Hierarchical validation rule

I have combobox field named "Building type" with options:

• Permanent
• Temporary
• Unknown

I have another field named "Notes". Field "Notes" is avalable for any value of field "Building type" but required only for "Unknown" value. Because if another building type is selected additional fields are opened by Webform conditional rules. If building typeis "Unknown" user should fill only "Notes" field wich is optional for any other building type.

I think it could be new option for validation rule wich is called "Parent validation rule". If any validation rule has one or more children validation rules it shouldn't rase error but (if parent rule is "true") should validate all of the children validation rules recursively.

P.S: I think this feature related to the core code of the module.