Before any retaining wall is designed or built, a qualified builder will carry out a thorough site assessment. This process protects your property, your neighbours, and anyone who uses the land — and it's a legal requirement under the New Zealand Building Code for walls above certain heights. Here's what a professional will look at and why it matters.
Initial Site Visit and Visual Inspection
The first step is a walk-around of the site. The builder will look at the overall lay of the land — how steep the slope is, what's sitting above and below the proposed wall, and whether there are any obvious signs of existing movement, erosion, or drainage problems. They'll note nearby structures such as houses, garages, driveways, fences, and boundary lines, because all of these affect the design.
They'll also check for surcharge loads — anything that adds weight above the wall, like a driveway, parked vehicles, a deck, or a building. These loads significantly increase the forces the wall must resist and must be factored into the design from the outset.
Soil Type and Ground Conditions
Soil is one of the most critical factors in retaining wall design. Different soils behave very differently under load and when wet. A builder — or the engineer they work with — will assess:
- Soil type: clay, silt, sand, gravel, or fill
- Bearing capacity: how much load the ground can support
- Stability: whether the soil is prone to slipping or swelling
- Fill history: whether the site has been built up with imported material, which may be less stable than natural ground
For larger or more complex walls, a geotechnical investigation (soil testing by a specialist engineer) may be required. This can involve test pits or bore holes to understand what's happening below the surface.
Drainage Assessment
Poor drainage is one of the leading causes of retaining wall failure in New Zealand. Water trapped behind a wall dramatically increases the pressure it must withstand. The builder will look at:
- Where surface water flows during heavy rain
- Whether there are any springs, high water tables, or boggy areas
- Existing stormwater infrastructure and where it discharges
- How drainage behind the wall will be managed (typically with a gravel backfill and agricultural drain pipe)
Getting drainage right is non-negotiable. BRANZ provides good guidance on drainage requirements for retaining structures.
Wall Height and Building Consent
Height is a key trigger for consent requirements. Under Schedule 1 of the Building Act, some retaining walls are exempt from needing a building consent — but the rules are specific and depend on height, soil conditions, and surcharge. As a general guide:
- Walls up to 1.5 m retaining ground with no surcharge may be exempt in many situations
- Walls over 1.5 m, or any wall with a surcharge (like a driveway), will typically require a building consent and often engineering design
These thresholds can vary depending on your local council's district plan, so always confirm with your council before starting work. The MBIE Building Performance website has detailed information on consent requirements and Schedule 1 exemptions.
Boundary and Legal Considerations
The builder will check the wall's proximity to property boundaries. Retaining walls near boundaries can affect neighbours — both in terms of physical impact and legal responsibility. Resource consent may be needed in some cases, separate from building consent. The builder may ask to see a site plan or certificate of title to confirm boundary locations.
Engineering Requirements
For any wall that requires consent, or that is in challenging ground conditions, the builder will involve a structural or geotechnical engineer. The engineer will:
- Calculate the loads the wall must resist
- Specify the wall type, materials, footing depth, and reinforcing
- Produce stamped drawings for the consent application
Common wall types assessed at this stage include concrete block, timber (treated pine), concrete panel, and boulder walls — each with different engineering requirements.
Material and Access Assessment
The builder will also consider practical logistics:
- Access for machinery: can an excavator or concrete truck reach the site?
- Material storage: is there space to stage blocks, timber, or concrete?
- Proximity to services: underground power, water, gas, and drainage lines must be located before any digging begins. Contact Dial Before You Dig — this is a standard step on any site.
When to Call a Professional
Retaining walls are structural building work under the New Zealand Building Code. Any wall over 1.5 m, or one with a surcharge, must be designed and built to meet the Code. Poorly built walls can collapse, damage property, and injure people.
Always use a Licensed Building Practitioner (LBP) for restricted building work where required. You can check a tradesperson's licence at Licensed Building Practitioners. For walls requiring engineering, ensure the engineer is a Chartered Professional Engineer (CPEng).
You can find qualified, local retaining wall builders on tradedirectory.co.nz — a great starting point for getting multiple quotes and checking credentials.
Licensed Building Practitioner (LBP)
