It is currently not possible to make an entities multiple-attribute index unique.
You can enforce uniqueness in other ways, by adding a BCo_Entity_PreventDuplicates to the entity combined with OCh_Entity_Attribute added everywhere you can change one of the attributes that are part of the index but that is cumbersome and errorprone.
Another option: If the index has only one attribute you can create a validation rule with the boolean 'unique' set to true. But that is not possible when the index has multiple attributes.
Therefor: Allow validation rules to apply to multiple attributes and/or give entity's index a checkbox 'unique'.
An alternative is to add an extra attribute to the entity and filling this BCo_Indexattribute with the concatenated values of Postalcode, Housenumber and HousenumberExtension.
Disadvantage of this method is the creation of redundant data.