Digging near your house foundations is one of those jobs that looks straightforward but can go seriously wrong without the right knowledge. Done carelessly, it can undermine your home's structural support, damage buried services, or put you in real danger. Here's what every NZ homeowner should know before picking up a spade.
Understand the Risks Before You Start
Foundations transfer the weight of your home into the ground. When you remove soil close to them, you reduce the lateral support holding that soil in place, which can cause:
- Foundation settlement or movement — even a small shift can crack walls, jam doors, and damage plumbing.
- Trench collapse — unsupported trenches deeper than 1.5 m are particularly dangerous. WorkSafe NZ treats trench collapse as a leading cause of serious injury and death on worksites.
- Damage to buried services — water, gas, stormwater, sewer, and electrical lines often run close to foundations.
Before any digging, take these risks seriously. If you're in any doubt, stop and get professional advice.
Check for Buried Services First
This step is non-negotiable. Contact Dial Before You Dig (0800 248 344 or dialbeforeyoudig.co.nz) to request a plans search for underground services on your property. Your local council may also hold drainage and water supply records. Even with plans in hand, treat all locations as approximate and hand-dig carefully when you get close to any marked service.
Remember: striking a gas line or live electrical cable can be fatal.
Do You Need a Building Consent?
In New Zealand, excavation work that could affect the structural stability of a building may require a building consent under the Building Act 2004. Schedule 1 of the Act lists some exemptions for minor work, but these have conditions and limits. As a general rule:
- Shallow, small-scale digging for garden beds well away from foundations is usually exempt.
- Any excavation that could affect the foundation bearing capacity, retaining walls, or drainage around the building is likely to need consent.
Always check with your local council before starting. You can find general guidance at MBIE Building Performance.
Safe Digging Techniques for Homeowners
Keep Your Distance
As a practical rule of thumb, avoid digging closer than 1 metre to any foundation edge without professional assessment. The deeper you need to go, the further back from the foundation you should start. Soil type matters enormously — clay, sandy soils, and fill behave very differently.
Work in Stages and Hand-Dig Near Foundations
- Use a spade or mattock rather than machinery when within 1–2 m of foundations or known services.
- Dig in small sections rather than opening a long trench all at once.
- Never undercut the soil beneath a footing — this removes the bearing support directly.
Manage Water and Drainage
Water is a major enemy during excavation. Saturated soil loses strength rapidly. If you hit groundwater or the weather turns wet, stop work and allow the area to drain before continuing. Poor drainage around foundations is already a common cause of foundation movement in NZ's wetter regions — don't make it worse.
Trench Safety
If your work requires a trench deeper than 1.5 m, you are legally required under NZ health and safety law to use shoring, battering, or benching to prevent collapse. For most homeowners, this depth signals that the job has moved beyond DIY territory. Review WorkSafe NZ guidance on excavation safety before proceeding.
Soil Types to Watch
| Soil Type | Risk Level | Notes |
|---|---|---|
| Firm clay | Moderate | Can crack and collapse when dry |
| Sandy or gravelly soil | High | Collapses quickly without support |
| Made ground / fill | Very high | Unpredictable, often loose |
| Rock | Lower | Still check for fractures |
Protecting the Foundation Itself
- Never remove soil from below the bottom of the footing (the underside of the concrete foundation).
- If you expose the footing, inspect it for cracks, crumbling concrete, or signs of movement. If you see any, stop and consult a structural engineer or licensed building practitioner.
- Backfill and compact properly when you're done — poorly compacted fill can settle and pull moisture toward the foundation.
For guidance on what good foundation practice looks like, BRANZ publishes research and best-practice information for NZ conditions.
When to Call a Professional
Some situations should always be handed to a qualified excavator or structural engineer:
- You need to dig deeper than 1.5 m anywhere near the house.
- The soil is sandy, loose fill, or waterlogged.
- Your home is on a sloping site or near a retaining wall.
- You suspect the foundations are already compromised.
- The work involves underpinning (extending or strengthening existing foundations) — this is specialist structural work requiring engineering sign-off and almost certainly a building consent.
- You need to relocate or work near gas, electrical, or plumbing lines — gas and electrical work near your home is restricted work that must be carried out by licensed tradespeople registered with the PGDB or EWRB respectively.
A qualified local excavator will have the equipment, experience, and insurance to do the job safely. You can find licensed excavation contractors in your area at tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
