Building savings - does it make sense? If yes, what amount?

For immediate financing, it probably makes no sense because it will no longer become eligible for allocation. However, you can secure the interest rate for the future in case it should become higher and thus partially protect your remaining debt later. But you must also be aware that you will have to continue paying into this alongside your financing for it to work. Therefore, do not choose the installment too high.

The problem with this option is that you practically earn no interest on the money in the building savings contract during the saving phase. Using the money as a special repayment at 3-4% interest has a significantly bigger effect (namely those 3-4% guaranteed interest savings per year). I have calculated this here as well as Hyponex. Despite low interest rates for the building savings loan, interest rates would have to rise extremely for this to be useful:

The problem with this option is that you practically earn no interest on the money in the building savings contract during the saving phase. Using the money as a special repayment at 3-4% interest makes a lot more sense (namely those 3-4% as guaranteed interest savings per year). I have calculated this here as well as Hyponex. Despite low interest rates for the building savings loan, the interest rates would have to rise extremely for this to be sensible:

Exactly. Meanwhile, I have caught up with my own calculation. Actually, securing the interest rate makes more sense the lower the interest on the annuity is. The more you already pay there, the more special repayments bring instead of investing in the building savings contract.

The problem with this option is that you practically earn no interest on the money in the building savings contract during the savings phase. Using the money as a special repayment with 3-4% interest makes a much bigger difference (namely those 3-4% as guaranteed interest savings per year). I calculated this here and so did Hyponex. Despite low interest rates for the building loan, interest rates would have to rise extremely for this to make sense:

I would also never insure the entire remaining debt. We only took out two building savings contracts in 2010 when we still didn’t know that we wanted to build and we are now incorporating them into the financing. They are also only 2 small ones. One for over 20K and one for over 40K, among other things to take advantage of the employer’s savings allowance. At €26.60 employer contribution, that is €4700 for one contract by 2025 without compound interest effect. That at least significantly exceeds the fees. Of course, one would not have done worse with ETFs, but back then I wasn’t familiar with them and now I still find it quite elegant with the building savings contracts. Depending on the current interest rate, you could also let them continue running or not and use them later for a renovation, a pool, or something else. It’s not always necessarily bad.

Whether you want to secure partial amounts or the remainder, the math for the partial amount comes out the same. Currently, you practically get no interest for the [Bausparer] but pay 3-4% on the main loan that you could otherwise repay with the money. So you are running a negative interest differential business over years, and the necessary market interest rate for you to make that up in the follow-up financing simply has to be too high.

Old contracts are different. In 2010, you probably still got quite decent interest rates, and I also still have a [Bausparer] from 2013 with 3% that I, of course, have not liquidated but simply continue saving. By the follow-up financing in 2031, it will be almost fully funded, and then I will repay part of the remaining debt with the money.

Whether you want to secure partial amounts or the remaining balance, the math for the partial amount comes out the same. Currently, you practically get no interest on the building savings contract but pay 3-4% on the main loan, which you could otherwise repay with the money. So you are running a negative interest differential business over years, and the necessary market interest rate to make that up in the follow-up financing simply has to be too high.

Old contracts are something else. In 2010, you probably still got quite decent interest rates, and I also still have a building savings contract from 2013 with 3%, which I of course have not terminated but continued to save into. By the follow-up financing in 2031, it will be almost full, then I will repay part of the remaining debt with the money.

Well, good interest rates were not really available then either. Only 1%.

The math is the same, that's clear, but with smaller amounts, the share of the fee for the capital-forming benefits (VWL) is smaller and thus somewhat more profitable. Also, it does not take as long until it matures for allocation, and you only have to save small sums. In our case, €100 in addition to the VWL.

1% doesn't sound like much but it's already 10 times the usual rate today and reduces the interest difference by at least one third. That makes a big difference in the long run.
Also, not everyone gets [VWL] ;)

edit:
To not drag the discussion out... we're talking about two completely different scenarios here. You have 1% interest and also [VWL] which you otherwise assume as 0 (probably not quite right, but whatever). That makes it much more likely that your building savings contract underbids the market interest rate.
But the OP is asking if they should sign a building savings contract now. And since there's nothing about [VWL], we're not including that in the calculations.