Yes, you can build a concrete block retaining wall yourself in some situations — but New Zealand's Building Code and local council rules set clear limits on when a DIY approach is legal and safe. Get this wrong and you could face costly remediation, or worse, a wall failure that injures someone. Read on to understand exactly where the line is drawn.
What the NZ Building Code Says
Retaining walls are regulated under the NZ Building Code, administered by MBIE Building Performance. The key question is whether your wall requires a building consent.
Under Schedule 1 of the Building Act 2004, some retaining walls are exempt from building consent — but the exemptions are specific:
- The wall retains no more than 1.5 metres of ground, and
- There is no surcharge (e.g. no vehicle loads, structures, or slopes above the wall that add extra pressure), and
- Failure of the wall would not affect any other property (neighbouring land, buildings, or public spaces)
If your wall meets all three of those criteria, you may be able to build without consent. However, you must always confirm this with your local council before starting — exemptions can vary and councils interpret conditions differently.
If your wall exceeds 1.5 m, sits near a boundary, or has any surcharge loading, a building consent is almost certainly required, and the design will likely need to be signed off by a Chartered Professional Engineer (CPEng).
Restricted Work and Licensed Practitioners
Retaining wall construction is not classified as restricted building work in the same way that structural framing or weathertightness work is — so you are not legally required to use a Licensed Building Practitioner (LBP) for a consent-exempt wall.
However, if your wall does require consent, the design and inspection process may involve an LBP or engineer, depending on your council's requirements. Always check before you start.
Planning Your Concrete Block Wall
Choosing the Right Block
For DIY retaining walls, concrete masonry blocks (standard hollow blocks) or segmental retaining wall blocks (such as interlocking garden wall blocks) are common choices. Hollow concrete blocks used in structural walls typically need to be core-filled with concrete and reinforced with steel rod (rebar) — this is essential for walls over about 600 mm tall.
You'll find a good range of blocks and materials at Bunnings, Mitre 10, and PlaceMakers.
Key Design Considerations
| Factor | Why It Matters |
|---|---|
| Footing depth | Must be below the frost line and into firm ground — typically 300–600 mm deep |
| Drainage | Poor drainage is the #1 cause of wall failure — install ag pipe and gravel backfill |
| Batter (lean-back) | Walls should lean slightly into the retained soil for stability |
| Reinforcement | Rebar and core-fill concrete are essential for structural block walls |
| Surcharge loads | Driveways, fences, or buildings above the wall dramatically increase required strength |
Drainage Is Non-Negotiable
Water build-up behind a retaining wall creates hydrostatic pressure that can topple even a well-built structure. Always install:
- A layer of free-draining gravel (chip or scoria) directly behind the wall
- Agricultural drainage pipe (ag pipe) at the base of the wall, sloped to an outlet
- Weep holes through the base course of block walls if ag pipe isn't used
BRANZ has useful guidance on drainage and ground moisture management that applies to retaining structures.
Step-by-Step Overview for a Consent-Exempt Wall
- Check with your council — confirm the wall is exempt and note any conditions
- Mark out and excavate your footing trench — typically 300–400 mm wide and 300–600 mm deep
- Pour a concrete footing and allow it to cure (at least 24–48 hours)
- Lay your first course of blocks on the footing, checking for level
- Install rebar vertically into cores as you build up each course
- Core-fill with concrete every 1–2 courses, or as your block manufacturer recommends
- Install drainage — gravel backfill and ag pipe as you go, not as an afterthought
- Cap the wall and finish the surface as desired
Always follow the block manufacturer's installation instructions, as requirements vary between products.
NZ Climate and Site Considerations
New Zealand's varied climate matters. In high-rainfall regions (West Coast, Northland, parts of Wellington), drainage design is even more critical. In earthquake-prone areas — which covers much of NZ — retaining walls above about 1 m should be carefully assessed, as seismic loading adds significant force. This is another reason to consult an engineer for anything beyond a modest garden wall.
If your site has expansive or unstable soils (clay, fill, or peat), a DIY approach may not be appropriate regardless of wall height.
When to Call a Professional
You should engage a qualified retaining wall builder or structural engineer if:
- Your wall will be taller than 1.0–1.5 m (even within the exempt range, taller walls carry more risk)
- There is any surcharge from vehicles, buildings, or steep slopes above
- The wall is near a boundary or neighbouring structure
- Your site has poor or unstable soils
- You are in an earthquake-prone zone and the wall is anything more than a low garden feature
- You are unsure about any aspect of the design or construction
A failed retaining wall can cause serious harm and significant property damage. The cost of getting professional advice upfront is far less than the cost of a collapse. You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
