Corner plot city villa 150 sqm + conservatory, E-W

What the development plan hardly prescribes, of course, are the squareness of the floor plan and the symmetrically distributed windows. Existing twin girls would be the last thing I would have thought of; I would have suspected another only child and was about to point out that child 2 would probably feel like an unwanted child since their room is so "leftover" when comparing the two children's rooms. The most feasible (in the sense of "minimally invasive") solution to "save 5 sqm on the price" I see here is to reduce the floor plan by 25 cm in both dimensions, namely at the bottom and the right of the plan.

therefore compromise on the location of the bathroom and office window on the ground floor to maintain a symmetrical exterior appearance.

I would decide "inside before outside" in case of doubt.

you can do without: carport instead of solid garage, dressing room in the bedroom and children's rooms

good list to strike off, giving up dressing rooms (which I find amusingly positive for the twins) won't bring you much.

Have the price per sqm from the construction company and would get along well again with this saving

That is too rough and wrongly calculated, because the whole technology and equipment is included in the sqm price, you don't reduce that. If you decide to build the conservatory-representative dining area possibly later, you are on the savings target and still have a great family dining area, for large gatherings it's then either summer (outside) or you improvise.

If the limit is still 400K€, then at today's prices we're talking about a total of at most 133sqm (400K€/3000€ per sqm). That is almost 10sqm less on each floor.

If that reduction is too much, in my opinion the only option left is a variant with a lot of own effort to lower the price per sqm.

Or just increase the budget ...

Where is the construction supposed to take place? Prices do vary regionally.