Is a building savings contract with a high outstanding debt sensible as partial security?

The disadvantage of the building savings contract is that although you secure the good interest rate of 1.19% for part of the remaining debt, you "invest" money at 0.01% every year for the next 7 years (to make that part eligible for allocation), which you could invest at significantly better returns. You have to take that into account in the calculation.

Just for understanding: With the building savings contract, I invest around €100k at 0% interest over the next 7 years and pay €3,200 for that. Afterwards, I get €100k at 1.19% interest. I pay this back over 7 years. That results in €4,200 interest.

Total costs approx. €7,400.

If you compare that 1:1 with investing and calculate with 3% interest on fixed deposit: For the next 7 years, I save about €1,200 per month to also reach €100k. With interest, I then have approx. €112k (+€12k). Afterwards, I finance the remaining €88k at, say, 3.5% and pay it back over 7 years. That results in about €11,300 in interest.

Total costs approx. €11,300.

Even with 5% interest on the account balance, I would still be above the €7.2k of the building savings contract. That would only be worthwhile if I invested a larger sum immediately and did not save up, or if interest rates later were around 2.5% or lower?

I do not want to favor the building savings contract with this; I’m just wondering if I miscalculated or missed something?

I just wonder whether I miscalculated or am overlooking something?

Is there still a premium?
Is a land registry entry necessary?

Just for understanding:
With the building savings contract, I invest around €100k over the next 7 years at 0% interest and pay €3,200 for it.
Afterwards, I get €100k at 1.19% interest. I pay this back over 7 years. This generates €4,200 interest.

Total costs approx. €7,400.

If you compare that 1:1 with investing and calculate with 3% interest on fixed deposit:
For the next 7 years I save about €1,200 every month to also reach €100k. With interest, I then have about €112k (+€12k).
Afterwards, I finance the remaining €88k at say 3.5% and pay it back over 7 years. That results in approx. €11,300 interest.

Total costs approx. €11,300

Even with 5% savings interest, I would still be above the €7.2k of the building savings contract.
That would only be worthwhile if I invest a higher sum immediately now and don’t save up, or if interest rates later are around 2.5% or lower?

I don’t want to prefer the building savings contract with this, I’m just wondering if I made a calculation mistake or if I’m overlooking something?

Purely from the numbers, I don’t see any calculation errors; it also always depends on the constellations and interest rates you calculate with what the outcome will be.
But if you look at your situation as of 2030 – you need more than €200k for your residual debt, which is quite a value pulled out of thin air? Then, by 2030, with the investment method you own €112k, and you owe the remaining residual debt minus €112k.
With the building savings method, you own €96,800, and you owe the remaining residual debt minus €96.8k, of which you actually get €100k at the interest rate of 1.19%. Whether that will be good or bad in 2030 no one can say today.

Alternatively, you could put part of your savings contribution into a fund calculator, for example, and play with the return values. That could result in quite different values – however, this is also associated with risks over the short period of 7 years. If the market is down when you have to access your money, that would of course be unfortunate.

Is there still a premium?
Is a land register entry necessary?

No premium. A land register entry for the building society saver is not necessary due to the same bank. Unless I would take out the building society saver at another institution.

But if you consider your situation as of 2030 – you need more than 200k for your remaining debt, that is quite an arbitrary value, right? Then as of 2030 you would own 112k€ with the investment method, and you owe further remaining debt minus 112k.
With the building society saver method you would own 96,800€, and you owe further remaining debt minus 96.8k, of which however you get 100k€ at an interest rate of 1.19%. Whether that will be good or bad as of 2030 no one can tell you today.

Alternatively, you could also put part of your savings contribution, for example, into a fund calculator and play with the return values. Completely different values can come out of that – but given the short period of 7 years, that also involves risks. If the price is bad when you need your money, that would of course be unfortunate.

The values now referred only to the building society saver. In total, that would be 570k€ remaining debt if I would not make any special repayments.
With the building society saver, I would have saved 96,800€, 100k€ at 1.19%, and the remaining 373k€ at the current interest rate.
With the investment option, I would have saved about 112k€ and the remaining 458k€ at the current interest rate. Assuming 3% return.

In recent years, I have also invested some money in ETFs on the side. But that showed me that with the current fluctuations there are periods when the return is rather poor and periods when it is significantly better. For long-term investment, that is okay for me. But if you need your money at a certain time, it can backfire.

I am currently inclined towards diversification:
10k€ per year into a building society saver
7.5k€ per year as special repayment
7.5k€ per year to invest/investment

For me, the calculation error is that you only include interest and closing costs, but omit the €12,000 lower loan amount.

As a result, the second option with the numbers would be about €60 cheaper per month during repayment than the [Bausparer].

I am currently leaning towards diversification: 10k€ per year into a home savings contract 7.5k€ per year for special repayments 7.5k€ per year to save / invest

Diversifying is great, but the special repayment is of course nonsense. Better make it fixed deposit. That will save you roughly 3,000€.