First calculation - realistic?

I think it's good, first of all, that you apparently approach the idea very openly and honestly and are not just looking for confirmation of your crap idea. Thumbs up for that!

Otherwise, one has to realistically say that renting out probably won't help but rather have the opposite effect, aside from the fact that you would then need an (even) larger plot. Building a semi-detached house or possibly a two-family house from scratch is likely to be extremely tight and will definitely exhaust any possible contributions from your relatives. It is probably easier to buy a used semi-detached house (complete) or a two-family house and then renovate/modernize it with [EL]. Even then, things won't be easy, but that seems the most feasible.

Using the parents' equity while letting them stay in their apartment would of course be another option, but it depends on whether that would really be okay for your parents. For me, that wouldn’t necessarily feel right, but that also depends on the parents' attitude.

Okay, I already thought so. I have to apologize, I just did some calculations without much background knowledge. I also looked at some prefabricated house prices and was probably a bit misled. I am aware that the catalog prices do not reflect reality, especially since many things are not included (for example, the foundation slab). Of course, renting makes the whole thing much riskier, but that is only optional anyway. And yes, a large part of the equity comes from the parents, basically as an advance inheritance to realize the whole thing. Would it perhaps be more sensible to refrain from building and look for existing properties?

Hello, sorry, the project including renting is a few leagues above your level. Allegedly, you have an equity balance of €1,000 and then no equity but only installment loans. I would also be cautious with advance inheritance. If care is needed, it can also be reclaimed.

Thank you first of all for the many replies.

Of course, it is still very naive at the moment, as we neither have experience nor can we really rely on people who have done this.
Only a family of friends who have been living on a construction site for years. For example, the staircase to the front door was only recently finished; before that, it was pallet boards. So not really suitable advisors, at most a bad example in this context.

Maybe a little more precisely about the finances. We made some mistakes there and are now paying for them dearly for several more years.
Now the living situation has become very bad (previously a well-kept multi-family house, then a change of ownership, now moving in of very noisy neighbors, for example) and we have to change that. Renting with the kids (and dogs) is very, very difficult. We would have to put at least 300€ more on the table than now. But then possibly experience the same crap as with every one of our previous rental apartments? No, the topic is done.
If the situation were different, we would first repay the loan, possibly save a bit more, and then start.

Be that as it may, I find it very difficult at the beginning to ask the right questions. Building/buying alone without parents is out of the question; we agree on that, they have to be involved.

Does it make sense to already contact a financial advisor at this point and first clarify the budget there?
Or should we first check out various home construction companies and see what is possible with a rough budget?

It definitely makes sense to first roughly assess the possible budget to avoid later finding out that the dream property is simply unattainable. If you like, I can give you the contact of a very good and above all independent financial advisor from your area.

Does it make sense to already contact a financial advisor at this point and first discuss the budget there?
Or should we first check out different house construction companies and see with a rough budget what is possible?

What do you want to hear from the house construction companies? They won’t be able to tell you much more than here in the forum.

If the parents have to come along, then a single-family house with a granny flat is most suitable.

140 m² for you and 70 m² for the parents would then be 210 m² of living space.

That means: 2,200 EUR/m² x 210 m² living space = 462,000 EUR construction costs (without garage)
plus an estimated 15% incidental building costs = 69,000 EUR
plus land in the Cologne/Bonn area = 500 m² x 400 EUR/m² = 200,000 EUR (assuming a friendly price from the municipality/city because of being an employee)
plus property transfer tax NRW 6.5% = 13,000 EUR

...which sums up to in total: 744,000 EUR with a simple finish

Corresponds with a 3.5% annuity (equity contribution 70,000 EUR): 1,965 EUR loan installment

Does it make sense to consult a financial advisor at this point and first discuss the budget there?
Or should we first check out various house-building companies and see what is possible with a rough budget?

Even though it has already been said, once again for confirmation:
Go to an independent financial advisor, disclose your situation to them, and have financing offers calculated. Then you will see how much credit you can take out. Then start your search with that.

The calculation by gives you a rough framework. However, the €2,200/m² in some areas (Baden-Württemberg) are more of the lower standard, and with some fittings, you have to reckon with €2,400 or more. And we are not talking about luxury here. If you do some work yourself, of course below that.

You can also get a repayment subsidy of €18,000 per residential unit through the KfW with a KfW55 (or better) house. So with a granny flat, €36,000 is possible. But the financial advisor will calculate that for you as well.

From my experience with prefabricated house providers, they always somehow adjust the figures to make it fit.