The switch from home-grown British Standards to a panEuropean alternative formally begins when the final draft of a Eurocode is approved by European standards drafting body CEN. This follows the consideration of comments from industry on the first draft, which is published as an ENV code, and which is supposed to be trialled by designers in all EU countries against the local codes.
In its final approved form the new code may be markedly different from the ENV as a result of the feedback received. It is passed to the British Standards Institution, which then has a maximum of two years to prepare the crucial National Annex and publish the Eurocode as a new British Standard.
There then begins the coexistence period, when Eurocodes and earlier British Standards may be used as a means of demonstrating compliance with regulatory requirements - Building Regulations, for example.
But once all the Eurocodes needed to design a particular type of structure, such as a concrete bridge, have been published, the co-existence period will last no more than three years. Then BSI must withdraw the comparable British Standards.