Connection for washing machine was forgotten in the new building

That is certainly the best idea for now. However, it is quite surprising that all the other craftsmen kept silent about it and we had to notice it ourselves first.

Well, the trades involved afterwards have no interest in what connections you ordered there. The plumber does that during the rough installation. After that, the plasterer covers what is there, and the tiler tiles over it.

That is correct, but even the plumber was constantly on site. Well, that’s how it is now. We will make the best of it.

That is correct, but even the plumber was constantly on site. Well, that's how it is now. We will make the best of it.

Feel free to let me know how it turned out.

A washing machine has three connections: fresh water, wastewater, and electricity. I assume it’s about fresh water or wastewater or both? Would the machine then have to be placed somewhere other than planned?

Presumably, the general contractor will want to make the connection at the washbasin. Do we have to accept this, or are we allowed to request a relocation inside the wall?

Presumably, he will use the supply and drain of the washbasin, yes.
I think that the effort for a flush-mounted relocation is no longer proportionate. Apart from that, the washing machine, like the washbasin, will cover most (if not all) of it, so there's no need to be overly picky now.
However, I would try to negotiate something, for example, a separate circuit for the washing machine or a built masonry pedestal. Something. That would provide a clearer added value.

That is certainly the best idea for now. However, it is rather surprising that all the other craftsmen remained silent about it and we had to notice it ourselves first.

Naive assumption. The other craftsmen do their trade. None of them know if you perhaps ordered it that way for cost reasons. As an electrical engineer, I don't care what the trades next to me are doing. Why should I? The customer pays me for my part, not for me to uncover the mistakes of the competition free of charge... I benefit if I can say afterward in case of problems, "Well, Company XYZ prepared/performed this poorly, we couldn’t do better. The preliminary work was your responsibility. We can correct it, but we would have to handle this via a change order." My experience from 15 years in the plant business: The customer does not thank you if you inform them beforehand; in the end, you get stupid comments, and the extra effort for the adjustments on my side is brushed off because "it could still be corrected early enough on the plant side."

Even the fitter only knows what his boss tells him. Because the boss is present at the on-site briefing, not the journeyman who actually performs the work in the end. If the boss says "Just do the rough installation there, he didn’t want a washing machine." Then the journeyman doesn’t ask. That’s how it is, no washing machine, fine.

Therefore, you should address the fitters directly if you notice errors and not proceed with the motto, "Well, they didn’t do it now. The boss will certainly take another look and correct it."

No. As a rule: If a job is not carried out (i.e., not at all; poor execution is something else), the boss hasn’t told his fitters. Then address someone immediately. This usually allows you to solve the problem cheaply. Demanding that it must be recessed after the final acceptance is unrealistic. Completely unrealistic.