At what stage should I speak to a building certifier?

You should speak to a building certifier before applying for Building Approval (BA) and before construction starts.

For a dual occupancy that requires BA, you must appoint a licensed building surveyor as the building certifier. The certifier reviews the building plans for compliance, decides the Building Approval application and carries out required inspections during construction.

It can also be helpful to involve a certifier earlier in the design process, particularly where there are complex building-code or compliance issues, so these can be identified before the plans are finalised.

Reference: ACT Building Approvals - https://www.planning.act.gov.au/applications-and-assessments/building-approvals