Regulation 4 of the Building Regulations 2000 states that "building work" should comply with the applicable requirements contained in Schedule 1. Regulation 4(2)a then goes on to state that, "after the work is completed, the building as a whole should comply with the applicable requirements of Schedule 1 or, where the building did not previously comply with any such requirement, is no more unsatisfactory in relation to that requirement than before the work was carried out."
Where an existing shop is extended such that the final floor area is greater than 2000m2 (whether it exceeded this value previously or not) then the building as a whole may be less satisfactory in relation to requirement B3(3) than before the work was carried out.
Therefore, the building would have to be either subdivided to limit the compartment size, fitted with sprinklers or some other solution would be necessary in order to satisfy regulation 4(2) in relation to requirement B3.
Regulation 4(2) must be judged against the requirements set out in Schedule 1 rather than the Approved Document. B3(3) requires sub-division of the building "to an extent appropriate" to its size and intended use and it may be that some buildings will still comply with B3(3) by virtue of its intended use even though they have been extended without further capitalisation.