Is a development plan change necessary? Who bears the costs?

Even if you don't see any advantage as an owner right now - it will come at the latest when there is a need for building land or a sale of the property.

Yes, seen that way I could actually ask everyone on the street. They can't say more than no... thank you all very much for your answers.

Have a nice day

Show me how you imagine it.

Something like this...

It's about converting garden land into building land. And with that, the value of every property that has garden land behind it increases.

And what do the neighbors really gain from this now? The property remains unchanged, the house remains unchanged. Bank balances decrease. Only the feeling of being richer changes. But that is not tangible.
As a neighbor, I would do absolutely nothing and wait.

Even if you currently see no advantage as an owner – it will come at the latest when there is a need for building land or a sale of the property.

A higher valuation of your own property is only positive if you actually plan to cash it in wholly or partially in the near future. Otherwise, especially with regard to inheritance or taxes (particularly with the prospect of a potentially upcoming wealth tax), it is disadvantageous.
Also, some neighbors will not like the idea of increased density.
So don't be surprised if you encounter differing opinions about your idea.

And what do the neighbors really get out of it now? The property remains unchanged, the house remains unchanged. Bank balance decreases. Only the feeling of being richer changes. But that is intangible. As a neighbor, I wouldn’t do anything and just wait.

If it is indeed going to be implemented in 3-4 years anyway, then I agree with you.

So don’t be surprised if you encounter different opinions about your idea

??? I don’t care at all because I don’t even live there :)