Is on-site stormwater detention required?
If you're converting a normal residential block into a dual occupancy, you do not automatically need an OSD tank.
The important question under the current Residential Zones Technical Specifications is the size of the site.
For example:
- 800 m² block + dual occupancy → the >2,000 m² OSD rule does not apply.
- 1,200 m² block + dual occupancy → the >2,000 m² OSD rule does not apply.
- 2,500 m² block + development that changes the stormwater runoff → the stormwater detention requirements apply.
This is important because the older WSUD material contained a 100 m² increase in impervious-area trigger. I would not use that old 100 m² trigger as the current Territory Plan requirement for your 2026 dual-occupancy assessment.
You still need to properly design the property's stormwater drainage — roof water, gutters, downpipes, connection/discharge points, etc. Not requiring OSD does not mean there are no stormwater requirements.
Reference: Planning (Residential Zones) Technical Specifications 2026 (NI2026–303), Assessment Outcome 24.5 Stormwater Retension and Detention – Planning (Residential Zones) Technical Specifications 2026 | PDF
Short Answer
No — on-site stormwater detention (OSD) is not automatically required merely because the development is a dual occupancy.
Under the current Planning (Residential Zones) Technical Specifications 2026 (NI2026-303), the specific stormwater retention and detention requirement is triggered for development on a site greater than 2,000 m² (other than major roads) where the works have the potential to alter the site's stormwater regime.
Therefore, for a typical dual-occupancy block under 2,000 m², that particular OSD requirement is not triggered simply because you are adding a second dwelling.
Detailed Explanation
The current applicable instrument is the Planning (Residential Zones) Technical Specifications 2026 (NI2026-303), made under s 51 of the Planning Act 2023 and effective from 1 July 2026. The ACT Legislation Register confirms NI2026-303 replaced the previous NI2025-493.
The relevant Stormwater Retention and Detention provision applies to development on sites greater than 2,000 m² (other than major roads) involving works with the potential to alter the site's stormwater regime.
Where triggered, the detention component requires:
- runoff to be captured and directed from the site;
- 1 kL detention storage per 100 m² of impervious area;
- detained stormwater to be released over 6 hours; and
- 50% of rainwater-tank volume may be counted toward detention where the stormwater is used on site.
Those requirements were already present in the immediately preceding Residential Zones Technical Specifications. The ACT Government also now publishes the ACT Practice Guidelines for Water Sensitive Urban Design 2026, confirming that the WSUD guidance itself has been updated for the 2026 planning framework.
Conclusion for dual occupancy:
Dual occupancy itself does not trigger OSD. For the current Residential Zones Technical Specifications, the key OSD trigger is a site greater than 2,000 m² with works capable of altering the stormwater regime.
So if your dual-occupancy site is, for example, 700–1,000 m², I would not show an OSD tank merely because the proposal adds more than 100 m² of roof/driveway area. Normal stormwater drainage requirements still need to be satisfied.
No minimum block size does not remove the need for planning assessment.
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