Estimate of construction costs for a single-family house in the Tübingen area

No, that shrinks a project down to a normal size. If it destroys the project, it was already too expensive before.

That's how we see it too. Exactly why we keep asking "how much does it cost" like idiots – we already know what it is allowed and what we can afford.

So, 10sqm more storage space instead of taking some stairs?

Without a basement, where else would you put it? Storage space on the upper floor? I gladly take that.

Then you do it like everyone else (you haven’t just invented celebrating): set up beer benches and standing tables, buffet, equip the dining table with folding chairs, and everyone picks their place, wherever. If necessary, standing by the kitchen island will do.

You may do it that way, but "everyone" certainly does not. We have 15 to 20 people at family celebrations (adult children with families, siblings, parents). There are also older people among them; no one would accept standing tables there. The house is designed so that there is space between the dining table and the living room. The grandchildren use that space to play, and at celebrations a longer table is set up there. There are no inns in our village either, and the older people or small children might also want to lie down in between or in the evening.

This has flowed into our wish list – plus there are wishes that one has had all their life.

Nowhere does it say that one can or should fulfill all wishes in life. Many things subside quickly again, which is why you should take due time for house planning. I like your way of presenting things; that you initially feel overwhelmed or first need to clarify your own feelings and possibilities is, by the way, completely understandable.

But honestly, you end up chasing cool things you could do eventually… the schedule that distinguishes between must-haves and nice-to-haves is important at first.

A really good insight that you should repeatedly recall, namely to carefully examine things or needs, especially regarding costs. Investing where it is permanently sensible and brings added value would be smart; paying attention to so-called must-haves or trends long-term is rather silly.
Listening to neighbors or other builders/friends only makes limited sense, because you usually mostly hear praises of their own decisions instead of critical comments. Moreover, you (hopefully) are completely different people than others and should incorporate exactly that into your house construction.

a living area that is so long that a table for about 12-14 people fits (max. 1 - 2 times a year)

Sorry, but that is really outdated. You build multiple square meters for €3,000/sqm so that once a year you can supposedly sit better there? As a young person, I once invited 120 people in the summer; outside there was a fire, and inside everyone spread out wherever it fit. That is much easier and nicer if you just let it happen and with people you like (you shouldn’t invite others anyway), it works wonderfully. When it's a family gathering for grandma’s birthday, only 6-8 sit at the main table, some prefer the bar, or we put up folding tables and 20 people fit there. The days when you keep the good living room ready for Sundays should be over… at least I thought so.
Recently, a young couple told me they absolutely need such a multipurpose room because twice a year they want to set up a straight table; an L-shape is out of the question. Luckily, no one forces me (anymore) to attend such compulsive events.
You would be amazed how nicely things spread out in a single-family house if you simply allow it. You are young people; man, we talked our parents out of that and now you come back with that rigid thinking.

a guest room on the ground floor, which can be furnished as a bedroom in old age, a normal 180x200 bed and a “classic .” Initially used as a home office or when grandma and grandpa come on weekends and sleep here.

I find that sensible, we also have such a multifunctional room. A child can also set up a racetrack or something else there. Fortunately, children usually look for places other than the ones assigned to them; as a child, I had “built” a cave behind the corner bench, my room didn’t matter to me back then.

Children’s room size is important to us so they can also pursue their hobbies and both must be the same size.

Another must-have. What hobbies do your children have (or must have) that set them apart from other builders? Children might want more closeness now; someday they will avoid you or you will avoid them. Are parents with 13 sqm rooms bad parents just because the sqm number differs? Break away from such things; there are no solid reasons for that except maybe perceived social pressure. Trying to achieve fairness through that is downright nonsense, as is the hope or intention to always treat your children equally. That is neither necessary nor possible, let alone sensible, because they are completely different individuals.
Children need a certain degree of privacy, but that begins already with how you interact with them, when/how you enter their room, and also how the children should treat their parents’ privacy. Room sizes cannot achieve what proper interaction does not.

Bedroom size has no priority and can benefit the children.

To me, that always sounds like a bit of self-sacrifice. Why should adults, who are responsible for the children, live less comfortably, i.e., not have a really nice bedroom? Children could rather see it as inspiration for their own growing up. Nothing “benefits” the children here, that’s just what you think. Square meters mean nothing here.

The staircase should not be too steep, a landing would be important; I’m clumsy.

You can gladly “waste” space for that because a comfortable staircase means a plus for everyone and permanently so.

I would like to have an area somewhere where I can practice boxing (punching bag, a weight bench, some space for jump rope)

Yep. If you like to do that regularly, you must plan accordingly. Good. That’s exactly what I often miss in a truly individual assessment of needs. Deduct 2 x 3 sqm from the children’s rooms and you also have a nice corner for that, which doesn’t even have to be separated, maybe just optically or with movable partitioning; then changing usage (children/teenagers) is always easy to implement again.

The bathroom upstairs should have a T shower toilet combo and a roof window / if 2 VG above the bathtub (bathing is important to us)

Then plan a nice bathing place and completely erase the unnecessary, apparently trendy “T” from your mind. A nice bathroom can be done in many ways. What exactly is important to you!

Storage for bicycles, Bobby cars, suitcases, and all the bulky stuff you have with at most 2 children.

This can also be stored covered on the side of the house or in a garden shed/carport.

a living-dining area that is so long that a table for approx. 16 people fits (about 5 times a year)

Now it’s suddenly 16 people and already 5 times a year. I see you’re stuck there!

That’s the issue; if we only celebrate one birthday, we quickly end up with 12 people, and it’s important to us that we are home and in the garden with the family.

Do you really think you have a unique position in your requirements with that? “Chill out,” 15 people is nothing, everyone has that. Why should you not have a nice celebration with friends/family in a single-family house? I had that in my small 50 sqm house in Norway. So far, that sounds rather stiff and therefore unnecessarily expensive. The problem I keep pointing out is that people run out of steam when it comes to other important spots in building. An acquaintance is building a fence for €30,000, huge foundations for his garden hut, and other things, but the terrace doesn’t get a roof.
At this point, I bring up again the phrase of my unfortunately just deceased family doctor and friend: What is frequent is frequent and what is rare is rare.
Shape the things you actually need and also like to use or that bring you joy (sports etc.) also with the necessary attention to detail and for the children (and not size). I’m thinking of blinds, a nice terrace, a well-insulated house, nice window fronts, etc.

Again a must. What hobbies do (must) your children have that set them apart from other home builders? Children may now want more closeness, but eventually they avoid you or you avoid them. Are parents with 13sqm rooms bad parents just because the square meter count differs? Detach yourself from such things; there are no reliable reasons for this except perhaps felt social pressure. Trying to achieve fairness through this is downright nonsense, just like the hope or intention to always treat your children the same. That is neither necessary, nor possible, and certainly not sensible, because they are completely different individuals.
Children need a certain amount of privacy, but that already starts with how you deal with them, when/how you enter the children’s room, and also how the children deal with my parental privacy (should). Room sizes cannot achieve what the way you treat each other cannot.

That always sounds to me like a piece of self-sacrifice. Why should adults responsible for children live less comfortably, i.e., not have a really nice bedroom? Children could rather see that as an inspiration for their own growing up. Nothing “benefits” the children from this, you just believe that. Square meters mean nothing here.

I see that differently from my own experience. Children need space. They want to romp around, sometimes build Lego, the train set, have a great console or computer space, set up sports equipment, and bring friends over, sometimes to stay overnight.
I would always make a sleeping room, where I predominantly just sleep, smaller in favor of the children’s rooms if space is not endlessly available.
For children, their room is both living space and sleeping space at the same time. Eventually the first boyfriend/girlfriend comes along, and then they spend a lot of time together in there.

What hobbies do your children have (or need to have) that set them apart from other home builders?

Hehe. From the life of a father of three girls. Children's rooms are sufficient (for us) six square meters. Sleeping, cellphone, homework. Otherwise, all three follow their hobby. They spend about 25 hours a week at the stable with their horses. I want to say that you can't plan anything and it always turns out differently (with children).

I see that differently from my own experience. Children need space. They want to romp around, build Lego, play with the train set, have a great console or computer area, put up a sports device, and bring friends over, sometimes even to stay overnight.

However, the following very important questions arise for me: 1. What can I afford? It's great if you have a separate playroom for the train set, drums and/or piano and/or a particularly quiet room for the child who prefers to read alone. Fortunately, you can't plan for that and you have to be able to afford to reserve rooms for possible use, which also makes little sense from a caring perspective. Kids would much rather play their console games on a huge OLED TV and not on a 42-inch shaky LCD, which might still be okay for the older generation for a bit longer. I could not depict every imaginable situation (wish) and above all would not want to. Children especially need mental freedom as well as inspiration from their environment for their healthy development; having more square meters does not help with that. Children always have space in a reasonably normal single-family house anyway, and to doubt that because of a smaller room would seem completely exaggerated to me. We also raised two boys, one had 14 sqm and the other 11 sqm. Do I see a causal connection to any developments from this? Rather not. Did they play a lot? Yes, constantly and everywhere, happily also each alone behind a closed door. Extreme portrayals as if they had no space in the house I find not very helpful to find a healthy middle ground. We once had half of the elementary school class in the garden and due to heavy rain it was then decided that they would not stay in the tent as planned, but all stayed overnight in our living room. I thought that was all great but I would not orient my house construction expensively to that; at least I would not and the kids found it adventurous. 2. Do I have to make my house expensive for every completely unforeseeable option? If I do that, I build compromises (structural and financial) on other sides and that is exactly what I would never want to do. Do both children have the right to permanently place, for example, a large sports device somewhere in the house? No, that simply might not be possible. Friends staying overnight is welcome but that has always been possible and will continue on the air mattress in the child's room, a folding bed, or elsewhere.

I would always make a bedroom, in which I mostly only stay to sleep, smaller in favor of the children's rooms if space is not unlimited.

Everyone does that differently but I like to spend time in nice rooms myself and my bedroom is just as much part of that; I can’t make sense of the sometimes-read statement "it’s just dark there anyway." Currently, we also purposely did not build our bedroom particularly large but paid close attention to making it nice for ourselves. Actually, I cannot understand this frequently read "obligation" (sorry for the maybe not quite fitting word) to make it as easy and good as possible for the children by size or supposed equality. The chosen phrase "in favor of the children" suggests that otherwise it would be too little or not sufficient and that is not true or provable in this way.

For the children, their room is both living space and sleeping space. Eventually the first boyfriend/girlfriend will come, and they spend a lot of time there together.

But they are not adults initially and therefore have a completely different set of requirements. Otherwise, one would necessarily also need the often-cited children's bathroom. The relationship in their own children's room is probably also limited in time and the adult is disturbed anyway. The topic of privacy cannot be created by 3 sqm more space in the room, but rather by prior appropriate planning, which I would find sensible in turn. , for example, solved that nicely and gave the two boys privacy. As I said, everyone may see and do that differently, I just find the statement that "others" would be too little, not sufficient, and virtually "better" for the children not appropriate or helpful. That is simply not true.