Can I apply alone without my partner?
Whether you can apply for a mortgage without your partner depends on three factors:
Are you married or not?
What arrangement form do you have?
At which bank will the application be made?
If you are married, most lenders will require your spouse to be included in both the mortgage and purchase contract. Their reason is that your spouse may have a right to stay in the house if you separate, and therefore they want to have a claim on your partner as well. Having a prenuptial agreement according to § 1365 BGB can exempt you from this condition.
Finally, there are exceptions and some lenders may not necessarily require you to both be in the purchase contract.