A mate of mine bought a place up in Newlands a while back — one of those sloping Wellington sections where the back lawn is basically a cliff held up by hope and a few sleepers the previous owner banged in. After the first big winter the whole lot started bulging, and by spring it was leaning like a drunk at a barbecue. He wanted to know if he could just prop it up.
I’ve spent forty years building and repairing retaining walls on Wellington’s hills, and the one thing I’ll tell you upfront is that a retaining wall is an engineering job pretending to be a landscaping job. Treat it like the former and you’ll be right. Treat it like the latter, like my mate’s previous owner did, and you’ll be paying twice.
Why steep clay sections like Newlands chew through walls
Newlands, and most of the northern hill suburbs around it — Khandallah, Ngaio, Johnsonville, Karori — sit on steep clay over greywacke. A lot of the housing on these hills went up from the early 1900s through to the 1970s, often on sections that were cut and filled rather than left at their natural slope. When it rains, and it rains a lot up here, that clay soaks up water and gets heavy. A retaining wall isn’t just holding back dirt, it’s holding back saturated dirt that wants to slide, plus the water pressure building up behind it. Add our seismic zone into the mix and you’ve got a structure that has to cope with weight, water and the odd shake all at once. That’s why the sleeper-and-hope approach fails so reliably — it was never built for the load.
The wall you can see is only half the job. The half that matters, the bit that makes it last, is behind and below it.
The corner almost everyone cuts: drainage
If you take one thing from a retired builder, take this: water pressure kills retaining walls, not soil weight. Nearly every failed wall I’ve pulled out on these hills failed because water built up behind it with nowhere to go. A proper wall has:
- A layer of free-draining metal (drainage aggregate) behind the wall face
- A perforated drainage coil (novacoil) at the base, running out to daylight or a stormwater point
- Geotextile cloth so the drainage layer doesn’t silt up over time
- Weep holes or gaps so any water that does get through can escape
Skip the drainage to save a few hundred dollars and you’ll be rebuilding the whole thing in five years. It’s the single most common corner cut on hillside retaining work, and the most expensive one. If you’re planning wider drainage work on a wet section, our piece on why hillside drainage matters in wet Wellington winters is worth reading alongside this.
The 1.5 metre rule, and why it doesn’t always save you
This is where people get into strife. In New Zealand, a retaining wall generally needs a building consent once it’s over 1.5 metres high, and it needs consent at any height if it’s carrying extra load on top — a driveway, a building, or a slope surcharging above it. On a Wellington hill, that surcharge situation is common, so plenty of walls under 1.5 metres still need consent and engineered design. Don’t guess. The rules are set out plainly on the government’s Building Performance website, and Wellington City Council will want engineered plans for anything structural on a steep section.
For anything of height, or anything holding up something valuable, you want a chartered engineer to design it and specify the footing. On our clay, footings and any piles have to be sized for the actual ground, which is exactly why a geotechnical report often pays for itself before you build a single course.
Timber vs engineered concrete: what it actually costs
Timber, treated H5 poles with retaining timber, is the common, cost-effective choice and does the job well when it’s built right. Concrete crib, block, or a poured concrete wall costs more but lasts longer and suits higher or heavily loaded situations. Here’s the Wellington ballpark I quote off:
| Wall type | Typical use | Cost per m² of wall face (incl. GST) |
|---|---|---|
| Timber retaining wall | Standard hillside sections, moderate height | $450 – $750 |
| Engineered concrete or block (crib) wall | Higher walls, heavy loads, hard-access hill sites | $900 – $1,500 |
Access is the hidden cost up here, and it’s a big one. That concrete wall figure factors in engineering, drainage and the cost of getting gear up a steep drive, which on some Newlands and Khandallah sections is half the battle before a single post goes in.
My advice to my Newlands mate was the same I’ll give you: do it once, properly, with drainage and, where the rules say so, an engineer and a consent. A retaining wall you cut corners on doesn’t just cost money to fix, it can take a chunk of your section with it.
Questions people ask me about retaining walls on Wellington’s hills
Do I need a building consent for a retaining wall in Wellington? Generally yes if the wall is over 1.5 metres high, and at any height if it’s carrying extra load such as a driveway, building or a slope above it. On Wellington’s hills that surcharge situation is common, so many walls under 1.5m still need consent and engineered design.
Why do retaining walls fail on Wellington hills? Almost always because of water pressure, not soil weight. Without proper drainage behind the wall, saturated clay builds up hydrostatic pressure and pushes the wall over. Good drainage aggregate, a perforated drain to daylight and geotextile cloth are what make a wall last.
How much does a retaining wall cost in Wellington? A straightforward timber wall runs about $450 to $750 per square metre of wall face including GST. An engineered concrete or block wall on a steep, hard-access site can run $900 to $1,500 per square metre including GST once engineering, drainage and access are included.
Can a leaning retaining wall be repaired, or does it need replacing? It depends on why it’s leaning. If the wall itself is sound but the drainage behind it has failed, sometimes you can retrofit drainage and pull the wall back with tie-backs. But if the timber’s rotten, the footing’s undersized, or it was never built for the load in the first place, like my mate’s Newlands wall, you’re better off pulling it out and starting again with proper engineering than patching a structure that’s already failed once.
Is timber or concrete better for an older Wellington hill property? On older sections around Newlands, Khandallah and Ngaio, timber is often perfectly adequate for moderate-height walls and matches the character of the property, but where a wall is carrying a driveway or sits above a house, I’ll generally push a client towards an engineered concrete or crib wall regardless of the era of the home, because the load, not the house’s age, is what decides the spec. For the smaller repairs those sections throw up — a rotten sleeper, a loose handrail — a Wellington handyman is usually a faster call than a full wall crew.