There is an agreement between CEN and the national standards bodies (NSBs) that CEN standardisation work shall be considered before starting national standardisation work. If CEN work is already started, this line shall be followed and national work should not be done. It is called a "Stand still". After the ENs are published, existing and possibly conflicting national standards shall be withdrawn within a certain time frame. If national legislation is referring to these national standards, the NSB can get some years to repair this. A three to five years period is considered as the maximum deviation period in which national standards shall be withdrawn.
Note, however, that in some Member States the building regulations do not always refer to national standards, but e.g. have (part of) the assessment procedures included in the regulations. In the case of the Construction Products Directive, the European Commission issues mandates to CEN to make use of CEN standards mandatory for all Member States. The EPBD allows a national/regional differentiation. See also the question on the status of the EPBD related CEN standards.
Author: Dick van DIJK (TNO)