There is no single universal depth that automatically triggers council notification for all excavations in New Zealand — it depends on what you're digging for, where it is on your property, and what the NZ Building Code and your local council require. As a general guide, excavations that are part of building work, affect foundations, or exceed certain dimensions will likely need a building consent or at minimum a careful check against Schedule 1 exemptions.
The NZ Building Code and Building Consents
Under the Building Act 2004, administered by MBIE Building Performance, almost all building work — including significant excavation — requires a building consent unless it falls under a specific exemption. Excavation is considered building work when it relates to the construction, alteration, or demolition of a building or structure.
If your excavation is part of a larger project (like a new foundation, retaining wall, or basement), the consent for that project will cover the dig. You should never assume the excavation alone is exempt without checking.
Schedule 1 Exemptions — What's Allowed Without Consent?
Schedule 1 of the Building Act lists work that is exempt from building consent. For excavation and earthworks, the key exemptions relate to:
- Retaining walls that retain no more than 1.5 metres of ground and do not support any surcharge (e.g., a driveway, vehicle load, or building) — these may not need consent.
- Drainage work for existing systems in some circumstances.
- General garden landscaping that does not affect the structural integrity of the ground near buildings or boundaries.
However, even exempt work must still comply with the NZ Building Code. And critically — Schedule 1 exemptions do not override your council's district plan, which may have its own earthworks rules around volume of soil moved, proximity to waterways, or slope stability.
Always confirm with your local council before you dig. Exemptions vary by council and site-specific conditions.
Retaining Walls: The 1.5 Metre Rule
The most commonly cited depth threshold in NZ excavation is the 1.5 metre rule for retaining walls. If your excavation requires a retaining wall that:
- Retains more than 1.5 metres of ground, OR
- Retains any height of ground but also supports a surcharge (vehicle loads, a building, or a slope above)
…then a building consent is required. This is outlined in Schedule 1 and is consistent with NZS 3604, the standard for timber-framed buildings, which also informs retaining structure design.
District Plan Earthworks Rules
Separate from the Building Act, your council's district plan (now called a Resource Management Plan in many areas) may require resource consent for earthworks above a certain volume or area — regardless of depth. Common triggers include:
| Trigger | Typical Threshold (varies by council) |
|---|---|
| Volume of soil moved | Often 50–250 m³ (check your council) |
| Area disturbed | Often 500–2,000 m² |
| Proximity to waterway | Usually within 10–50 metres |
| Steep or unstable land | Site-specific |
These thresholds differ significantly between councils — Auckland, Wellington, Christchurch, and smaller territorial authorities all have their own rules. Contact your council's planning or building department to get the specific rules for your site.
Underground Services: A Critical Safety Step
Before any excavation — even shallow digging — you must locate underground services. Striking a gas line, water main, or power cable can be fatal and is your legal responsibility.
- Contact Dial Before You Dig (0800 248 344 or beforeudig.co.nz) to get service plans for your area.
- Allow several working days for responses.
- Hand-dig carefully within 500 mm of any marked service.
WorkSafe NZ has clear guidance on excavation safety, including requirements for shoring or battering the sides of any trench deeper than 1.5 metres to prevent collapse — this is a legal health and safety requirement, not optional.
Trenches and Drainage Work
For plumbing and drainage trenches, the depth is often dictated by pipe fall and cover requirements under NZ Building Code clause G13 (Foul Water) and G12 (Water Supplies). Drainage work connecting to the public system is restricted work that must be carried out or supervised by a licensed drainlayer registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). Homeowners cannot legally do this work themselves.
Proximity to Boundaries and Neighbouring Structures
Excavating close to your property boundary or near neighbouring foundations can undermine their structural support — this is called loss of support and carries legal liability. As a rule of thumb, if you're digging within a horizontal distance equal to the depth of your excavation from any boundary or structure, get engineering advice first.
When to Call a Professional
You should engage a qualified excavator operator or geotechnical engineer if:
- Your excavation exceeds 1.5 metres in depth for any reason.
- You're working near foundations, retaining walls, or boundaries.
- The ground is unstable, sloped, or has fill material.
- Your project requires a building or resource consent.
- You're unsure about underground services or soil conditions.
For drainage and plumbing trenches, always use a licensed drainlayer. For structural excavation tied to building work, an LBP (Licensed Building Practitioner) should be involved.
You can find qualified local excavators and related tradespeople on tradedirectory.co.nz — look for operators with experience in residential earthworks and a good understanding of local council requirements.
Licensed Building Practitioner (LBP)
