Buying a house - Are we creditworthy? Building savings contract?

At the moment, it is better than any savings account. I would continue to invest in it. You do not need to generate loans from it.

As a building society savings contract, it is currently not sensible, that's true. But it offers 1.5% interest on credit balance, which is great at the moment. I would continue to save into it. You don't have to take the loan when it is ready for allocation. How high is the building savings amount again, how high is the regular savings performance and how much minimum capital is required for allocation? Maybe a split would be a solution if you want to get it allocated faster.

Who advised you to "renew" it? And what is meant by that? Terminate and take out a new one? Taking out a new one can possibly make sense because it secures the current interest rates for you, but the setup fee is also quite significant. And the promotion ties up capital in case you build/buy faster than the new contract becomes allocation-ready.

Let me guess, the bank advised you to renew it :-)
They have been trying for some time because the credit interest rates are significantly higher than the current investment interest rates. Even though 1.5% for a contract concluded in 2009 is very low (we concluded ours in 2012 at 4%), it is worthwhile since overnight money, etc., currently yields less than 0.5% interest. Save the maximum amount so that you can get it ready for allocation quickly. Either then fully save it or have it paid out. The loan interest rate will probably be too high compared to current annuity loans, so an allocation of the loan makes no sense.

Whether it makes sense to take out another [Bausparvertrag] for your "new" savings is hard to say. If the money otherwise "rots" in an overnight account, it can certainly be considered. If you can invest it a bit more profitably, I would rather go that way because it is more flexible. A [Bausparvertrag] currently brings you no credit interest (the 0.1-0.2% interest is eaten up by the fees), but you can still secure favorable loan interest rates. So if you can wait until the allocation, that may be a good option to protect yourself against interest rate increases. If they are still low then, you won't have earned any interest during the saving period, but you are free to choose. If interest rates have risen, you can be happy about the [Bausparvertrag]. You should actually know the basic model of a [Bausparvertrag] if you have trained in the industry.

PS: Even in the smallest hick town you won't get a semi-detached house for €250,000 that was built after 2000. That is more the price for one half of a semi-detached house. Moreover, it is very, very rare that a complete semi-detached house is for sale. A two-family house would probably be the "cheaper" alternative for you, but even there I fear it will be difficult to stay under €300,000, which leads to a repayment of at least €1000 per month. Also, €400 ancillary costs for an existing property are very tight. We are currently already at €250 with property tax, water, and residential building insurance. That does not yet cover heating, electricity, garbage bin, etc.