Building a retaining wall near a boundary is one of those jobs that can quietly cause big problems if it's not thought through carefully. The good news is that with the right planning, design, and drainage, you can protect your neighbour's fence — and your relationship with them — while getting the job done properly.
Understand Why Retaining Walls Can Affect Fences
When you build a retaining wall close to a boundary, you're changing how soil loads and water behave near that line. The main risks to a neighbouring fence are:
- Lateral soil pressure pushing against fence posts, causing them to lean or fail
- Loss of soil support (undermining) if excavation removes the ground that fence posts rely on
- Water redirection sending runoff toward fence footings, rotting timber or rusting steel
- Vibration and disturbance during construction loosening fence post footings
Understanding these risks is the first step to preventing them.
Check Whether You Need Building Consent
In New Zealand, retaining walls are not always exempt from building consent. Under Schedule 1 of the Building Act 2004, a retaining wall may be exempt if it retains no more than 1.5 metres of ground and does not support a surcharge (such as a driveway, vehicle load, or structure). However, this threshold can be lower depending on your local council's rules, and walls near boundaries often attract extra scrutiny.
Always check with your local council before you start. If your wall is close to the boundary and could affect your neighbour's property, consent is likely required. You can find general guidance at MBIE Building Performance.
Talk to Your Neighbour First
Before any design or digging begins, have an honest conversation with your neighbour. Let them know:
- What you're planning to build and roughly where
- How close the wall will be to the boundary
- How long construction will take
- That you'll take steps to protect their fence
This goodwill goes a long way and can prevent disputes later. If the fence sits on the boundary, it may be jointly owned under the Fencing Act 1978, which means you may have obligations around how you affect it.
Design the Wall to Protect the Boundary
Good design is your best protection. Key principles include:
Set the Wall Back from the Boundary
Where possible, position the wall at least 300–600 mm from the boundary. This gives room for drainage, access, and means fence posts are less likely to be undermined during excavation.
Use Appropriate Wall Type and Footing Depth
Different wall types (concrete block, timber, concrete panel, boulder) have different footing requirements. Make sure your footing excavation doesn't extend under the boundary line or remove soil that supports fence posts. A qualified retaining wall builder or structural engineer can advise on the right footing design.
Account for Surcharge Loads
If there's a driveway, shed, or other structure near the top of the wall, this adds extra load. This must be factored into the design — it's a common reason walls fail and push into neighbouring properties.
Get Drainage Right — It's Critical
Poor drainage is the number one cause of retaining wall failure in New Zealand. Water trapped behind a wall builds up hydrostatic pressure that can push walls over and saturate fence post footings.
Essential drainage measures include:
- Coarse aggregate (gravel) backfill directly behind the wall to allow water to move freely
- Ag pipe (agricultural drain pipe) at the base of the wall, sloped to daylight or a stormwater outlet
- Weep holes in concrete or block walls at regular intervals (typically every 1.2–1.8 m)
- Directing all drainage away from the boundary, not toward your neighbour's fence
Check that your drainage outlet connects to an approved stormwater system — discharging onto a neighbour's property is not acceptable and may breach your council's rules. BRANZ has useful technical guidance on retaining wall drainage.
Protect Fence Posts During Construction
Excavation is the riskiest phase for existing fence posts. Before you dig:
- Identify exactly where fence posts are and how deep they're set
- Avoid excavating within 300–500 mm of a post footing without temporary support
- If posts must be temporarily removed, agree this with your neighbour in writing beforehand
- Compact backfill carefully in layers to avoid settlement that could tilt the fence
Use Quality Materials
The materials you choose affect long-term performance. Treated timber (H5 for ground contact), quality concrete block, or engineered concrete panels are all common in NZ. Consult your local Mitre 10, PlaceMakers, or Bunnings for product options, but always follow manufacturer specifications and any engineer's recommendations.
When to Call a Professional
Retaining walls over 1.5 metres, walls on unstable ground, walls near structures, or walls close to boundaries in complex situations should always be designed and built by a qualified professional. Look for:
- A Licensed Building Practitioner (LBP) for supervised or restricted building work — find one at Licensed Building Practitioners
- A structural or geotechnical engineer if the ground conditions are uncertain or the wall is substantial
- Note that plumbing connections for drainage must be done by a licensed plumber — see Plumbers Gasfitters & Drainlayers Board
You can find experienced local retaining wall builders on tradedirectory.co.nz who know your region's soil conditions and council requirements.
Licensed Building Practitioner (LBP)
