Is financing a new single-family house feasible?

btw no more KfW 55 funding. Save, have your child and start searching for existing properties with more equity.

I think the OP applied for and was granted the subsidy early.

But with or without KfW... financially, it would be way too tight for me. Your incidental construction costs already include kitchen, furniture, lamps, etc. That 35,000 will be gone faster than you can blink. And the general contractor will also hold out their hand for one thing or another. Although the house price as a shell construction seems quite high to me. To what extent are you doing the finishing work yourselves?

10000 for the outdoor facilities seems sufficiently adequate to me, if you are willing to do it yourselves (and that is fun after all).

If 10k is supposed to be enough for the terrace, possibly terrace roofing, decking + fence, driveway and access path as well as garden and irrigation, you probably have to buy the materials used on eBay Classifieds.
Just a fence alone would start at a minimum of 5k (material), then half is gone.

My question would be whether this is generally feasible

Generally definitely feasible, however…

Rate > at least €1000 higher than the current cold rent, additional costs definitely higher than the current €100, possibly additional childcare costs for the child and if one plans to have another child soon (and it actually works out), then less income again.

The mentioned balance of currently about €2100 will probably approach €0 in the next few years until both work full-time again.

It can be done, but you have to think it through carefully.

You can always buy a used kitchen first if necessary, just like all furniture, lamps, etc., if in the end it’s not enough.

Saving is okay, but who wants to put or hang used "junk" in their new home...
And just doing the outdoor facilities like that is also a thing... I always see it on Saturdays when I walk to the bakery through the new development area where the builder was active on his own.
(Note: Exceptions prove the rule)

I see it the same way. Due to the sharply rising interest rates, your net income is unfortunately too low for a loan of €400,000. Currently, I see an interest rate of just under 2.6% for a 15-year fixed interest period with a loan-to-value ratio of 100%. This would be a monthly rate of €1,686.

Most banks assume costs of €2,350 per month for 2 adults plus 1 child. In addition, the warm rent is calculated. Let’s assume €700 here, then your disposable income for the loan installment with a net income of €4,000 would be €950.

The €2,350 with a child roughly matches our expenses. We spend just under €2,400 per month on everything except fixed costs like rent, additional rent costs, etc. That can really add up with a child.

Regarding our situation: We are currently looking for a new build with a financing amount of just under €600,000 and have already wavered due to the high interest rates.

What always strikes me in the forum with similar questions is the, in my opinion, far too optimistic view of "child costs". When I look at our own situation, having children is financially a significant setback. Fortunately, we can afford it and still love them anyway :)

We have two boys, just 6 and almost 4.

Monthly costs for two children
- Kindergarten for both -300 EUR
- Clothing, toys, doctor, miscellaneous -400 EUR
- Babysitter -50 EUR
- Additional costs energy, nutrition, etc. -200 EUR
Total = -950 EUR

Child benefit 438 EUR

This already results in a shortfall of around 500 EUR per month for us, it will probably be much higher (additional costs vacations, insurance, replacement of furnishings, renovations, clubs...)

In addition, my wife can no longer work more than 14 hours per week with two children. And that is already very exhausting for everyone (child sick, vacation). My wife will not go back to full-time work as long as the children are at home. Children do become more independent, but the household also requires attention, the children need to be driven around, etc. And when the children were very small, of course my wife did not work. And the parental allowance for the second child was also much lower because there was no full-time employment before.

Therefore my tip:
- If children are planned, I would permanently calculate the salary of the parent staying at home at a maximum of 50% (if previously full-time). And for the next 10 years. Don’t be too optimistic about returning to the workplace, children are not robots, constant unpredictable things happen.
- And per child I would calculate additional costs of 250 EUR.

Applied to the specific case, my very simple calculation:
Salary 2,300 + part-time salary 1,500 EUR + child benefit 220 + pro-rated bonus 200 = 4,220
- Child costs 250 EUR
Income approx. 4,000 EUR
- Mortgage payment for 500,000 loan (40 years, full repayment, 2.5%) = 1,650 EUR
= 2,350 EUR.

That is the amount left for living expenses. With increasing inflation, the rate will become smaller in relation to the salary. But the remaining living costs will also be higher.

Is that enough? For food, car, vacation, renovation, gifts...?