Townhouse Stormwater Drainage Plans: Strata & Council Checklists

Townhouses: Private Roofs, Shared Drainage

Townhouse developments sit in an awkward middle ground for stormwater. Each dwelling has its own roof and courtyard, but the water almost always ends up in a shared drainage network – common pits, pipes, detention and a single discharge point – that will eventually belong to an owners corporation. Councils review these schemes closely, and the stormwater plan is one of the most heavily conditioned documents in a townhouse DA. Here is what builders and developers need in the package.

The Strata Dimension: Who Owns Which Pipe?

Under a strata subdivision, drainage infrastructure generally splits two ways:

• Within a lot – gutters, downpipes and pipes serving only that dwelling are usually the lot owner’s responsibility.
• Common property – the trunk drainage: collector pits, shared pipes under driveways, the on-site detention system and any pump station. The owners corporation maintains these forever.

Good stormwater design respects that split. Trunk lines should run within common property (typically the driveway), not under private courtyards where future access becomes a fight. Pits need to be located where they can actually be lifted and cleaned. Councils commonly require a positive covenant over the OSD system obliging ongoing maintenance – factor that into the strata documentation from the start.

Common Property Pits and Pipes: Design Essentials

• Sized by calculation – pipe sizes, grades and invert levels designed for the developed catchment, using methods consistent with Australian Rainfall & Runoff 2019 and the council’s DCP criteria.
• Driveway drainage – the central driveway is both a major catchment and usually the overland flow path; grated drains at the low points and at the street boundary stop runoff surging into garages or the street.
• Access and maintenance – every junction and change of direction at a pit, with pit lids accessible after landscaping is complete.
• Overland flow – a documented path for storms exceeding pipe capacity, graded so excess water exits the site without entering dwellings.
• OSD – storage and discharge control designed to the council’s current requirements, with the discharge control pit in common property and protected by the positive covenant.

Roof Drainage Still Has to Stack Up

The shared network only works if each dwelling’s roof drainage feeds it correctly. Townhouse roof design should follow AS/NZS 3500.3:2021: eaves gutters and downpipes sized for each catchment, overflow provisions, and specific design for any box gutters between attached units – the same party-wall cautions that apply to terraces apply here. Where BASIX requires rainwater tanks, each dwelling’s tank, connected roof area and overflow into the site system must appear on the drainage plan, not just the BASIX certificate.

The Council Checklist: What Assessors Look For

Most NSW councils publish engineering or stormwater checklists in their DCPs. While details vary by council – always confirm the current local requirements – townhouse stormwater submissions are typically assessed for:

• A site plan showing all pits, pipes, sizes, grades and invert levels.
• The nominated lawful point of discharge and connection details.
• OSD calculations, storage details and the discharge control structure.
• Overland flow paths and finished surface levels relative to floor levels.
• Driveway longitudinal sections showing drainage at grade changes.
• Easements or covenants for any drainage crossing other land or requiring maintenance.
• Erosion and sediment control during construction.
• Works-as-executed (WAE) survey requirements as a condition of consent – plan for this at construction stage.

Sequencing Tips for Builders

• Engage the stormwater engineer at concept stage – driveway grades and dwelling floor levels are drainage decisions as much as architectural ones.
• Lock in the discharge point and any easements before DA lodgement.
• Build the OSD early in the program and keep it clean; certifiers will inspect it before the final occupation certificate.
• Photograph and survey pipes before backfilling – the WAE survey is much cheaper that way.

Get a Coordinated Townhouse Package

Contrive Consultants prepares council-ready stormwater and roof drainage plans for townhouse and multi-dwelling projects across Sydney and NSW – roof drainage, common property networks, OSD and checklist-compliant documentation. See our services or contact us on +61 497 848 111 or info@contriveconsultants.com.au.