You may be able to build a retaining fence on a steep NZ section yourself — but whether it's legal to do so without a building consent depends on the height of the retained soil, the surcharge load, and your local council's rules. Get this wrong and you could face costly remediation, liability issues, or a structure that fails dangerously. Here's what you need to know before you start.
Retaining Fence vs Retaining Wall — What's the Difference?
In everyday language, people often use "retaining fence" to mean a structure that both holds back soil and acts as a boundary fence. Technically, the retaining function is what triggers building regulations — not the fence part. If your structure is holding back earth, it is legally treated as a retaining wall under the NZ Building Code, regardless of whether it also has fence panels on top.
This distinction matters because retaining walls are regulated structures, while standard boundary fences generally are not.
When Do You Need a Building Consent?
Under Schedule 1 of the Building Act 2004, some retaining walls are exempt from requiring a building consent. As a general guide, a retaining wall may be exempt if it:
- Retains no more than 1.5 metres of ground, and
- Does not support any additional surcharge load (such as a driveway, vehicle traffic, or a building foundation nearby)
However, this is not a blanket rule. Your local council may have additional requirements, and the exemption does not apply if the wall could affect neighbouring properties, drainage, or public safety. Always confirm with your council before starting work — never assume the exemption applies to your specific site.
For walls over 1.5 metres, or any wall with a surcharge load, a building consent is almost certainly required, and you will likely need engineering input. Check MBIE Building Performance for guidance on consent exemptions and the Building Act.
NZ Building Code Requirements
Retaining structures must comply with NZ Building Code Clause B1 (Structure) and Clause E1 (Surface Water). On steep sections, managing water runoff behind the wall is critical — poor drainage is the number one cause of retaining wall failure in New Zealand.
If your section is particularly steep or the soil is unstable (common in areas with clay soils or volcanic material), you should consult a geotechnical engineer before designing anything. BRANZ publishes useful guidance on earthworks and retaining structures for residential properties.
What Can a Homeowner DIY?
If your retaining fence falls within the consent-exempt category and you're confident in your abilities, a homeowner can legally carry out the physical construction work themselves. Common DIY-friendly options for modest retaining on steep sections include:
- Timber retaining walls using treated H5-rated timber posts and rails (suitable for ground contact)
- Concrete block or segmental retaining block systems (such as Allan Block or similar interlocking systems available at Bunnings or Mitre 10)
- Railway sleeper walls for lower-height applications
Key construction principles to follow:
- Use correctly treated timber — H5 treatment for in-ground use
- Install drainage — a gravel backfill and agricultural drain pipe (ag pipe) behind the wall is essential
- Set posts deep enough — a common rule of thumb is one-third of the total post length in the ground, but check with your supplier or an engineer
- Don't overload the wall — keep driveways, heavy planters, and structures well clear of the top of the wall
Fencing on Top of a Retaining Wall
If you plan to add a fence on top of the retaining wall (common on steep sections for privacy or safety), the fence posts must be properly integrated into the wall structure — not just surface-mounted. The combined height of the retaining wall plus fence can also affect whether a pool fencing or Fencing of Swimming Pools Act compliance is needed if a pool is nearby.
For boundary fences specifically, the Fencing Act 1978 governs cost-sharing with neighbours, so it's worth having a conversation with your neighbour before building anything on or near the boundary.
Drainage and Waterproofing
On steep NZ sections — particularly in wetter regions like Auckland, Wellington, or the West Coast — water management is critical. Poor drainage behind a retaining wall builds up hydrostatic pressure that can cause even well-built walls to fail. Install:
- A layer of free-draining gravel (chip) directly behind the wall
- Ag pipe (perforated drainage pipe) at the base of the wall, directed to a suitable outlet
- Geotextile fabric to prevent soil migrating into the drainage layer
When to Call a Professional
You should engage a licensed building practitioner (LBP) or structural/geotechnical engineer if:
- The wall will retain more than 1.5 metres of soil
- There is any surcharge load (driveway, building, vehicle access) near the top of the wall
- The ground is unstable, has a history of slipping, or is on a very steep slope
- A building consent is required
- You are unsure about any aspect of the design or construction
A qualified fencer or retaining wall specialist will understand local soil conditions and council requirements. You can find experienced local fencing professionals on tradedirectory.co.nz to get quotes and advice specific to your site.
For LBP verification, visit Licensed Building Practitioners.
Safety Note
Retaining wall failures can be sudden and dangerous, particularly after heavy rain. If you notice bulging, cracking, or leaning in an existing retaining structure on a steep section, stop using the area below it and get a professional assessment immediately. Report any concerns about public safety to your local council.
