I did the math again briefly: 1.35% on €100,000 is €112.50 interest per month. That leaves €152.50 to save into the [Bausparvertrag]. After 15 years, that amounts to €27,450, without interest. This might be just enough for 30% [Bausparguthaben] by the time of allocation maturity. So it would be a possible model. Please take a close look at what is being offered to you. With a [Bauspardarlehen] of €70,000, the monthly burden could easily double, just for this component.
And the [BHW] is a [Bausparkasse], so the model is quite likely.