Can I Build a Simple Garden Retaining Wall Using Concrete Blocks?

Yes, many Kiwi homeowners successfully build their own concrete block retaining walls for garden beds and gentle slopes. However, there are important rules around height, drainage, and building consent that you need to understand before you start — getting these wrong can lead to wall failure, property damage, or legal headaches.

What the NZ Building Code Says About Retaining Walls

Retaining walls are covered under the NZ Building Code, and whether yours needs a building consent depends primarily on its height and what's near it.

Under Schedule 1 of the Building Act, retaining walls that retain less than 1.5 metres of ground and do not support any surcharge (such as a driveway, vehicle load, or structure) are generally exempt from building consent. However, this is not a blanket rule — your local council may have additional requirements, and site-specific factors like unstable soil or proximity to boundaries can change things. Always confirm with your local council before you start.

For walls over 1.5 m, or any wall near a boundary, driveway, or building, you will almost certainly need consent and may need engineering input.

Choosing the Right Concrete Blocks

For DIY garden retaining walls, the most common options available at stores like Bunnings or Mitre 10 include:

• Segmental retaining wall blocks (e.g. Allan Block, Versa-Lok style) — these are designed specifically for retaining walls, with a built-in batter (backward lean) and interlocking design.
• Standard concrete blocks (hollow or solid) — these can work for low garden walls but require mortar, a concrete footing, and often reinforcing for anything over a couple of courses.
• Besser blocks — a common NZ hollow concrete masonry unit, suitable for mortared walls with steel reinforcing.

For a simple, low garden wall (under 600–800 mm), dry-stack segmental blocks are the most DIY-friendly option. For taller walls, mortared construction with a proper footing is strongly recommended.

Planning and Preparation

Good preparation is the difference between a wall that lasts decades and one that leans or collapses after the first wet winter.

Site Assessment

• Check the soil type — clay soils retain water and create significant lateral pressure. Sandy or loose soils may need additional stabilisation.
• Identify any underground services (water, gas, electrical) before digging. Call Dial Before You Dig (dialbeforeyoudig.co.nz) or check with your local council or check with your local council.
• Consider surface water flow — where does rainwater go during heavy rain?

Key Design Principles

• Batter (lean): Most segmental block walls are designed to lean back into the slope at roughly 1° per course. This improves stability significantly.
• Base depth: Bury the first course at least 100–150 mm below ground level, or deeper on soft ground.
• Footing: For mortared block walls, a concrete footing (typically 200 mm deep × 300–400 mm wide as a guide) is essential. Check manufacturer specs for your specific block.
• Wall length and steps: For longer walls on a slope, step the base down with the terrain rather than building one tall section.

Drainage — The Most Critical Step

Poor drainage is the number one cause of retaining wall failure in New Zealand. Water build-up behind a wall creates enormous hydrostatic pressure that can push even a well-built wall over.

• Backfill with free-draining aggregate (e.g. AP20 chip or similar) for at least 300 mm behind the wall.
• Install agricultural drain pipe (ag pipe) at the base of the wall, sloped to daylight or a stormwater outlet.
• Include weep holes every 1–1.5 m along the base of mortared walls to allow water to escape.
• Avoid using clay-heavy soil as backfill directly behind the wall.

For guidance on drainage and moisture management, BRANZ is an excellent NZ resource.

Step-by-Step: Building a Simple Dry-Stack Block Wall (Under 800 mm)

1. Mark out your wall line with string and pegs.
2. Excavate a trench 150–200 mm deep and slightly wider than your block.
3. Compact the base with a hand tamper or plate compactor.
4. Lay a level base course, checking for level along the length and ensuring the blocks are set back into the slope.
5. Backfill behind the first course with free-draining aggregate as you go.
6. Lay subsequent courses, staggering the joints (like brickwork) and maintaining the batter.
7. Install ag pipe at the base before backfilling fully.
8. Cap the top with a suitable capping block or flat stone.

When to Call a Professional

Some retaining wall projects are beyond safe DIY territory. You should engage a qualified landscaper or structural engineer if:

• Your wall will be over 1.0 m tall (and especially over 1.5 m)
• The wall is near a boundary, driveway, or building
• The ground is unstable, steep, or has poor drainage
• You're on a hillside property or in an area with known land instability
• A building consent is required

For walls requiring consent, you may need plans prepared by a Licensed Building Practitioner (LBP) or engineer. Find qualified LBPs at lbp.govt.nz.

A skilled local landscaper can assess your site, advise on the best block system, and build a wall that meets code. You can find experienced landscapers in your area right here on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance — Retaining Walls & Consent
• BRANZ — Building Research
• Licensed Building Practitioners Register
• Bunnings NZ — Landscaping & Retaining
• Mitre 10 NZ — Garden & Landscaping
• Standards New Zealand