New construction costs - Is the financing in order? Experiences?

Uneducated people have demands too! … ;)

Certainly. But since we have a child, we are out of my IT bubble in that respect. And the difference regarding the length of the parental leave is quite noticeable.

Here I want to drop a bomb:
We have now decided on a different house in the same new development project. The following data NEW:

What is the reason for the other house? Just the price? How does the price difference come about? Only due to 80 sqm less land (430 sqm is quite tight for a detached house) and 7 sqm less space or are there other differences as well?

… also, what I notice:
if the construction site is supposed to be in southern Lower Saxony: as far as I know there is a plot of land at least 650sqm in size. So if it is smaller, as you say, then it is either a semi-detached house plot or a plot divided by shares, i.e. ideologically split.

Good hint. I have asked the question to the project developer.

sounds reasonable, but I would keep the annuity at €2,300 as originally planned. Then you’ll at least make some progress with repayment. Since you are already expecting extra costs anyway, best to put part of those immediately into the loan; whether you then take out €25k more doesn't really matter anymore. Don’t fixate on the special repayment either, as there will still be plenty to pay at the beginning until everything is “finished”.
The children discussion is classic house-building forum stuff again. You just do it when the time comes, the way you think is right.

Yes, we will have a variant calculated again with less equity contribution.

[...] Do you still have any other buffer in case something unexpected occurs? [...]

We still have a nearly five-figure buffer for extreme emergencies.

[...] What is the reason for the other house? Only the price? How does the price difference come about? Only due to 80sqm less land (430sqm is quite tight for a detached house) and 7sqm less living space or are there other differences?

Yes, only the price and the idea that we don’t need a separate office just for work and that room would eventually just stand empty when the children move out and we are retired. 138 sqm is enough for us, even if there are four of us. The floor plan is basically the same, only the living area downstairs is bigger in the smaller house. For that, the “guest room” on the ground floor is omitted.

Yes, only the price and the thought that we don't need an extra office just for work and that this room would eventually just stand empty once the children have left the house and we are retired. 138 sqm is enough for us, even if there are four of us. The floor plan is basically the same, only the living area downstairs is larger in the smaller house. However, the "guest room" on the ground floor is omitted.

I wouldn't base myself solely on the price in the initial phase. We did that a lot at the beginning, because the costs can be overwhelming... but I definitely wouldn't do that anymore. Of course, the whole thing has to be affordable, but the most important thing is that the house fits your requirements. The appropriate house size is more than the sum of the square meters. If you mainly work from home, it should indeed be a study, and (which bothers you now) one for each person. I wouldn't want a desk in the bedroom as a permanent solution, and working on a laptop at the kitchen table is not good for your back... absolutely not. Yes, it is more expensive then, but that is ultimately the price you pay for home office. A study can certainly also double as a guest room with a sofa bed. I would recommend that you first get an overview of the real costs and your real needs. And then take another look at your planning.

What actually surprises me a bit about your post are the current low total monthly costs.

You actually only have expenses of 1500€ including rent (600€)?
So you only spend an additional 900€?
That seems personally low to me. What about insurance, garbage fees, groceries, gifts, other leisure activities, vacations?
You don't mention any of that. Of course, it could be that you live frugally. The two of us spend more money per month.

What actually surprises me a bit about your post are the current low total costs per month.

You actually only have expenses of 1500€ including rent (600€)?
So you only spend 900€ in addition?
That seems a bit low to me personally. What about insurance, garbage fees, groceries, gifts, other leisure activities, vacations?
You don't write anything about that. Of course, it may be that you live frugally. The two of us spend more money per month.

I thought that too. The three of us are already at around 600€ per month for groceries & drugstore items. Two years ago, 500€ was still enough, but you can slowly feel the inflation. I recently paid 3.64€ for 3 stalks of leek. o_O Then there is "everything else", we are significantly higher there as well. Not everyone lives the same way as others, but 900€ seems tight to me.

I would never want to go down to zero reserves with house financing.

I did it and I would go down to zero euros again anytime. Unplanned additional costs of the renovation had eaten up all buffers. Better that than a refinancing. If not for something like that, what else is money saved for?