Financing of construction projects

The last question only you can answer yourself. If there is still room to live, yes. If not... then better not.

so offer from the bank is here.

15 years, 1.56%, 2.5% initial repayment, special repayments annually up to €12,000 possible. Rate €812

After 15 years remaining debt of approx. €149,000 (without special repayment)

Building society savings plan 2.35%, €100 monthly, savings amount €53,000. Option to increase to €166 and then savings amount of €90,000

What do you think? Is this feasible with our salary?

I also come to around €139k remaining amount.

I still don’t understand the purpose of this building society savings plan in the offer, unless the Sparkasse has a problem with the loan-to-value ratio.
With the building society savings plan, you are merely buying interest rate security for €36k out of €121k (139k less building society savings 15x12x100€). That’s neither fish nor fowl. The security costs you at least €2000. If the follow-up interest rate does not rise above 3%, you will definitely do badly.
Depending on special repayments and rate after 15 years, the actual break-even should settle at a follow-up interest rate around 4%.

How high is the expected total rate after the 15 years?

Why not put all the money directly into the loan, without the building savings contract nonsense...

Also, I calculated it. With 2.5% repayment, you would have less than 139K€ debt left (don’t know how you get 149k€, either typo or forgot compound interest) and if you also repay the 166€ directly (total then 3.33% repayment) then after 15 years you would only have about 105K€ left without a single € of special repayment. And you also save nearly 4k€ interest + closing fee from the building savings contract

We have 1 year until 30.06. interest-free period of the loan. And pay. Repayment only from 01.07.

We have 1 year until 30.06. interest-only period of the loan. And pay. Repayment only from 01.07.

It's about 145,000 ....