Quick answer
Royal Oak and Three Kings sit on basalt. Te Tātua a Riukiuta erupted about 28,500 years ago, Maungakiekie / One Tree Hill long before that, and their lava flows are the ground your house stands on. That rock is why your quote jumped when you asked to move the sink. Gravity drainage needs continuous fall, fall needs depth, and depth here means breaking rock by the hour. The cheapest good kitchen on this ground leaves the waste pipe and gully trap where they are and moves everything else: layout, storage, benchtop, appliances. Sliding a sink along its own wall is usually minor. Sending it to an island across a slab reprices the job.
Key points
- Te Tātua a Riukiuta erupted about 28,500 years ago, and its lava ran three kilometres to Western Springs, leaving tunnels still sitting under private properties.
- Maungakiekie / One Tree Hill, east of Royal Oak, sent flows in every direction — around 20 square kilometres, second only to Rangitoto by area in the Auckland field.
- Winstone quarried Three Kings for red scoria until 2015, so the aggregate under Auckland's roads is the rock under your kitchen floor.
- A 100 mm drain at 1:60 drops about 17 mm per metre, so every metre you move the sink buys depth you may have to cut out of basalt.
- Schedule 1 calls shifting a gully trap a short distance minor work, but MBIE says extending a branch drain 16 metres needs a consent.
Fall needs depth. Depth here means rock.
Two quotes for the same Royal Oak kitchen, and they agree on almost everything. Same carcasses, same Melteca doors, near enough the same benchtop. Then you reach the drainage line and the numbers stop describing the same job. You asked for one thing: bring the sink off the window wall into the new island, four metres across a room you walk through daily. Four metres. It cannot be the expensive part of a kitchen. On this ground it usually is, and the reason is under your floor, roughly 28,500 years old.
This is the ground under Royal Oak, Three Kings and the streets running towards Onehunga and Hillsborough. It is not a geotechnical report — the lava is lobed and uneven, and two houses on one street can have different answers. What follows is why the drainage line behaves the way it does, where the consent line sits, and how to get the kitchen you want without paying a drainlayer to fight basalt for two days.
What is actually under your floor
The Auckland volcanic field is roughly 53 volcanoes over about 600 square kilometres, all fed by basaltic magma. Two decide what happens under a kitchen here. Maungakiekie / One Tree Hill erupted roughly 67,000 years ago, and two of its three craters were breached by lava. Those flows went in every direction, many towards Onehunga, covering something like 20 square kilometres — by area the second largest flow field in Auckland behind Rangitoto. Royal Oak sits on its western side.
Te Tātua a Riukiuta — Three Kings — is younger, and dated with unusual confidence: six boreholes drilled through its lava in 2006 brought up charred wood, and carbon dating returned six very similar dates around 28,500 years. Its tuff ring was breached and lava ran three kilometres towards Western Springs, building tunnels as it went. Most collapsed, but sections survive as caves, some reached from private properties in the area.
Then people spent a century and a half digging it up. Commercial scoria quarrying started around Three Kings in the 1870s, Winstone bought in during the 1920s, and by the 1980s the five original cones were down to one. Winstone decommissioned the quarry in 2015; Fletcher is building hundreds of homes in the hole now. The aggregate under Auckland's roads and the rock under your floor are the same material.
Why the drain is the expensive part
A waste pipe has no pump. It runs on gravity, so it must fall continuously from fixture to gully trap to lateral, evenly the whole way. G13, the Building Code clause covering foul water, is blunt: drains are to be laid at an even gradient with no obstructions to flow, and installed at the maximum practicable gradient. The acceptable solution spells out the intent — a pipe should not be laid at 1:80 if it can practicably be laid at 1:60.
Now the arithmetic that reprices your kitchen. A gradient of 1:60 drops about 17 mm per metre. Push the sink four metres further from the gully and the run needs roughly another 70 mm of depth. Six metres and you are hunting for 100 mm. Your floor level is fixed. The gully invert is fixed. So the trench goes down, and down here is basalt.
| Gradient | Drop per metre | Extra depth over 6 m | What G13 says |
|---|---|---|---|
| 1:40 | 25 mm | 150 mm | Comfortable fall; up to 255 discharge units |
| 1:60 | About 17 mm | 100 mm | The sensible default; up to 205 units |
| 1:80 | 12.5 mm | 75 mm | Up to 149 units; levels need a verifiable levelling device |
| 1:100 | 10 mm | 60 mm | Up to 122 units; only where steeper is impractical |
| 1:120 | About 8 mm | 50 mm | Flattest for 100 mm; no margin for a sag |
You can flatten the grade, and people do. A 100 mm branch still carries a domestic kitchen at 1:120 on paper. But G13 asks for the maximum practicable gradient, so flattening purely to dodge rock is a call your drainlayer must defend — and at 1:80 or flatter, levels must be set with a verifiable levelling device, because a sag of a few millimetres becomes a blockage. Flat drains are a trade, not a saving.
Rock does not care about your programme
Breaking basalt is not a line item anyone can fix-price honestly. It is a breaker or a jackhammer, charged by the hour because the hour is the only unit that means anything, and nobody knows how many before the first cut. The flow changes over a few metres: a dense sheet that takes all day, rubble that comes out like gravel, or a void where a tunnel ran. A lava tube saves you nothing — the drainlayer must bed a pipe over something that will not hold it, and G13 requires rigid pipes to be jointed flexibly against differential settlement.
Then there is what sits above the rock. A 1930s Royal Oak bungalow on piles gives you a subfloor void to work in, which is why the same four-metre move can be reasonable in a bungalow and brutal next door. Slab-on-ground — most recent building around Three Kings, and the townhouse stock running down through Onehunga and Penrose — means saw-cutting the floor, digging under it, then meeting whatever is below. Much of what we do in the bungalows one suburb over applies here, because the bungalow belt and the lava field overlap almost exactly.
The four moves, ranked by pain
Almost every kitchen conversation here reduces to one of four moves. The gap between the second and the third is where most of the money quietly disappears.
| The move | What happens under the floor | Rock exposure | Consent position |
|---|---|---|---|
| Stays on its existing run | New tap and tail, reuse the trap; gully untouched | None | Nothing to think about |
| Slides along the same wall | Re-pitch a short branch; gully may shift a short distance | Low | Often inside the Schedule 1 exemptions |
| Crosses to the opposite wall | New branch across the room holding fall; gully probably moves | Real — depth grows every metre | Ask first; may be past minor |
| Lands on a new island | Break the slab, trench under it, hold fall back to the gully, vent past 10 m | Highest | Assume a consent conversation |
Those are patterns, not a ruling on your house. Each turns on your building and your council's read of it, so confirm before anyone orders a carcass.
Nobody prices a rock job off a plan. I can tell you what the first hour looks like. After that the ground tells us, and anyone who says otherwise is guessing with your money.
Four ways to move a sink. Two are cheap.
Where the consent line sits
Schedule 1 of the Building Act 2004 decides this. Clause 34 covers minor alteration to drains, and the wording is short enough to quote: alteration to drains for a dwelling if the alteration is of a minor nature, for example, shifting a gully trap. Dwellings only, and an authorised person must do it. MBIE's guidance gives the example that matters: a homeowner has the plumber relocate the kitchen sink to an adjacent wall, and the gully trap shifts a short distance to receive the discharge. That sits inside the exemption. Moving a sink is not automatically a consent job.
Clause 35 covers alteration to existing sanitary plumbing, and it holds as long as the total number of sanitary fixtures is not increased and no specified system is affected. Add a second sink in a new scullery and the count rises, so the exemption stops applying — worth knowing before you fall for a scullery whose drainage nobody has costed.
The far end is drawn just as clearly. MBIE's worked example of drainage that does need consent is a branch drain extended 16 metres: past 10 metres triggers venting under G13, and length and venting together are more than minor. Where your job falls is a conversation with Auckland Council, not your kitchen company, and MBIE's advice when unsure is to seek an exemption or apply for consent rather than risk applying it wrongly. Two things are not optional: drainlaying is restricted work, so a licensed practitioner must do or supervise it, and pipework that could affect the structural performance of joists or framing can pull a consent in on its own.
What we would spend it on instead
Nobody visiting your house will notice the sink is under the window rather than in the island. They will notice a benchtop gone dull along the front edge, drawers that bang, a bin you bend double to reach, and nowhere to put the toaster. Drainage is the only part of a kitchen budget that is invisible when finished and miserable while it happens.
So we design around the drain. Keep the sink on its run and the island becomes what it should have been anyway: prep space, seating, storage. Getting the island sizing and clearances right does more for how the room works than the sink's postcode ever will, and once you stop buying trench the same money stretches across the whole kitchen. Sometimes the sink genuinely must move. Fine — move it the cheap way: along the wall it sits on, or towards a wall the branch already runs to.
What goes wrong
The failures are predictable, which is the good news. The quote gets accepted with a provisional sum against drainage and nobody reads the word provisional. The drainlayer hits rock on day one, the number moves, and the homeowner feels ambushed by a variation disclosed in writing three weeks earlier. If the drainage line says provisional, ask what triggers it and what the hourly rate is.
The sink gets moved on the drawing before anyone has found the gully trap. This is the big one. The gully is a physical object at a physical level, and it anchors the design. We find it at site measure, which is a different exercise from measuring off a plan — by then the drawings are signed and everyone is committed to an island. Close behind: someone flattens the grade to dodge rock and says nothing. It passes. Then in four years the run blocks every winter because of a sag nobody can see.
The programme forgets the drainlayer is on the critical path — a kitchen installs over five to seven days, and two unexpected days of rock breaking reschedules every trade behind it. The saddest one: the homeowner pays for the move, gets the island sink, then finds the dishwasher stayed on the old run because nobody costed a second waste line. Two wet zones in a kitchen designed for one. Ask where the dishwasher goes before you sign.
- Where is the gully trap, and what level is the top of the dish? If nobody can answer, the drainage number is a guess.
- Is the drainage line fixed, provisional, or excluded? Get it in writing, with the hourly rate for rock.
- Has anyone been under the floor, or is this priced off the plan? A subfloor void and a slab are different jobs.
- How long is the new branch, and does it pass 10 metres? That is the G13 venting threshold.
- Are we adding any sanitary fixture — a scullery sink, a second dishwasher? One moves the job outside clause 35.
- Who is laying the drain, and are they registered? Exempt work stops being exempt when the wrong person does it.
Frequently asked questions
Why is moving my kitchen sink four metres so expensive in Royal Oak when my cousin in Hamilton paid almost nothing?
Your cousin's drainlayer was digging soil. Yours is quite possibly digging basalt from the Maungakiekie and Three Kings flows, which cover much of this part of the isthmus. Drains fall at a fixed rate — about 17 mm per metre at 1:60 — so distance from the gully becomes depth, and depth here becomes hours of rock breaking charged by the hour. The cabinets cost the same in both cities; the ground does not.
Do I need a building consent to move a kitchen sink in Auckland?
Often not, but it depends how far it moves and what else changes. Schedule 1 exempts minor alterations to drains for a dwelling — MBIE's example is relocating a sink to an adjacent wall and shifting the gully trap a short distance — and exempts alterations to sanitary plumbing provided the fixture count does not increase and no specified system is affected. It stops being exempt if the work is more than minor, an unauthorised person does it, or it makes a new connection to the network utility operator's service. Confirm with Auckland Council or your LBP before ordering anything.
Can the drainlayer just lay the pipe flatter so we do not dig into the rock?
Sometimes, within limits, and never free. G13 sets minimum gradients against discharge unit loading, and a 100 mm domestic branch has room to flatten. But the acceptable solution asks for the maximum practicable gradient, so flattening purely to avoid rock is a call your drainlayer must justify. At 1:80 or flatter, levels must be set with a verifiable levelling device, because a small sag becomes a permanent blockage.
We are on a concrete slab near Three Kings. Is an island sink off the table?
No, but price it as its own project rather than a detail of the kitchen. A slab job means saw-cutting the floor, trenching under it, holding fall back to the gully, and meeting whatever rock is underneath — plus venting if the branch passes 10 metres. The question is whether it beats what the same money buys across the benchtop, hardware and storage. Usually it does not.
Could there really be a lava cave under my house?
Around here it is not far-fetched. The Three Kings flows built tunnels running towards Western Springs; most have collapsed, but sections survive as caves and some are reached from private properties in the area. For a kitchen a void matters as a bedding problem, because a drain needs continuous even support and G13 requires rigid pipes to have flexible joints against differential settlement. If your drainlayer finds a void, let them redesign rather than backfill and hope.
Getting a number that means something
Send us the scope and we will send a trade-priced number back inside 24 hours. On this ground the useful thing to send is not a mood board. It is where the sink is now, where you want it, whether you are on piles or a slab, and a photo of the gully trap if you can find it in the garden. That is enough to tell you the honest thing on the phone: keep it, move it a little, or budget properly for moving it a lot.
We build in our own workshop in East Tamaki, supply and install under one contract and one invoice, and turn out ten-plus kitchens a week — so what you get is what the work costs us, not a showroom's margin. Twenty-three years and more than two thousand kitchens means we have had this conversation on this rock more times than we would like to count. Before you sign, know which lines on a kitchen quote hide the surprises — on the isthmus, the surprise is almost always underneath you.