Give everyone the same specification
The designer, builder and door installer need a consistent description of the proposed assembly. Record the intended door dimensions, style and operation, then have the relevant professionals confirm the installation requirements. An early sketch is not a final site measurement.
Resolve interfaces before work is hidden
Discuss who is responsible for the surrounding structure, electrical provision and any finishing around the opening. The door installer should identify the requirements of the selected equipment; other trade work should be assigned explicitly rather than discovered at handover.
Set a sensible order of decisions
Confirm the selected product and site readiness before treating a preferred date as a firm installation appointment. Keep changes visible to everyone involved. A late switch in door construction, glazing or opener type may require another review of the plan.
Your questions
Can the garage door be chosen after the opening is finished?
It may be possible, but early coordination reduces the risk of selecting a product that does not suit the available space or project requirements.
Who should confirm the final opening measurements?
Have the responsible professionals agree on the finished dimensions and installation requirements. Do not rely only on a homeowner’s approximate measurement to place the order.