Financing plan - What is realistic?

Good morning,
we would preferably like to build something new and would like to know what the maximum amount would be that we could/should borrow as a loan.

General information about you:

[*]We are 29 and 31 years old.
[*]Currently no children.
[*]However, children are planned in 3-4 years (1 child) / wife would go back to work after 1 year, probably 30 hours per week
[*]We are employees and both work full-time 38-39 hours.

Income and asset situation:

[*]We currently earn 2300 and 2350 € net / + 1x 13th month salary + approx. 3000 € net annual bonus
[*]We currently own 1 terraced house (worth approx. 250,000), which we would offer as collateral to the bank for the purpose of land registry entry, expected rental income later min. 900-1000 €
[*]We could currently put in approx. 50,000 - 70,000 € (max) in equity.

Mobility costs:

2 cars, no car loans or other loans outstanding. One car will be sold soon because there is no need for 2 cars.

We would like to build a single-family house with approx. 140-170 m². For this we need a plot of land with approx. 400-550 m², I think. I think the house would cost us approx. 250,000 € + additional costs approx. 50,000 €. The land prices vary greatly here in Cologne and the Cologne surrounding area between 150,000 € and 400,000 €, so the question is rather what we can realistically afford.
I think no matter how we look at it, we will not get under 500,000 €, I think. The loan would run for at least 25-30 years, I think.

Thank you very much in advance for your answers.

Excellent starting position. I will put it differently, what would you like to afford? It should be possible up to a 700k loan.

Please just imagine building a house with 170 sqm in the western part of Cologne for 250K. Even 140 sqm will be more than tight.

Excellent starting position. I'll put it differently, what do you want to afford? A loan up to 700k should be possible

I find that a bit too much loan, with decreasing income (due to children) and rent not fully taken into account.

... and rent that can’t be fully accounted for.

What does "can’t be fully accounted for" mean? You can’t really account for anything from the rent. At most (if things go well) the five percent return that the four-rental property generates. But the €50 per month doesn’t make much of a difference.

Of course, the house helps with the bank, to get good interest rates and a high amount. And if things get tight, you can always sell.

So: add 20% to the house price, another 15% as a buffer, and go to the bank with your request. I think that’s doable.

What does "not fully countable" mean? You can't count anything from the rent at all. At best (if it goes well) the five percent return that the subletting generates. But the 50,- € per month doesn't make much of a difference.

Rental income is completely normal income. You just have to offset corresponding expenses and risks against it. Interest and repayment seem to no longer exist, then there would still be maintenance, maintenance reserves, non-allocable costs, and the default risk, which must be priced in at X%. Depreciation is indeed an expense, but not an outflow of cash.
If the rent were not taken into account, you would not be able to finance a yield property.