Calculation of Property Purchase and Renovation

A house should play the role of a home in the family setting. However, I would rather see a one hundred and twenty-year-old house combined with the renovation concept "changing tires while driving" as a severely disabled third child competing for parental attention. It can be managed, but it costs a lot of energy – not least from the offspring. Smaller children's rooms in a more manageable house bring them more joy. By the way, on the side, I also read >10 years of professional stability as a warning sign to expect imminent changes. Will your calculation still work if one of the commutes is significantly longer in five years?

Please tell us a bit more about the house. Heating, energy consumption, age of the water pipes, windows, etc. Then make a list of all the things due in the next 1-2 decades, sorted by priority and their dependencies on each other.

the back part can still be used as a building plot, but should remain a garden

How about distancing yourself from the house and inquiring about the land separately? If you like the location so much and apparently have a relationship with the seller, couldn't that be an option?

By the way, "expanding the electrical system a bit" will probably not be enough. Old installations lose their existing protection, for example, through expansion (flush-mounted). That means: completely new electrical system (somewhere between 15 and 25K€) or surface-mounted expansion. Missing RCDs can be replaced by expensive single sockets in wet rooms… all doable but not cheap either. The roof MUST be insulated within 2 years after acquiring the property (or the top floor ceiling); whether anyone checks this is another question… if no new construction is planned, I would finance as much as possible because connection/small loans are significantly more expensive…

Year of construction 1900

Wow!
I can't say anything about the renovation costs.
I generally find it sympathetic when someone is not afraid to move in first, not wanting to have everything immediately in their desired design and then gradually implement what can be done later by themselves. But you also have to be the type for that. Not everyone can and wants to do that. In the end, the house becomes a hobby over years/decades and never really finished. With a year of construction 1900 and problems in the basement, I would be cautious. Then: where the electrical system needs to be redone, the water pipes will probably not be new either. Etc.

But some notes:
Keep in mind that
- with the second child the income will probably decrease.
- car costs must also include the new savings rate, since eventually you have to replace the old one.
- purchase incidental costs are not financed, so they must be paid from liquid assets.
- at the age of the heating system, you should definitely keep the budget now so that a replacement is possible. A common mistake is to finance too little and then have no cash left to pay for necessities.

That means you should borrow more to avoid getting into a financing backlog. There would still be some buffer upward if the salaries remain the same.

I would also advise either to negotiate the house price down at least by the demolition costs or,
Plan B, to reserve the free plot.

Thank you for your many replies.

Topic Demolition/New Construction
The buildable part of the property cannot be purchased separately. The building site is not easily accessible; an existing outbuilding would first have to be demolished, and in addition to the development costs, a new building with minimal requirements could certainly not be realized for less than 300k. I think that significantly exceeds our financial means.
We also see demolition as difficult. Expensive and not unproblematic due to boundary construction.
In general, the price/performance ratio is not ideal, a house with many construction sites and, if you want to see it that way, an expensive garden.

Insulating the top floor ceiling: we trust ourselves to do this on our own, provided the roof cannot be addressed in the near future. The new roof has top priority, also with regard to gas consumption. I think it won’t be possible to get below 25,000 kWh annual consumption without a new roof.

We do not expect any professional changes that would have a negative impact. But it is clear that with a second child, income and available money will at least temporarily be less.

We will reconsider our plan to move in first and then save, and afterward tackle all measures step by step instead of financing everything directly. We actually liked the idea of paying thousands of euros less interest to the bank quite a bit.
Possibly the strategy you suggested, to increase the loan and complete some items before moving in, would actually be the better one.

I will address the further points regarding the renovation plan and what exactly needs to be done in more detail later.