Preparation of monthly additional costs. Can I find something like this somewhere?

They write something about annual reserves, for me that would be 12€/m2 per year...

Of course, 1€/m² monthly or 12€/m² yearly.

But then calculate it over 30 years
12€/m² x 30 years x Xm²

For me, with 125m², that is 45,000€ in 30 years. (without interest and inflation)

And then think about what might be due within the next 30 years. In 30 years, heating, bathrooms, maybe also windows or roof, etc. might need to be replaced. If the painter is also covered, then they might easily go through the house 3-5 times.
In my opinion, 45,000€ is far too little; you should easily have reserves of 2.00-2.50€/m².

How do you do it in practice? Do you have a separate (savings) account for the house reserve and a standing order? Or do you "virtually" create reserves that are recorded in accounting and then remain as a base amount in a general account? Or fixed deposits? Or something else?

So far, we do not create explicit reserves for the house. Nor for the children's education, and not for a possibly needed new car. But we regularly set money aside (savings). So far, only private retirement provision and, of course, the house annuity (now in the second year) run really systematically.

Ouch, out of sight, out of mind. So, a daily allowance or something similar.

However, money for insurance, vacation, and other reserves also goes there. For that, there's an Excel list.

This way, it's easier to keep track of the salary account.

How do you do this in practice? Do you have a separate (daily allowance) account for the house reserve and a standing order? Or do you "virtually" build reserves that are recorded in an accounting system and then remain as a base amount in a general account? Or fixed deposits? Or something else?

So far, we have not formed explicit reserves for the house. Nor for the education of the children, nor for a possibly necessary new car. But we regularly set money aside (daily allowance). Really systematic so far are only the private retirement provision and of course the house annuity (now in its second year).

If you carry this through consistently, it’s okay.
For my part, I explicitly will not form reserves for repairs in the first 10 years, why should I!?

I have an amount X of special repayments in mind that I will regularly pay in the next years. The rest will be saved and kept on call in the checking account.

Nor for the education of the children, nor for a possibly necessary new car.

But here you should reconsider whether this is wise.
- Child/children will want a scooter or a driver’s license.
- Then perhaps a car to get to the place of education or university.
- There are training courses that cost several thousand euros in fees per year.

If we didn’t have grandparents who have been paying into a savings account for exactly these things monthly since the children’s birth, we would have done it ourselves like this.

I do not specifically set aside an amount X each month, but simply put surplus capital into the checking account or ETFs, so that there is always only a base amount of 1000-2000€ in the account.

For this, I have an Excel spreadsheet and virtually allocate reserves for the car, house, and vacation.
The total balance from accounts, daily allowance, ETF, and building savings credit should then at least correspond to these reserves. What I do not do is additionally set aside 3 months' salary.
Since we currently make a 5% special repayment every year, we could also skip one year if necessary.

The child receives a small amount monthly into their account so that they will have a small capital stock later. In terms of overall household wealth, however, special repayments are prioritized.
Hopefully, the loan will be gone before the child is 18, so that enough remains monthly to finance studies or similar.
It also has the advantage that you can at least have some influence on what the saved money is used for.