You always need Building Regulations approval. You also need approval under Planning legislation (unless the design is within the limits of Permitted Development).
The Building Regulations are completely separate from planning legislation. So the processes for compliance are separate too.
A capable designer knows that some decisions at the design-stage for the planning application affect the building regulations approval design-stage later.
So that capable designer considers and resolves certain technical matters before the planning application. This insight and practice avoids the risk of an ineffective planning approval. (Which you might otherwise have to pay to amend, or replace, with a second application). It also avoids unexpected costs.
