Can I Install Subsoil Drainage in My Waterlogged Garden Myself?

Yes, in most cases you can install subsoil drainage in your waterlogged garden yourself — it's one of the more accessible DIY drainage projects for NZ homeowners. However, there are important distinctions between work you can legally do yourself and restricted drainlaying work that must be carried out by a licensed professional. Getting this right protects your property, your neighbours, and the environment.

What Is Subsoil Drainage?

Subsoil drainage (also called agricultural or French drain systems) collects excess groundwater from saturated soil and redirects it away from your garden, lawn, or building foundations. A typical system uses:

• Perforated flexible pipe (commonly 65 mm or 100 mm diameter) wrapped in a geotextile sock
• Drainage aggregate (clean, washed gravel — typically 10–20 mm chip)
• Geotextile filter fabric to prevent soil migration into the pipe
• A collection point such as a sump, soakpit, or outlet to a legal discharge point

These systems work by intercepting water moving through the soil profile before it saturates your garden or seeps into your home.

What Can a Homeowner Do Themselves?

Under the Plumbers, Gasfitters and Drainlayers Act 2006, homeowners are permitted to carry out drainage work on their own property for their own use, provided it does not connect to the public stormwater or wastewater network. This means you can legally:

• Dig trenches and install perforated subsoil drain pipes in your garden
• Install a soakpit or sump to collect and disperse water on your own land
• Redirect drainage to a legal on-site discharge point (e.g., a soakage area away from boundaries and buildings)
• Regrade garden areas to improve surface runoff

Always check with your local council before discharging to any watercourse, road drain, or neighbouring property — this may require resource consent under the RMA.

What Is Restricted (Licensed) Work?

Any connection to the public stormwater or wastewater reticulation network is restricted drainlaying work and must be carried out by a licensed drainlayer. This includes:

• Connecting your subsoil drain outlet to a council stormwater pipe or kerb drain
• Any work on or near the public sewer
• Installing new drainage within 3 metres of a building foundation in some circumstances (check with your council)

If you're unsure whether your planned outlet point is considered a public connection, contact your local council or a licensed drainlayer before you start.

Does This Work Need a Building Consent?

Most garden subsoil drainage does not require a building consent under Schedule 1 of the Building Act 2004, as it is generally considered routine maintenance or minor drainage work. However, this depends on your specific situation — for example, drainage work that affects a building's foundation or involves significant earthworks may require consent. Always confirm with your local council before starting.

For guidance on the NZ Building Code (particularly Clause E1 – Surface Water), visit MBIE Building Performance.

Planning Your Subsoil Drainage System

Step 1 – Identify the Problem Area

Walk your property after heavy rain and note where water pools or the ground stays saturated longest. NZ's variable climate — particularly in wetter regions like Northland, the West Coast, and parts of the Waikato — means many properties benefit from subsoil drainage.

Step 2 – Plan Your Pipe Layout

Common layouts include:

• Herringbone pattern — lateral pipes feeding into a central collector (good for large areas)
• Single run — one pipe along the lowest point of a slope

Pipes should be laid with a minimum fall of 1:200 (ideally 1:100) toward the outlet.

Step 3 – Dig the Trenches

Trenches are typically 300–500 mm deep and 200–300 mm wide. Use a spade or hire a trenching machine from a local hire centre for larger jobs.

Step 4 – Lay the System

1. Line the trench with geotextile fabric, leaving enough to fold over the top
2. Add a 50–75 mm bed of drainage aggregate
3. Lay the perforated pipe (holes facing down)
4. Surround with more aggregate to within 100–150 mm of the surface
5. Fold the geotextile over the top of the aggregate
6. Backfill with topsoil and reinstate lawn or garden

Step 5 – Outlet and Disposal

Your outlet options include:

• A soakpit (a gravel-filled pit that allows water to percolate into the subsoil — check your soil type first)
• Discharge to a legal watercourse (resource consent may be needed)
• Connection to the council stormwater network (licensed drainlayer required)

Materials and Approximate Costs

As a rough guide only, expect to spend on materials:

Prices vary by region and supplier. Check current pricing at Bunnings, Mitre 10, or PlaceMakers.

Safety Notes

• Call Before You Dig — always contact Dial Before You Dig (0800 248 344) to locate underground services before any excavation
• Be aware of underground electrical cables, gas lines, and water pipes
• Trenches deeper than 1.5 m require shoring — contact WorkSafe NZ for excavation safety guidance

When to Call a Professional

Consider hiring a licensed drainlayer (find one on tradedirectory.co.nz) if:

• Your drainage needs to connect to the council stormwater network
• You have persistent water ingress affecting your home's foundations
• The problem is complex, large-scale, or involves retaining walls
• You're unsure about your soil type, fall, or outlet options
• Your property is on a cross-lease or unit title with shared drainage

A licensed drainlayer can also issue a Certificate of Compliance for completed work, which may be important for future property sales.

Sources & Further Reading

• Plumbers, Gasfitters and Drainlayers Board – Licensing & Homeowner Work
• MBIE Building Performance – NZ Building Code Clause E1 Surface Water
• BRANZ – Drainage and Moisture Control Guidance
• WorkSafe NZ – Excavation Safety
• Bunnings – Drainage Products
• PlaceMakers – Drainage Supplies