When is the right time to hire an interior design agency?

Hiring an interior design agency should be done as below:

Private villas: once your structural and architecture plans are done and approved by municipality.
Shop in a Mall: once you get the lease agreement from the mall management they will provide you with LODs which will be the fully detailed set for the designer.
Standalone Shops: once the rent agreement is done, the landlord can provide you with a set of drawings to proceed with the design.

there might be other options for hiring an interior design agency but it is linked to the project’s requirement.