Pay off the home loan or use the money to build another apartment?

Hello. We moved into our new house on 01.06.20. Currently, two invoices are still open, which will amount to about €26,000 - €27,000. After that, we will have about €40,000 available for free disposal.

Regarding the rough data:
Loan of €290,000, effective interest rate 0.81%, term 15 years, monthly rate €1,100. After that time, a remaining balance of about €124,000 would still be left if we do not make any special repayments (which would be possible free of charge up to €10,000 per year). I don’t know the exact repayment rate off the top of my head. I can provide it later if needed. We can repay 10% of the loan amount free of charge. However, I actually would have to check how long that applies. We have already drawn the full amount into our account in order to start full repayment.

How would an early repayment affect the loan and the interest? We could possibly repay €29,000. And next year make a special repayment of €10,000. There will be interest savings and of course a lower remaining debt combined with a lower residual risk after 15 years for the follow-up financing.
How can I best calculate the savings and the remaining debt through the repayment?

I was aware from the beginning that I would struggle with myself if we had leftover money (resulting from good savings performance last year, selling the car (switch to a construction site vehicle), and more personal contribution than expected) to give the money back since the interest rate is already good. In the next years, a wedding is also planned. If I give the money back, and then have to take it out again at 5%, it would be really bad.
I cannot guarantee that I will continue to save diligently with our new house.

On the other hand, I am considering investing the money in a duplex bungalow. For the bungalow, I would most likely want to build it together with a friend or family (each owning one unit). This should lead to savings for both parties if heating, land, roof, etc. can be shared, or am I wrong here? I believe, for example, that the roof does not automatically cost twice as much despite the double size because positive synergy effects arise, right?
How would the land be handled? Can it be purchased jointly? Does one have to be related for that?

Is it not the case that a bungalow should be generally cheaper than a single-family house (with roughly the same living space) due to the lack of a first floor? The stairs, the concrete intermediate ceiling are eliminated, no load-bearing walls necessary (possibly steel supports), the complex construction of gables is also omitted, etc.

I haven’t asked for prices or looked into it in detail yet, but I think a duplex bungalow could be feasible for us for €280,000. That would also be my limit. Beyond that, I no longer consider it profitable. I initially base this on our house and deduct the garage, kitchen, and 30 sqm of living space. For a developed plot of land, about €30,000 are incurred as with the house. That would mean: €140,000 - €40K equity = €100,000. With about 60 sqm per unit, I can generate €390 rental income. The refinancing would take 21.4 years, without interest payments but also without any subsidy from us on the repayment. I find that borderline long, right? The first major renovations will already be necessary by then. Ideally, the duration would be 15 years so that afterwards the rental income could additionally amortize the loan from the house.
Is there a general rule for the construction budget and the rental income? Is it more profitable to build larger apartments despite higher investments to generate more rental income?

The whole test reads totally confusing. Sorry. That's why I am not addressing it. BUT: First, a bungalow is always more expensive per sqm than a "normal" single-family house due to its construction and the land area consumption, and second, investing capital in the new construction of a single-family house is the worst real estate investment you can make in terms of returns.

So you have drawn down a loan of 290k and at the end of the construction phase there is 40k equity. You can repay 10% of the loan amount free of charge.

If the annuity does not change, the repayment portion in the installment increases. You then repay more. Roughly calculated interest savings = 29,000 * 0.81 % * 15 years = €3,523 interest saved (probably more due to compound interest) and thus 4 installments saved at the end.

But if you already know that money will be tighter, wedding is coming up, car etc., I would hold on to the money. Financing is always more expensive.

How about solving it with the property? Can one purchase it as a community? Do you have to be related for that?

I'll pick out this "problem": It is completely irrelevant with which other natural or legal persons you acquire a property together, practically everything is possible.

As far as I understand, you have 40k€ left from the construction financing and want to put it into a semi-detached house. Depending on the bank and equity contribution, you might not get the 40k without submitting invoices. You can't finance either a car or a wedding with it. If you also want to invest in real estate, then finance completely separately with little equity so that the tax office still helps a bit. Presumably, a new bungalow does not bring the best return unless you can rent it to parents/children. A small condominium would be better there. Regarding the remaining debt after 15 years, you can make an Excel spreadsheet with special repayments and play around with it.

As far as I understand, you have €40k left over from the construction financing and want to put it into a semi-detached house. Depending on the bank and your equity contribution, you might not even get the €40k without providing invoices. You can't finance a car or a wedding with them either.

He already has it in the account. Saved during the house construction period through own work or contributions.
Back then, we also had quite a bit left over and used the special repayment right away in the year of the house construction and the following January. (We did not have a right of withdrawal.)
I would only return the money or make the special repayment if you really don’t need the money.
Some buffer for car, house, and other things (or in your case the planned wedding) should definitely be available.

Now putting all of that into one large single property, where you already have a concentration risk in the form of your property, I think is complete nonsense.