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

Technical Specifications = measurable numbers

Things like setbacks, building height, site coverage, private open space dimensions, etc.

Design Guide = quality of the design

Things like whether the development fits the neighbourhood, provides good privacy, has sensible entrances and landscaping, gives residents good amenity, and responds appropriately to adjoining properties.

So you could have a dual occupancy that ticks all the numerical boxes but still has a design problem.

For example, imagine the Technical Specification allows a particular setback and your proposal meets it exactly. However, the resulting second storey has a poor relationship with the neighbouring property.

The Authority can still look at the Design Guide and the underlying Assessment Outcome. Simply saying “we comply with the setback number” may not resolve the broader design issue.

On the other hand, not following a particular Design Guide suggestion does not automatically mean refusal. Alternative design solutions are possible if you can demonstrate that your proposal still comprehensively achieves the relevant Assessment Outcome. ACT Government guidance specifically recognises alternative approaches.

Reference: ACT Planning , The Territory Plan – https://www.planning.act.gov.au/professionals/our-planning-system/the-territory-plan

For a dual occupancy, the plans should give the planner enough information to understand where the existing services are and whether the new dwellings will interfere with them.

This can include things such as sewer, stormwater, water, electricity and other existing/proposed utility infrastructure, particularly where they affect where you can put the dwellings, driveway, retaining walls or landscaping.

You don’t necessarily need a separate complicated engineering utilities drawing for every DA. The utility information can sometimes be incorporated into the relevant plans. ACT’s DA naming guidance specifically says that a Utilities Services Plan details existing and proposed utility services.

The important practical issue is clearances. For example, if a sewer line or utility easement runs through the block, you can’t simply draw a dwelling over it because the building otherwise meets the planning setbacks. Utility access and protection requirements still need to be dealt with. ACT Planning expressly recognises easements, utility infrastructure access/protection spaces and plumbing/drainage clearances as separate constraints.

So for a dual occupancy DA, I would normally want the site documentation to identify any utility infrastructure that could affect the proposed development, rather than leaving it until building approval.

Reference: Planning (Minimum Development Application Documentation) Guideline 2026 (No 1) (NI2026–13)

Think of site coverage as how much of the block is physically covered by buildings or covered structures when viewed from above.

For a dual occupancy, the following are specifically identified as included:

• Balconies
• Basements
• Roofed structures such as carports, terraces, pergolas, patios, decks and garden sheds
• Cantilevered upper-floor elements

The following are specifically excluded:

• Awnings and eaves
• Unroofed decks, terraces, pergolas and patios
• Swimming pools
• Hard surfaces such as retaining walls and driveways

The lists are not exhaustive.

Site coverage is measured from the external faces of exterior and basement walls. For balconies, terraces, pergolas and similar structures, the furthest extent of the structure is generally used.

Reference: ACT Planning Advisory Note 08 – Working out gross floor area and site coverage – https://www.planning.act.gov.au/__data/assets/pdf_file/0011/2323001/an08-working-out-gross-floor-area-site-coverage.pdf

In practical terms, notification may be sent to:

• neighbours beside your property;
• neighbours behind your property;
• relevant neighbours directly across the street; and
• potentially other nearby neighbours if ACT Planning considers they could be affected by the proposal in a similar way.

The exact neighbours notified therefore depend on the shape and location of the block and the surrounding properties.

Reference: ACT Planning: Development Application Public Notification and Representations – https://www.planning.act.gov.au/__data/assets/pdf_file/0015/2323014/development-application-public-notification-representations.pdf

The Landscape Plan shows how the areas around both dwellings will actually be landscaped, rather than simply showing the buildings.

It should show relevant details such as proposed planting, plant species and quantities, planting sizes and locations, together with other required landscape information. ACT Planning also describes the Landscape Plan as showing trees to be retained, their size/height, protection measures and surface treatments.

So for a dual occupancy, landscaping shouldn’t just be labelled “landscaped area” on the site plan—the DA should include the required Landscape Plan information.

Reference: Minimum DA Documentation Guideline 2026 (NI2026-13)