300k budget for a 130 sqm single-family house?

I see the problem that on about 130m² the wishes cannot be realized.
On the ground floor you write spacious living area, closed kitchen, utility room, guest WC, office and/or guest room...???
In the attic you want 3 bedrooms, possibly plus dressing room plus bathroom.

I hadn’t seen that at all and I also consider it nearly impossible. My colleague has a house that almost meets your room requirements:
Ground floor:
- Living area
- Closed kitchen
- Utility room
- Guest WC (not extremely small but without shower/bathtub)
- Emergency children’s room with 9m² (possibly office with you)
Upper floor:
- 3 bedrooms
- Bathroom (should be about 11-13m²)

Overall it turned out to be a pretty nice house (built in 2005), but anything but spacious. He always describes it as square-practical-good and I think he is right. Don’t get me wrong, he’s not cramped in there, but the living room is about 24m² (not spacious), the kitchen 13m² (so that 5 people can also eat there) and the bedrooms on the upper floor are just within a comfortable range (I think between 13 and 17 m²). No space is wasted there and without garage and basement the two garden sheds are pretty fully packed. The interesting thing about the story is: This house has 147m² of living space. On 130m² that seems impossible to me as well. But maybe I just lack imagination.

I now see the space problem the same way. We started planning the floor plan a little, and it was always very tight. We took another close look at the plots, it will come down to the bigger one. Not just because of the size, the location appeals to us much more.

Originally, we wanted to build without a basement. Now I am wondering if that is wise. If we have to fit the technical/utility room completely on the ground floor, the rest will simply be too tight. Another consideration would be to move that outside, as an extension to the garage. But I think it is then almost the cheapest solution to just build a basement. It is definitely cheaper than increasing the total area for that reason. So we are clearly leaning towards a basement.

Regarding the total costs. Yes, I really have good contacts in the expensive trades. I am a craftsman myself and have worked with many for years. Especially for the shell construction and roof, there would be good prices. And also generally when purchasing.

I guess I should have written better on the topic of "building costs," but unfortunately, I only realize that now...

125 sqm city villa at a rather expensive construction company 200,000€ house price + 80,000 extras and incidental costs. 20,000 remain for the outdoor area. (land as you said is extra)

with 2000€/sqm you are already pretty well equipped and a few extra features are easily included. (we have a fireplace, electric shutters, underfloor heating, ventilation system, etc...)

Did you spend 280,000 € for 125 sqm (including the mentioned extras?) including incidental construction costs?

Well, instead of a basement you can probably build about 40m² larger, which should be enough. What I also saw once was a garage attached to the house and extended backward by about 2.5m. The technical equipment was in that area. Maybe that’s feasible?

This can definitely be implemented, the plot would allow it. Will I get by with about €300/sqm for the basement? Then the extended garage would probably be cheaper after all.

The extended garage is certainly (!) cheaper than the basement.