In many cases a homeowner can dig a drainage trench on their own property without needing a resource consent — but whether you need a building consent, a licensed tradesperson, or both depends on what the trench is for, how deep it goes, and where you live. Getting this wrong can be costly, so it pays to understand the rules before you pick up a spade.
Resource Consent vs Building Consent — What's the Difference?
These are two separate things, and people often confuse them:
- A resource consent (under the Resource Management Act) is about land use, environmental impact, and effects on neighbours or waterways. Most straightforward residential drainage trenches on private property don't require one — but if your trench is near a waterway, wetland, or in a flood-prone or environmentally sensitive area, your local council may require one.
- A building consent (under the Building Act) is about the physical work meeting the NZ Building Code. Drainage work connected to your stormwater or wastewater system often does require a building consent, though some minor work is exempt under Schedule 1 of the Building Act.
Always check with your local council before starting. Rules vary significantly between territorial authorities.
When Is a Building Consent Required?
Under Schedule 1 of the Building Act, some drainage work is exempt from building consent — for example, certain minor repairs and maintenance. However, new drainage connections to the public network, new stormwater systems, or alterations to existing drainage infrastructure generally require a building consent and must comply with NZ Building Code clauses such as:
- E1 (Surface water)
- E2 (External moisture)
- G13 (Foul water / sanitary drainage)
If your trench is for a new subsoil drain, a French drain, or a surface water channel that doesn't connect to the public wastewater or stormwater network, it may fall under Schedule 1 exemptions — but confirm this with your council first, as exemptions have conditions.
Restricted Work: What You Cannot Do Yourself
This is critical. Even if no consent is required, certain drainage work is legally restricted and must be carried out by a licensed professional:
- Drainlaying — connecting to or working on any sanitary (wastewater) drain or public sewer is restricted work under the Plumbers, Gasfitters, and Drainlayers Act. Only a licensed drainlayer can legally do this. Check the Plumbers Gasfitters & Drainlayers Board to verify a tradesperson's licence.
- Stormwater connections to the public network — in most councils, connecting to the public stormwater drain also requires a licensed drainlayer.
A homeowner can legally dig the trench itself (the earthworks), but the actual pipe connections to any public or sanitary system must be done by a licensed drainlayer. If you're unsure where the line is, ask your council or a licensed tradesperson.
Safe Digging: Practical Rules Before You Start
Locate Underground Services First
This is non-negotiable. Striking a buried power cable, gas line, or water main can be fatal or extremely costly. Before any digging:
- Contact Dial Before You Dig (now called beforeUdig in NZ) or check with your local council and network utility operators for service plans.
- Allow extra clearance around any marked services.
- Hand-dig carefully near marked lines — never use a mechanical excavator near them without confirmation.
WorkSafe NZ has guidance on excavation safety that applies to homeowners and contractors alike.
Trench Safety
- Trenches deeper than 1.5 metres are considered confined spaces and carry serious collapse risk. Shoring or battering the sides is required.
- Never enter a deep trench without proper support in place.
- Keep spoil (excavated soil) well back from the trench edge — at least 600 mm — to prevent it collapsing back in.
- If you're hiring a mini excavator, make sure you understand how to operate it safely and check whether your hire company requires a licence (most compact machines under 1 tonne don't, but always confirm).
Environmental Considerations
If your trench or drainage work could direct water toward a neighbour's property, a waterway, or a road, you may need to manage sediment and erosion. Your council's district plan will have rules about this. Discharging sediment-laden water to a stream can breach the RMA regardless of whether you have consent for the earthworks.
What Type of Drainage Trench Are You Digging?
| Type | Likely Consent Needed | Licensed Tradesperson Required? |
|---|---|---|
| French drain / subsoil drain (no public connection) | Possibly exempt — check council | Not usually, but confirm |
| Stormwater drain connecting to public network | Building consent likely required | Licensed drainlayer required |
| Wastewater / sanitary drain | Building consent required | Licensed drainlayer required |
| Agricultural or garden drainage (private land) | Often exempt — check council | Not usually |
When to Call a Professional
You should engage a licensed drainlayer or excavation contractor if:
- The trench connects to any public wastewater or stormwater system
- The trench is deeper than 1.5 metres
- You're working near existing underground services
- Your council requires a building consent (a licensed drainlayer will often manage this process)
- You're on a steep site, near a retaining wall, or in unstable ground
A qualified drainlayer can also advise on pipe grades, backfill materials, and inspection requirements to make sure the work passes council sign-off. You can find licensed local excavation and drainage professionals on tradedirectory.co.nz.
For rental properties, note that Healthy Homes Standards include drainage requirements — another reason to get drainage work done properly and documented.
Licensed Building Practitioner (LBP)
