Can we afford this?

It might also be helpful to know what you have included as additional costs.
We are currently in the planning phase as well.
However, our additional costs are significantly higher than 30,000 euros.
Just the kitchen and a few extras in the sanitary area already add up massively. Then you also need lots of extra sockets and light switches in addition to what is already included according to the building specifications. For us, that alone is around 1,500 euros. And who knows what else will still come.

So the furniture that we additionally need we have calculated separately, we went to furniture stores and for a kitchen, dining table with chairs, bathroom cabinet, wardrobe, and a new couch we estimated about €10,000, which should definitely match the local prices. However, we also still have this money in our savings accounts, but it has not yet been declared as equity by us. Now we are considering buying a cistern in addition, but we have no idea whether it makes sense to use it only for garden irrigation or also for the house water.

We had also considered installing something like that. However, it only really makes sense if you have a large garden. For us, it was absolutely not worth it despite having 678 sqm of space! However, it also depends on how much you have to pay for a cubic meter of water. If you want to use the water for toilet flushing, etc., the problem is that you still have to pay the actually expensive item "wastewater" because a separate calculation is (still) not possible. At least not here with us. We decided against a cistern but will make all the necessary preparations, like laying the required pipes, since the effort is naturally less than with a retrofit. And if it ever becomes worthwhile, we can relatively easily "upgrade".

And if I add everything up, we come to €303,000. Now our question is whether the incidental construction costs are realistically calculated? And do you think it is financeable? We have about €27,000 in equity which would also be immediately available.

I just looked at the numbers and plugged them into a sample calculator. If the numbers stay the same and you manage with the €303,000, then with the included equity, a 15-year fixed interest period and an associated interest rate of 4.4% (effective 4.49%) and 1% repayment, you have a monthly burden of €1,242.

If you now plan to have a child, then one income will be lost (well, the parental allowance comes, but that too is only temporary). I find it calculated too tightly and would not trust myself to build with these numbers...

So we have come to a total of 1010 euros with 1% repayment and that should be quite manageable. And you can make 5% special repayments annually. Why should this become tight? It would be nice if you could explain to me why you come to that conclusion. Maybe I am thinking something wrong...

So we came to a total of 1010 euros with 1% repayment

with which input parameters? The ones I listed here I just entered online into a calculator for fun and came to this total.

Why just under? Quite simple – I always calculate with the worst case – meaning my wife stays at home after the birth... so only one income for a long time, and even then the money has to be enough to live comfortably.