What is needed to merge an upstairs and downstairs apartment?

Questioner

We bought an upstairs apartment and a downstairs apartment 5 years ago. The then owner lived in the downstairs apartment for another 5 years and has since left. When we bought the apartments, they were notarially designated as 1 apartment and we notarially split them for financing purposes. Now that the tenant has left, we would like to include the downstairs apartment with the upstairs apartment and thus make it, as it were, 1 residential home. Is this possible? Does the apartment have to be notarially split again? Or what exactly is needed to achieve this. By the way, the apartment has 2 entrances and 2 house numbers. Thank you in advance for your answer.

Lawyer

This action may be a prohibited withdrawal of housing under the Housing Act. Please check with your municipality.