Planning house construction and considering the desire for children

Wow, I didn’t expect there to be so much feedback here! I hadn’t planned a specific direction for the thread, so I find all the topics here interesting.
What I really liked was the statement that it makes a difference whether you simply have one extra room (it’s clear that you can always fill that somehow) or whether you have already mentally designed it as a children’s room. We were so optimistic at the beginning that we even thought about furnishing the room. Fortunately, we didn’t do that, but as a result, we have a completely empty room in the house.
Many of you seem quite relaxed about whether or how many children there will be in the end. I am not that relaxed because I can’t imagine a life without children. That’s why I wonder if all of you would really be that relaxed if things had turned out differently...
Topic age: that is really not easy nowadays. I was lucky to have a permanent job with a good salary early on and was therefore able to get started early. I have friends who either don’t have a partner around the age of 30 or are still busy with their studies or trying to establish themselves in their jobs. Since fertility decreases, I hope everything works out for them because I know how long it can take...

...who already had planning security at 25...

That's exactly the mistake in the assumption!
Where do you get the certainty that the woman in the relationship can become pregnant immediately after stopping the pill?
Where do you get the certainty that the child will be born healthy?
Where do you get the certainty that the child will develop according to the usual average?
Such questions could be spun on indefinitely.

In short: There are too many unknowns in the equation, so the "planning" is completely absurd!
You have no other choice but to let everything come as it will!

We decided to build the house even though we assumed that having children might not work out. It turned out differently, and because we hadn’t planned anything, it was much easier to adapt to the changed situation, especially since our lives were turned upside down by the childbirth.
Anyone who wants/has to plan their life tends to perceive a child, if it comes unplanned, rather as a foreign body because their own self-fulfillment has not yet made room for it, and they are not yet ready to leave their own personality at the door of the delivery room.

Many of you seem to deal with it quite relaxed or have dealt with it quite relaxed, whether or how many children there will be in the end. I am not that relaxed, as I can't imagine a life without children. That's why I wonder if all of you would still be so relaxed if it had turned out differently... Topic age: that's really not easy these days. I was lucky to have a permanent job with a good salary early on and could therefore get started early. I have friends who either don't have a partner at around 30 or are still busy with studies or getting established in their jobs. Since fertility declines, I hope everything works out for them, as I know how long it can take...

Honestly? I have a plan B for that case. In the Netherlands, single women can also receive a sperm donation. If it becomes clear in 3-4 years that no partner is in sight, that would actually be an alternative for me personally.

The world population is already too large anyway, so childless couples wouldn't actually be a bad thing. But most can't cope without children, as their lives would otherwise be meaningless. A shame.

Large houses with many rooms for each child and each hobby are adult projects.

In my opinion, an adult project is more likely to be building a normal house as an average earner ("own home" instead of "dream house," keyword "flair"). Anyone who plans their house not according to their own financial circumstances, but according to the "standard of living" of their daily soap role models, I do not yet count as an adult.

The world population is already too large anyway, so childless couples wouldn't actually be so bad. But most can't handle it without children because their life would otherwise be meaningless. A pity.

well... if you take people like my mother who push "I do everything only for you" or "I live only for you" on you, that may be true, and of course that's not good, especially not for the children.
Our child is definitely not my hobby. Without a child, we probably wouldn't have this house, but a smaller one just for us, which would also be okay. I would have enough to do even without children, but we just wanted to.