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

Nonsense idea:
Is it actually possible to create a pond, place the house on a pontoon, and thereby bypass building regulations because you live on a boat?

Your house-boat must not be permanently connected to the property (no anchor, supply lines, etc.). So you would have to board your rowboat each time and row over to your "house".

Parallel to "checking what is needed and what it costs," I would reach out to potential financiers. Based on our experience, I suspect that sole financing carries an interest surcharge, which makes your installment more expensive. Especially if you are planning to have children, it makes sense to reconsider how you want to handle it.

The question I ask myself is, can I manage something for about 350k, with the few down-to-earth wishes mentioned in the opening post?

For that, you really have to get going and contact the providers! We are building near the border to Lower Saxony and do not have the extreme prices reported here.

Settlement of terraced houses offered with 110sqm. They have 2 children's rooms of 13sqm, a bedroom of 15sqm, and even an office of 9sqm. So you can definitely live with 110sqm!

I exactly agree! My terraced house even had only 106sqm and was still quite spacious, except for the kitchen, which was rather small (7sqm). And in the neighborhood, only families with 1-3 children lived in this type of terraced house!

The pure house would have cost 330,000 without upgrades.

Our house will be built with 153sqm according to the Energy Saving Ordinance including controlled residential ventilation and including upgrades or special wishes we are at 303,054.15 € for the pure house.

Contract signed in November 2020. If the area of the OP is not too overpriced, one should be able to build a nice little house for 350k.

So a price per square meter of around 2300-2500 €. Isn't that feasible nowadays?

That is definitely feasible. Have you ever been to a model home park? Maybe you can take a look there soon. They usually have brochures and offers that look affordable and probably are. These houses only get more expensive if you deviate from the standard, as my husband and I (unfortunately) have done by now. Say right away that you already own a plot of land, then they will try to win you as a customer. But don’t sign anything spontaneously, no matter what promises, discounts, or whatever else is offered to you.

By buying the plot of land, you have taken the first and right step. Congratulations on that and continued success. Feel free to keep me/us updated on how things go for you. And please don’t be discouraged, there are always one or two stumbling blocks.
(By the way, we are building for 2,500 euros per square meter, if I haven’t miscalculated.)

Hello,
where in RLP are you from?
We finished a week ago in northern RLP.
However, we had relatively a lot of own work (interior finishing: heating + water completely electrical, tiles, wallpapering, floor coverings, electrical (pulled cables ourselves, electrician connected later)), so probably hard to compare. A bungalow with 141 sqm and a 35 sqm garage (connected) was built on a 1005 sqm^2 plot, with rather upscale features.
(Tiles 60x60 and 60x120), bathroom with freestanding bathtub and completely Villeroy & Boch, guest WC also completely Villeroy & Boch, 5x blinds in the living room, otherwise electric shutters, 6 floor-to-ceiling windows.
The house with garage cost me about €255,000. The rest of the trades were done by self-contracting. Construction supervision by an independent expert.
Currently, prices in northern RLP I would describe as quite reasonable compared to prices in some other areas.
Therefore, depending on the region in RLP, feasible at these sqm prices.

Regards

I also see myself at a max of €1200 per month. Plus the option of special repayments at the end of the year

With a €350,000 loan and 1% interest, you would be finished in 28 years. Interest rates will tend to develop to your disadvantage by the time that is reached. The thing is, €1200 rate is financed pretty much on the edge. With the prospect of a significantly higher salary level, you could dare it, but as I said... borderline. Therefore, I would not only look at the financials but also thoroughly examine the topic of partnership. You write that your partner supports you, but is that really the case when you sit at the computer late into the evening analyzing construction descriptions? Going to 10 appointments (which she of course has to attend) with construction companies until you have decided? How does she feel about the topic "moving into your house"? How will you manage if she doesn’t work for 1, 2, or 3 years because of children? Building a house is a burden for both.