Stormwater and Drainage Site Plan Requirements NZ

A stormwater and drainage site plan in New Zealand shows how rainwater will be collected, conveyed, treated and discharged from a property. It helps the council understand the relationship between the proposed building work, roof areas, paved surfaces, ground levels, downpipes, private drains, soakage systems, public connections, overland flow paths and neighbouring land.

Stormwater planning is important because a new roof, driveway, extension, garage, granny flat or paved area can change how water moves across a site. A design that looks straightforward in dry weather can cause ponding, erosion, foundation moisture or flooding during a heavy rainfall event if the drainage route has not been considered properly.

What Is Stormwater?

Stormwater is rainwater that falls on roofs, driveways, patios, paths, parking areas, gardens and other parts of a property.

On an undeveloped or well-landscaped site, part of the rainfall can soak into the ground. Once roofs, concrete and other hard surfaces are added, more water can run across the surface or enter the drainage system quickly.

A residential stormwater system may use:

• Roof gutters and downpipes
• Private underground stormwater drains
• Grated channel drains
• Catchpits and sumps
• Rainwater tanks
• Detention or retention tanks
• Soak pits and infiltration trenches
• Rain gardens and vegetated swales
• Public stormwater connections
• Approved discharge points
• Safe overland flow routes

The correct system depends on the property, soil, rainfall, network capacity, ground levels, natural hazards and local council requirements.

Why Stormwater Must Be Shown on a Site Plan

New building work can alter both the amount and direction of runoff.

For example, a house extension can:

• Add a new roof catchment
• Remove an area where water previously soaked into the ground
• Move a downpipe
• Block an existing surface-flow route
• Require an existing drain to carry more water
• Redirect water toward another property
• Reduce access to a public or private drain

A drainage site plan helps the council and project team confirm:

• Where rainfall is collected
• Where each drainage pipe runs
• Whether the existing system can be reused
• Whether additional detention or soakage is proposed
• How surface water will move during an extreme event
• Whether the proposal affects easements or public infrastructure
• Whether runoff could damage the building or neighbouring land

Building Code Clause E1 Surface Water

New Zealand Building Code Clause E1 addresses surface water.

Its objective is to:

• Safeguard people from injury or illness caused by surface water
• Protect other property from surface-water damage
• Protect drainage-system outfalls

Buildings and sitework must be constructed to protect people and other property from the adverse effects of surface water.

The performance requirements address matters such as:

• Surface water entering buildings
• Surface water overflowing onto other property
• Drainage-system capacity
• Blockage and leakage
• Safe disposal to an appropriate outfall
• Gravity drainage where practicable

E1/AS1 provides one Acceptable Solution for demonstrating compliance with Clause E1. Alternative designs can be used, but they need sufficient evidence to show Building Code compliance.^[1][2]

Does Stormwater Work Require Building Consent?

Stormwater work is often included in the building consent for a new dwelling, extension, garage, minor dwelling or other project.

A building consent may be needed when the work involves:

• A new building with roofwater drainage
• A substantial new private drainage system
• A new connection to an existing drain
• Changes affecting Building Code compliance
• A large alteration to an existing private drain
• Drainage associated with foundations, retaining walls or siteworks
• Work outside the plumbing and drainage exemptions in Schedule 1

Some minor drainage alterations can be exempt when completed by an authorised person and all exemption conditions are satisfied. Exempt work must still comply with the Building Code and other applicable requirements.^[3]

The drainage scope should be confirmed before work begins. A builder, homeowner or landscaper should not install significant drainage work merely because the associated structure appears to be consent-exempt.

Can Resource Consent Also Be Required?

Yes. Building consent and resource consent are separate.

Resource consent may be required when the project involves:

• A stormwater discharge not permitted by the regional plan
• Earthworks exceeding a permitted threshold
• Work near a stream, wetland or coastal area
• Modification of an overland flow path
• Filling or building within a flood-hazard area
• Discharge of sediment or contaminants
• Stormwater infrastructure inside a protected natural area
• Changes that do not comply with district-plan impervious-area rules
• A soakage or low-impact system requiring substantial earthworks

Regional councils generally regulate discharges to land and water, while territorial authorities commonly regulate land use, building work and local stormwater networks.

A project can therefore require approval from more than one council department or authority.

Read Building Consent Site Plan vs Resource Consent Site Plan in New Zealand for a fuller comparison.

What Should a Stormwater and Drainage Site Plan Show?

1. Legal Property Boundaries

Show the complete legal property outline.

Label:

• The front boundary
• The rear boundary
• Both side boundaries
• The length of each boundary
• The adjoining road
• Shared accessways
• Drainage or utility easements

Legal boundaries matter because stormwater must not be redirected onto adjoining land without a lawful and appropriately designed discharge arrangement.

A fence, hedge, retaining wall or driveway edge does not automatically identify the legal boundary. Use reliable survey, title or deposited-plan information.

2. Existing and Proposed Buildings

Show all existing and proposed buildings affecting the drainage layout.

These may include:

• The existing house
• A proposed extension
• Garage or carport
• Granny flat or minor dwelling
• Sleepout
• Shed
• Covered deck or verandah
• Pergola or patio cover

Use separate hatching or linework to distinguish existing and proposed construction.

The proposed roof outline should match the architectural roof plan. A drainage drawing showing a different footprint or roof shape can produce incorrect catchment calculations and downpipe locations.

3. Roof Catchment Areas

The roof plan or drainage site plan should identify the areas contributing water to each gutter and downpipe.

Show:

• The complete roof outline
• Roof ridges, valleys and falls
• Gutter locations
• Downpipe positions
• Canopies and verandahs
• Existing and proposed roof areas
• Which downpipe serves each roof catchment

Downpipe and drain sizing depends partly on the catchment area and rainfall intensity. Decorative placement alone is not a sound basis for locating downpipes.

4. Gutters and Downpipes

Mark every downpipe clearly and use a consistent symbol or abbreviation.

The plan may identify:

• Downpipe diameter
• Gutter type
• Connection to the underground drain
• Discharge to a tank
• Overflow route
• Existing downpipes being retained
• Downpipes being relocated or removed

The design should ensure that overflows do not enter the building or create a hazard on access paths.

5. Private Stormwater Pipes

Show the route of all proposed and relevant existing private stormwater pipes.

Where available or required, label:

• Pipe diameter
• Pipe material
• Pipe gradient
• Flow direction
• Invert levels
• Inspection or cleaning points
• Junctions
• Connections to catchpits or tanks

Pipe routes should avoid:

• Building foundations
• Footings and piles
• Large tree roots
• Other utilities
• Areas inaccessible for maintenance
• Locations where insufficient fall is available

A schematic pipe line without levels may be adequate for early planning, but detailed consent or construction documents may require invert levels and gradients.

6. Public Stormwater Connection

Where the property connects to a public network, show:

• The known public main
• The private connection point
• The connection location at the boundary
• Relevant manholes or catchpits
• Pipe levels where available
• The private drain route
• Any required network approval

Council mapping can help identify public infrastructure, but mapped positions may be approximate. CCTV inspection, service locating, test pits or survey information may be required before final design or construction.

Do not assume that the existence of a nearby public pipe guarantees:

• A legal connection
• Available capacity
• A suitable connection level
• Permission to cross neighbouring land
• Permission to build over the pipe

7. Catchpits, Sumps and Grated Drains

Surface drains collect runoff from paved or low areas before it enters the underground system.

The site plan may show:

• Driveway channel drains
• Patio drains
• Yard sumps
• Catchpits
• Doorway or garage-threshold drains
• Connections to the private stormwater pipe

A grated drain should not be treated as a magic line that removes unlimited water. Its size, outlet capacity, maintenance needs and overflow consequences must be considered.

8. Driveway and Hardstand Drainage

Driveways, parking areas and other hard surfaces can generate substantial runoff.

Show:

• The driveway outline
• Surface material
• Driveway gradients
• Crossfall
• High and low points
• Channel or grated drains
• Discharge direction
• The vehicle crossing
• Any flow toward the road

A driveway sloping down toward a garage requires particular care. The design may need a grated drain, bund, suitable garage-floor level and safe overflow path.

Runoff should not be directed across a public footpath or onto a neighbouring driveway without approval.

9. Patios, Paths and Other Impervious Areas

The plan should identify existing and proposed hard surfaces, including:

• Concrete patios
• Paver areas
• Paths
• Parking spaces
• Pool surrounds
• Outdoor kitchens
• Compacted or sealed areas

These surfaces can increase runoff even when they are not roofed.

Their falls should direct water away from:

• External walls
• Door thresholds
• Crawl spaces
• Neighbouring properties
• Unstable slopes
• Areas vulnerable to erosion

10. Rainwater Tanks

A rainwater tank can collect roofwater for reuse, detention or both.

The plan should identify:

• Tank location
• Tank capacity
• Roof area connected to the tank
• Inlet pipe
• Outlet pipe
• Overflow route
• Detention outlet where applicable
• Pump and reuse system
• Maintenance access

A tank used only for garden irrigation may be full when a storm arrives. It should not automatically be counted as effective detention unless the design maintains the required storage volume.

The tank overflow must discharge to an approved location rather than simply spilling beside the foundation.

11. Retention Versus Detention

A single tank can provide both functions when it is designed with separate storage zones and suitable controls.

12. Soak Pits and Infiltration Systems

A soak pit or infiltration trench allows stormwater to soak into the ground.

The site plan should show:

• The proposed soakage location
• Dimensions and depth
• Connected catchment
• Pre-treatment or silt control
• Overflow route
• Distance from buildings and boundaries
• Distance from retaining walls and slopes
• Groundwater information where relevant

Soakage suitability depends on:

• Soil permeability
• Groundwater level
• Site slope
• Fill or contaminated land
• Nearby foundations
• Retaining structures
• Risk of slope instability
• Potential effects on neighbouring land

A soakage test or geotechnical assessment may be required. A soak pit should not be selected merely because no convenient public connection is visible.

13. Rain Gardens and Swales

Rain gardens and vegetated swales are forms of low-impact stormwater design.

They can:

• Slow runoff
• Filter sediment
• Support infiltration
• Reduce peak flows
• Improve the appearance of stormwater infrastructure

The plan may need to show:

• Inlet location
• Surface area
• Storage depth
• Filter media
• Underdrain
• Overflow
• Planting
• Maintenance access

A landscaped depression is not automatically a compliant rain garden. Its storage, soil, drainage, overflow and planting must work together.

14. Surface-Flow Arrows

Show arrows indicating how water moves across the property.

These arrows should reflect:

• Existing ground levels
• Proposed grading
• Driveway and patio falls
• Swales
• Retaining walls
• Buildings and fences
• Low points
• Safe overflow routes

Surface-flow arrows should not be decorative guesses. Where drainage direction is important, use reliable contours, spot levels or a topographic survey.

15. Overland Flow Paths

An overland flow path carries water across the land when rainfall exceeds the capacity of pipes, sumps or soakage systems.

It may follow:

• A natural depression
• A driveway
• A side yard
• A swale
• A road corridor
• A defined flood route

Show known overland flow paths and identify the relationship to:

• The proposed building
• Floor levels
• Garages and doors
• Fences and retaining walls
• Neighbouring land
• Public drainage infrastructure

Building over, filling or obstructing an overland flow path can increase flooding upstream or divert water toward another property.

16. Contours and Spot Levels

Contours and spot levels can be essential on sloping, flood-prone or tightly developed sites.

Show relevant levels such as:

• Existing ground levels
• Proposed ground levels
• Finished floor levels
• Door thresholds
• Garage floor level
• Driveway high and low points
• Catchpit lid levels
• Pipe invert levels
• Retaining-wall levels
• Boundary levels

Levels should use a consistent datum. Mixing assumed, surveyed and architectural levels without explanation can make the drawing unreliable.

17. Finished Floor Levels

The finished floor level helps demonstrate that surface water will not enter the building.

Its suitability depends on:

• Adjacent ground levels
• Paved-surface levels
• Known flooding
• Overland flow paths
• Minimum council floor-level requirements
• Foundation type
• Door and garage openings

A generic statement such as “floor above ground” may not be enough where the property is affected by flooding, ponding or sloping land.

18. Drainage Easements

Drainage easements can protect public or private pipes and provide rights of access, conveyance or maintenance.

The plan should identify:

• The easement location
• Easement width
• Purpose
• Benefited land or authority where known
• Relevant drain or service
• The proposed building’s distance from the easement

Building consent or resource consent does not automatically authorise construction over an easement.

Separate approval may be required from:

• The council
• A network utility operator
• The easement holder
• A neighbouring landowner

19. Public and Private Drains

The drawing should distinguish public infrastructure from private drainage.

This matters because responsibilities differ.

A public stormwater main is normally operated by the relevant council or network authority. Private drains generally serve an individual property or a group of properties under private arrangements.

Show:

• Known public mains
• Private stormwater pipes
• Shared private drains
• Manholes and connection points
• Easements
• The direction of flow

Responsibility cannot always be determined from pipe location alone. Council records, title documents and network information may need to be checked.

20. Natural Watercourses and Wetlands

Where a property contains or adjoins a stream, drain, wetland or coastal area, show:

• The water feature
• Top and bottom of banks
• Required setbacks
• Flood boundaries
• Riparian vegetation
• Proposed discharge point
• Erosion protection
• Any culvert or crossing

Direct discharge to a watercourse may require regional council approval and treatment to manage sediment or contaminants.

A standard residential site plan does not replace the ecological, hydraulic or erosion assessment that may be required.

21. Retaining Walls and Subsoil Drainage

Retaining walls can interrupt natural drainage and create groundwater pressure.

Where relevant, show:

• Wall location and retained height
• Ground levels above and below
• Subsoil drain
• Drainage aggregate
• Outlet location
• Surface-water diversion
• Nearby buildings and boundaries

A subsoil drain should not be connected casually to the stormwater system without checking the design, discharge point and council requirements.

Groundwater and surface water are related but not identical. Complex retaining-wall drainage may require engineering and geotechnical input.

22. Erosion and Sediment Control

Drainage planning during construction is different from the permanent stormwater system.

Construction can expose soil and allow sediment to enter:

• Roadside catchpits
• Stormwater drains
• Streams
• Neighbouring land
• Coastal water

An erosion and sediment-control plan may show:

• Stabilised construction entrance
• Silt fences
• Catchpit protection
• Stockpile locations
• Clean-water diversion
• Sediment-retention devices
• Temporary drainage
• Areas to be progressively stabilised

The owner and contractor must prevent sediment and construction debris from leaving the site or polluting waterways.

23. Stormwater Calculations

A drainage site plan may need supporting calculations.

Depending on the system, calculations can address:

• Design rainfall intensity
• Roof and paved catchment area
• Peak runoff
• Pipe capacity
• Gutter and downpipe sizing
• Tank detention volume
• Controlled discharge rate
• Soakage capacity
• Overland flow
• Pre-development and post-development runoff

The calculation method must suit the project, council standard and proposed compliance pathway.

A drafter can accurately document the proposed system, but hydraulic design and certification may need to be completed by an engineer or other suitably qualified professional.

24. Existing Drain Capacity

Reusing an existing drain does not automatically prove that it has adequate capacity or condition.

The project team may need to confirm:

• Pipe diameter
• Pipe material
• Gradient
• Existing catchment
• Additional proposed catchment
• Blockages or defects
• Connection level
• Legal discharge point

A CCTV drain inspection may be appropriate where the drain is old, damaged, inaccessible or serving multiple buildings.

25. Onsite Stormwater Disposal

Some properties cannot connect conveniently to a public stormwater network.

Possible onsite methods include:

• Soak pits
• Infiltration trenches
• Rain gardens
• Detention tanks
• Retention and reuse
• Swales
• Approved surface discharge

The site plan should explain how the complete system works, including what happens when the primary device reaches capacity.

Onsite disposal may be unsuitable on:

• Impermeable clay soils
• Steep or unstable land
• Sites with high groundwater
• Contaminated land
• Properties with limited separation from buildings or boundaries
• Land where infiltration could affect retaining walls or neighbours

26. Flood-Prone Properties

A flood-prone property may require more than an ordinary roofwater plan.

The application may need to show:

• Flood levels
• Floodplain boundaries
• Overland flow paths
• Minimum finished floor level
• Existing and proposed ground levels
• Flood-storage areas
• Structures or fill affecting flood conveyance
• Safe access during flooding

Engineering or flood modelling may be required where the project could displace water, obstruct flow or increase risk to other property.

27. Rural Property Drainage

Rural properties often rely on onsite stormwater management.

A rural drainage site plan may include:

• Long roofwater drains
• Water tanks
• Farm tracks and culverts
• Open drains
• Streams and watercourses
• Swales
• Soakage areas
• Steep slopes
• Onsite wastewater disposal fields

Stormwater and wastewater systems must remain distinct. Surface runoff should not enter a septic tank or wastewater disposal field in a way that impairs its operation.

28. Cross-Lease and Shared-Drainage Properties

Properties with shared drains require extra care.

Check:

• Which properties use the drain
• Who owns and maintains it
• Whether an easement exists
• Whether additional discharge is permitted
• Whether other owners must approve the work
• Whether the drain has enough capacity

A building consent does not automatically resolve private cross-lease or shared-drainage rights.

Building Consent Site Plan Versus Detailed Drainage Plan

For a simple residential project, both functions may be combined into one coordinated drawing. Complex systems are easier to read when drainage is placed on a separate plan.

Common Stormwater Site Plan Mistakes

Showing downpipes but no discharge point

Every drainage route should end at a clear, lawful and technically suitable connection or disposal system.

Using a soak pit without testing the ground

Poorly draining soil, groundwater or slope conditions can make soakage ineffective or unsafe.

Treating a rainwater tank as unlimited storage

The plan must show the outlet, overflow and any dedicated detention capacity.

Ignoring paved areas

Driveways, patios and paths can generate substantial runoff even though they do not have downpipes.

Drawing surface-flow arrows without levels

Drainage direction should be based on verified site information rather than visual guesswork.

Blocking an overland flow path

A new building, fence or retaining wall can redirect floodwater toward the house or neighbouring land.

Omitting existing public drains

Buildings and foundations may need clearances or approval when public infrastructure crosses the property.

Building over an easement

Council consent does not automatically override drainage rights or permit interference with the protected service.

Failing to distinguish stormwater from wastewater

The two systems should be labelled separately and must not be connected incorrectly.

Showing pipes without gradients or levels

A pipe route may look possible in plan view but fail because there is insufficient fall to the outlet.

Discharging toward a neighbour

Stormwater should not create flooding, erosion or nuisance on adjoining land.

Leaving construction sediment uncontrolled

Permanent stormwater plans do not replace temporary erosion and sediment-control measures.

Submitting inconsistent plans

The roof plan, site plan, drainage plan and architectural drawings should show the same building footprint and downpipe arrangement.

For more examples of council issues, read Why Site Plans Get Rejected in New Zealand—and How to Fix Them.

When Is an Engineer Likely to Be Needed?

Engineering input may be required when the project involves:

• Large roof or paved catchments
• Stormwater detention calculations
• Network-capacity constraints
• Floodplain or overland flow
• Complex soakage design
• Steep or unstable ground
• Retaining walls and subsoil drainage
• Discharge to a stream or coastal area
• Significant earthworks
• Commercial or multi-unit development
• Alternative Building Code solutions

The council may request calculations, a design report, producer statements or signed drawings from a suitably qualified professional.

Information to Gather Before Ordering a Drainage Site Plan

• Property address
• Record of title
• Lot and deposited-plan details
• Existing survey or site plan
• Topographic survey or spot levels
• Existing and proposed building plans
• Roof plan
• Driveway and paving layout
• Known downpipe locations
• Existing stormwater-drain information
• Public network maps
• Known connection points
• Drainage and utility easements
• Rainwater-tank information
• Soakage-test results, where applicable
• Flood and overland-flow information
• Retaining-wall details
• Photographs of the property
• Any engineer or council correspondence
• Any request for further information

Order a Stormwater and Drainage Site Plan

NZ Site Plan can prepare a clear property drawing showing the legal boundaries, existing buildings, proposed work, roof catchments, downpipes, paved areas, private stormwater routes, surface drains, tanks, soakage systems, drainage easements and known discharge points.

The drawing can be prepared using an existing survey, title information, council mapping, architectural plans, drainage sketches, project measurements and photographs supplied by the property owner or designer.

Where the project requires hydraulic calculations, soakage testing, civil engineering, flood modelling, geotechnical assessment, public-network design or a resource consent assessment, those services must be obtained separately from suitably qualified professionals.

Frequently Asked Questions

What must a New Zealand stormwater site plan show?

It should show the property boundaries, existing and proposed buildings, roof areas, downpipes, private drains, surface drains, connection or disposal point, paved areas, levels, flow directions, overland flow paths and relevant easements.

Does every residential site plan need detailed drainage?

The required detail depends on the project. A small addition may need a simple connection layout, while a new dwelling or constrained site may require a separate engineered drainage plan and calculations.

Can roofwater discharge onto the ground?

Only where the proposed disposal method complies with the Building Code, council rules and site conditions. Uncontrolled discharge beside a foundation or toward neighbouring land is not acceptable.

Can I use a soak pit?

Potentially, but the soil, groundwater, slope, available area and effects on buildings and neighbouring land must be suitable. Testing and engineering may be required.

Does a rainwater tank solve the stormwater requirement?

Not by itself. The design must address tank capacity, available storage, outlet, overflow and the lawful destination of excess water.

What is an overland flow path?

It is a route followed by surface water when rainfall exceeds the capacity of the normal drainage system. Buildings and siteworks should not obstruct or dangerously redirect it.

Do driveways need drainage?

They often do, particularly where the driveway slopes toward a garage, house, footpath or neighbouring property. Show gradients, flow direction and any grated or channel drains.

Can stormwater connect to the wastewater drain?

Not unless a lawful combined system specifically permits it. Stormwater and foul wastewater are normally separate systems and should be clearly distinguished.

Can I build over a stormwater pipe?

Approval may be required, especially where the pipe is public, shared or protected by an easement. Building over a drain can also affect maintenance access and structural design.

Does stormwater work require resource consent?

It can. Discharges, earthworks, work near waterways, floodplain changes and other activities may require regional or district council approval.

Do I need a topographic survey?

A topographic survey may be necessary where drainage depends on accurate levels, contours, flood information, driveway gradients or overland flow paths.

Who can design a stormwater system?

Simple work may be documented by the project designer or authorised drainlayer. Complex hydraulic, detention, soakage or flood designs may require a civil or hydraulic engineer.

Can NZ Site Plan provide engineering calculations?

The service described here is the site plan drawing. Engineering calculations, hydraulic design, geotechnical testing, flood modelling and producer statements must be obtained separately where required.