How expensive can the property be?

and I know surgeons who earn less than teachers with an A14 salary

Sorry, I have just now gone through all the answers and will summarize.
Thanks already for the lively participation.

The living expenses for 3 people are probably estimated higher by the bank, more towards 1000€ - 1100€. You don’t know the expenses for the child, but they are certainly about the amount of child benefit, rather more. Very frugal people manage with 50€, but I don’t assume that here.

How high is your continuous saving rate – or have you kept a household budget? Have you already saved 1,500€ every month for months? And have you included all the additional costs for the house? You don’t pay just the installment. The rule of thumb is max. 1/3 of the income for housing, and additional costs are better included in that. Without your partner’s job, I consider your idea of up to 1,300€ installment unrealistic. But even with 3,900€ it gets tight if you haven’t included holidays, all insurances, or a car reserve, as you wrote.

1) I really can't estimate the expenses for the child. But your assessment could be right, then I come to about 1,100 EUR.
2) I have been keeping a household budget for years and have an exact overview of my finances and how much I spend on what.
3) I save about 1,500 EUR monthly. However, only for about 1 year now, before that I earned much less.
4) My wife is currently pregnant, as soon as possible she will take a part-time job. Then it should be no problem to pay an installment up to 1,300 EUR. BUT my plan is not to take the maximum installment amount.
5) If the rent for a future apartment costs 1,000 euros, then I can just as well pay an installment of 1,100-1,200 EUR, right? ^^

Too poor creditworthiness. Not even assets after buying the plot (that goes for additional costs, the rest is balanced with the Bafög loan). And I already said something about the installment. For full financing most banks require more than 2% repayment – for that you also get poor conditions.
Calculate with a factor 4 annuity = 330€ per 100,000 loan.

1) What do you mean by 4 annuity = 330 Euro per 100,000 loan?

Means conversely:
You invest 8,000 € to immediately and sustainably increase your household surplus by 130 €.
For 130 € you currently get about 40,000 € more loan from the bank.
So why not repay immediately?

1) Does it really make such a difference to the bank that I have to repay 130 Eur Bafög monthly? Do I have worse conditions because I already have a loan to repay?
2) I mean, if I repay the 8,000 all at once, then I have correspondingly less equity available.

Why desperately look for options?
A house is expensive... without equity anyway... hardly affordable on one income.
For me it starts at 4,000€ to even think about it. But there are different constellations (equity, EL, other circumstances) that make many things possible. But here at 3,100 I don’t see that.
100,000 plot, 320,000 house with basement, 50,000 everything else ... as sole earner you won’t get a loan.
Also not as a civil servant. That status is outdated.
And just take your presumed 1,200, add additional costs of 300€, and then you are missing entire items like daycare, child costs, your clothing, advertising costs, new purchases, reserves... vacation is also a thing...

And how many children will it be then? One or two children? You speak of “baby”, but the baby will become an expensive toddler, a schoolchild and then a teenager with smartphone and “Hilfiger sneakers”... Your calculation doesn’t add up. Also don’t understand how nowadays someone doesn’t work at all. Your wife has no employment except with you? Each to their own, but then it’s rent or apartment.

1) If the rent for a future apartment costs 1,000 euros, then I can just as well pay an installment of 1,100-1,200 EUR, right? ^^
2) Forget the basement. Plot for 100,000 + house (250-300,000)
3) When the child is a teenager, then my wife will also be able to work full-time. At the moment she is pregnant, later she will work part-time.

If there is no building obligation on the plot, it is your dream location, and such plots are rare on the market, then you can just buy it first and pay off with your desired rate of 1,500 Euro within 5 years. Then even with a second child soon, your wife can work again and you have the plot as equity and can fully get started.

1) The plot must be built on within 3 years.
2) For the location and the area it is a good offer, hence the consideration.
3) Currently 1,500 EUR are available. If I subtract a buffer of 300 Eur monthly, I still have 1,200 EUR available. Viewed that way, any property whose rent is not even close to that is worth it and rent is now almost rising to 1,000 EUR. So why not just own something right away?!

Nobody wants to hear this, but try saving 1,000€ monthly for 2 years.
If that works without problems, you can repay a 400k€ loan and then have about 50k equity.
Then your wife might work again, and then it sounds better.

1) For one year now I have saved about 1,500 EUR monthly. Before that it was not possible because I was employed and earned less.
2) Theoretically it makes more sense to continue that and then have enough equity, but there is an attractive interesting plot here, hence the consideration.
3) I have kept a household budget for about 3-4 years.

So you prefer to live like in the USA with 3-4 credit cards fully loaded? The main thing is to have a house and a nice car.
The problem is that for such a tight budget the costs seem very low/not properly calculated:
1. 100k for the plot including development. What about additional costs like: property transfer tax, notary, authorities...?
2. 800 € other living costs. So clothes, internet, insurances, car, reserves. For three people it will more likely be 1,500 € - 1,800 €. We ourselves calculate with over 2,000 €.
3. Private health insurance as civil servant already considered? What about child/wife?
4. Housing. Is that warm or cold rent?

It’s not about whether the OP can afford the house. For me personally it’s mainly that with these vague details we can’t judge it at all.

1) Yes, including property transfer tax, notary etc., I come to 100k. If you take it exactly it’s about 95,000 Euro, I rounded up.
2) I have kept a household budget for a long time. 800 Euro is the average of the last 7 months. Average of the last 12 months is 1,000 Euro. As said, I can hardly estimate that with the baby. We apparently have different lifestyles, I won’t come to 2,000 Euro.
3) Health insurance is already deducted from the net salary.
4) Housing is warm rent.

New construction currently not feasible, but I agree with some that you have to get creative. You are young and a civil servant, so you can stretch it. ING, for example, offers 1% repayment, after that you can increase.

Also neglected is the factor that construction may only start in 1-2 years, you can also stretch such a plan into the 3rd year, then the baby is 3 and wife can work...
As of today, you can’t manage it, which is not a problem as this is a long-term thing and that is the problem with many, that’s why we have this renting mentality in Germany and everyone wonders why citizens in other countries have more wealth. There ownership is normal...

My suggestion to you. Finance the plot, try to work with a special repayment option and plan the construction in 2 years. Not everyone can have 5k net at least and then build.
If you change your mind, you can easily sell a plot in such an area...

1) You summarized it well. This is also the rough consideration.
2) First finance the plot, delay the start of construction. Until then the wife can start working again. This would increase the net household income.
3) And as already mentioned, why pay someone else 1,000 Euro rent if you can invest in your own home. An installment of 1,200 Euro should not be a problem. Within the last year I have saved on average 1,500 Euro per month.

1) What do you mean by 4er annuity = 330 Euro per 100,000 loan?

4% interest and repayment combined.

5) If the rent for a later apartment costs 1,000 Euro, then I could just pay a rate of 1,100-1,200 EUR, right?

The landlord pays for maintenance, the owner themselves. In addition, the ancillary costs for a house are always higher than for an apartment.

But your assessment could be correct, then I come to about 1,100 EUR

The bank will definitely apply higher flat rates.

When the child is a teenager, my wife will also be able to work full-time.

But you cannot spend that money today.

A rate of 1,200 Euro should not be a problem. Within the last year, I have saved an average of 1,500 Euro monthly.

Good luck, you will need it.

The generalization of civil servants does not exactly demonstrate a pronounced ability to differentiate. Apart from teachers (what would parents actually do if they were not civil servants and went on strike once) and civil servants in administration, people in the professional fire brigade, police, judges, etc. are also civil servants. Please keep this in mind.

Otherwise, I stick to my statement... 1500€ all in will simply be difficult to impossible. The 300€ are already going towards incidental costs. That doesn’t even include maintenance or similar expenses.

2) I have been keeping a household budget for a long time. The 800 euros are the average of the last 7 months. The average of the last 12 months is 1,000 euros.

Here it already starts... you see that you have 1000€ expenses, but you first state 800€ here. Why???

And as already mentioned, why pay someone else 1,000 euros rent if you can invest in your own home. A payment of 1,200 euros should not be a problem.

1000€ warm rent vs. 1200€ installment + 400€ additional costs. Difference 600€!

You really have to be careful not to lie to yourself with your numbers. Therefore, check very carefully whether this statement corresponds to reality:

Within the last year I have saved on average 1,500 euros monthly.

You always calculate with the warm rent and compare it to the repayment. Please, please, take the cold rent on the credit side and add higher additional costs for the house on the debit side, otherwise you are only deceiving yourself!