Do I need to design strictly within the Technical Specifications?

Designing to the Technical Specifications is generally the clearest pathway to demonstrating compliance, but they are not all automatically “hard rules.”

For example, if a Technical Specification provides a particular design benchmark and your proposal does not meet that benchmark exactly, this does not necessarily mean the DA must be refused. An alternative design may be proposed if you can adequately demonstrate how it still achieves the relevant Territory Plan Assessment Outcome. ACT Planning expressly describes Technical Specifications as providing guidance and possible approaches or benchmarks

The important distinction is:

Assessment Requirement → mandatory and must be met.

Assessment Outcome → the proposal must demonstrate that the outcome is achieved.

Technical Specification → provides a benchmark/possible solution for demonstrating how an Assessment Outcome can be achieved; an alternative approach may be possible where the relevant Territory Plan provisions are still met.

Reference: ACT Planning Technical Specification - https://www.planning.act.gov.au/professionals/our-planning-system/the-territory-plan/technical-specifications