Purchase, construction, and full renovation of a single-family house affordable?

How much remaining debt do you have here?
Also cross out the €25,000 own contribution. That's not money you have and can spend in the sense of reducing the loan amount.

After deducting kitchen, furniture and credit, you actually have €0 equity, even negative, since you want to pay your mother (illegally) €28,000 on the side.

But with €6k net everything is relatively irrelevant, you won’t overextend yourselves, even if the renovation costs double ;)

Remaining debt €35,000, car and Z

I want to calculate again very differently.
Consciously very conservative.

So still €128,000.

How much living space might that be on two floors, 60? 60m²*€3000/m² = €180,000

New roof, facade insulation (mandatory due to extension?) and the whole rest... I would say 100m² * €2000/m² = €200,000

Makes a total of €508,000.
Equity goes for kitchen, furnishing and buffer.

Loan amount therefore rounded €500,000. A bit more than your estimate:

So currently some friends of mine are building new, and they are just at the price you are estimating.

I’m not saying it has to be wrong, but in my opinion a full renovation and a simple extension should be significantly less!

If you let everything be done, a renovation is actually more expensive than a new build, because demolition and disposal costs are added. Your advantage is the low purchase price. Still, I believe it won’t be quite that expensive, because the house looks to me like it’s from northern Germany, doesn’t it? But I don’t see it as cheap as you think either.

In southern Bavaria, I find the 500,000€ without personal contribution quite realistic, but in your region it can certainly be cheaper. Friends of ours have paid more for an extensive renovation than we did for our new build, so this "significantly cheaper" is not always true. Overall, you can also afford a 500,000€ loan. We have that in a very similar situation as well.

It's quite simple. Our salary looked completely different six months ago! I've held my position for 6 months now, my wife has been permanently employed for 9 months!

We have saved most of the €30,000 in recent months. Also, the opportunity with the house only arose at short notice (grandpa passed away).

That then explains the equity, your spending behavior also matches the information. As I said, with that salary it's doable. If it should get more expensive, it won't break your neck either.

Friends of ours paid more in total for an extensive renovation than we did for our new build, so this "significantly cheaper" is not always true.

I have personally done an extensive renovation with new dormers (but without an extension) and can only confirm that renovating is not as cheap as hoped at the beginning of the thread. And also consider that a renovation does not even achieve new build standards. I could not accommodate a residential ventilation system. I also decided against underfloor heating because my screed was only 4 cm thick, and in room corners it was even down to 2 cm. If the layering had been made much higher there, the stair steps wouldn't fit at all anymore. Door heights also become problematic. How expensive it will get here is still completely open. I haven’t followed the floor plans in the parallel thread very closely. In any case, a first step is to get an architect for the building application. They will also be able to provide a proper cost estimate. If plots of land are free and available locally, I would advise considering a new build in parallel.