Maximum and reasonable credit amount

Assumption net income after the birth of the child: €5500, without considering the income from the [Sonderobjekt]. Then, with €200k equity and the 30% rule at 3.5% interest and 2% repayment, just under €600k as a purchase price/construction costs can be realistically represented. It is better to invest the income from the [Sonderobjekt] abroad.

Questions:
What is the financial situation after the birth of the child ([Elterngeld], change of tax classes, etc.)?
What is the timeline for planning to return to work?
Number of hours planned when returning to work with anticipated income?
Choice of tax class?
When is the second child planned?
The expenditure side is at least relevant for the period until both are working (full-time) again.
Costs for childcare?
Repayment plan for the financing? - does it necessarily have to be repaid by retirement (which I only consider sensible if there is really enough money); can it be a minimal repayment of 1% because savings are being accumulated in parallel until old age; repayment of the remaining loan at 65 through the sale proceeds of the rented property (makes little sense in old age).
Can the rented property be used as additional collateral, thus possibly making the financing significantly cheaper? Considered?
...

Any tip regarding possible financing is correct. That can be 300k or 800k.

I will try to answer the questions one by one.

Questions:
What is the financial situation after the birth of the child (parental allowance, change of tax classes, etc.)?
During parental leave, we will probably have income of almost 8.5-9k from a tax perspective.

What is the timing plan to return to work?
Number of hours planned when resuming work with planned income?
Choice of tax class?
My wife plans to take 12 months of parental leave and then return to full-time work. Honestly, one does not know how and what kind of childcare will be available. Let's conservatively assume 150% combined working hours (it has been discussed that I will reduce my working hours depending on the childcare options).
The tax class could be changed, but this will be corrected anyway in the tax return.

When is the second child planned?
Desired soon, within 2 years.

The expense side is relevant at least for the period until both are back to (full) work.
Childcare costs?
Maximum rate 450€.

Repayment plan of the financing? - Does it necessarily have to be repaid by retirement (which I only consider sensible if you really have enough money); could it be a mini repayment of 1% because you save in parallel until old age;
We would basically be for

Repayment of the remaining loan at 65 through the proceeds from the sale of the rented property (makes little sense in old age).
I do not understand that.

Can the rented property be used as additional collateral, so that the financing could possibly become significantly cheaper? Planned?
I discussed this with some banks. I got the impression in the discussions that the willingness for this extra effort was very low. It’s not a run-of-the-mill property.

Assumption net income after the birth of the child: €5500, without considering the income from the special property. Then, with €200k equity and the 30% rule at 3.5% interest and 2% repayment, just under €600k as purchase price/construction costs can be represented healthily. It is better to invest the income from the special property abroad.

Then my assumption is correct that €1 million (land €400k + house €600k) is still too much for us at the moment..

Hello,

Then my assumption is correct that 1 million (land 400k + house 600k) is currently still too much for us..

It's all a question of how much you want to take on. The bank will certainly go along with up to 4000 euros monthly installment. Equity capital could be a bit tight, depending on the cost volume of the project, you will probably have to prove more here. With that, you can easily calculate what loan volume is feasible.

The 3000 euros from the rental property can actually not be taken into account. When the "tenants" have moved out, the place is run-down and needs renovation. For that, reserves are needed. Unless the taxpayer also pays for the renovation.

Best regards

Repayment of the remaining loan at 65 through the proceeds from the sale of the rented property (makes little sense at that age).
I don't understand that.

I mean that a) the remaining loan can be paid off with the sale proceeds and b) a rented property is no longer particularly interesting, as the tax component is no longer applicable.

During parental leave, we will probably have income of almost 8.5k-9k from a tax perspective.

With the rental income? Let's just exclude that as not sustainable (what would be sustainable after the lease expires in 2027?) and there remains 6k. Then I would still imagine a 600k financing – that is temporarily 50% of the net income. But whatever.
Your wife is a civil servant and thus has no job problem.
It’s only about parental leave. After that, things look better again with 7.2k after childcare and before additional rental income.
600k financing with a 5% annuity is a 2.5k installment and thus 35% of the net income (before rental income). That leaves you 4.7k per month for living expenses, which is still more than ample.
As a bank, I would also give you significantly more credit. As a rule of thumb, 50% (with such incomes) of the net income as an installment is quite acceptable. That would be 800k financing. And honestly? You would still have more net income left than the average family even earns and from which housing costs still have to be paid.

In the end, it’s a question of which risk perspective you want to adopt. Personally, given your wishes as well as the expensive land prices, I would first significantly increase my equity until the end of parental leave (or until the second pregnancy) and only then start building the house to meet all wishes.