t start immediately but only once the loans are disbursed, so in about 2 years. Then we’re easily at 62 when the house is paid off. If interest rates rise until the conclusion somewhat later, or somewhat earlier if special repayments work out.

If it no longer fits, it will simply be sold. It is and remains a new build. And relatively cheap. You can definitely get rid of it and look for something else if all else fails.
There will surely be a couple looking for a cozy little cottage at a fair price.

Sure, also an option.
One just should be at least aware of that beforehand.