Do I need separate approval to demolish the existing house and other structures on the site?

It depends on what is being demolished. Demolition of the existing Class 1a dwelling will generally require Building Approval (BA) before demolition starts. A separate DA may not be required if the single-dwelling demolition meets ACT Planning’s exemption criteria, including that it does not share a party wall, satisfies the general exemption criteria, and the required information is given to adjoining neighbours.

For Class 10 buildings or structures on the site—such as certain garages, sheds, carports or other minor structures—the demolition may be exempt from both DA and BA if the particular structure and demolition satisfy the applicable exemption criteria. For BA, ACT Planning states that demolition is exempt where the building would be BA-exempt if it were built today and all other applicable general rules are met.

Reference: ACT Planning – Check if you need a DA → Demolition - https://www.planning.act.gov.au/applications-and-assessments/development-applications/check-if-you-need-a-da & Check if you need a BA → Demolition - https://www.planning.act.gov.au/applications-and-assessments/building-approvals/check-if-you-need-a-ba