Quick answer
The granny flats building consent exemption caps a small standalone dwelling at 30 fixture units across the whole building, and a kitchen barely dents that. Under Table 2 of G13/AS1 a domestic kitchen sink rates 3 discharge units — single bowl or double, with or without a waste disposal unit — and a dishwasher adds 3 more. A realistic one-bedroom 70m² flat lands near 18, so the cap is almost never what stops you. The condition that breaks kitchen layouts is the ban on pumped systems inside the building: an island sink needing a macerator or waste pump to reach the drain puts the build outside the exemption and back into a building consent.
Key points
- The exemption allows a maximum of 30 fixture units across the entire dwelling, and the numbers come from Table 2 of G13/AS1 — the table your drainlayer already works from.
- A domestic kitchen sink is 3 discharge units whether it is a single bowl or a double, and a waste disposal unit adds nothing to the rating.
- A one-bedroom 70m² flat with a dishwasher, one bathroom and a laundry totals about 18 fixture units, leaving genuine headroom under the cap.
- You can only pass 30 by stacking a second bathroom, a bath and a scullery sink into 70m², at which point floor area gives out before drainage does.
- The rule that actually kills kitchen layouts is "no pumped systems inside the building", because an island sink without fall to the main has nowhere to go.
A typical granny flat spends 18 of its 30 fixture units.
Picture a 70m² flat going up on the back section of a Manurewa property. The slab is booked for Thursday. The plans show a tidy island with the sink in it, because the owner saw one on a home show and the designer drew what was asked. Nobody has asked the drainlayer where that island's waste is going to go. That question — not the fixture unit count — decides whether this build stays consent-free.
This covers the plumbing and drainage conditions attached to the granny flats exemption that took effect on 15 January 2026, and what they mean for the kitchen. It is not drainage design. Your registered plumber and drainlayer own that, and they sign a Record of Work for it. Read it so you can talk to them before the layout is locked, not the week after the concrete truck has been.
What a fixture unit actually measures
A fixture unit measures the load a sanitary fixture puts on the drainage system — how much water it uses, how often, and for how long. Add them up and you have the total demand the dwelling places on whatever it discharges into. It is not pipe size, and it is not litres.
That condition is blunt: no more than 30 fixture units across the dwelling. MBIE's stated reason is to keep the fixture count low enough that the load on the outfall system stays manageable. The cap protects the council's sewer and keeps these builds simple. It is not a calculation about your own drain.
Fixture units are formally defined in AS/NZS 3500.0:2021. The guidance gives you a more practical door in: a design using G13/AS1 and G13/AS2 meets the requirement by limiting discharge to 30 discharge units as defined in Table 2 of G13/AS1. That table is short, public, and it is your budget sheet. The third column below is the minimum trap and discharge pipe diameter — a separate requirement from the unit count.
What each fixture actually costs you
| Fixture or appliance | Discharge units | Min trap and discharge pipe (mm) |
|---|---|---|
| Kitchen sink (domestic, single or double, with or without waste disposal unit) | 3 | 40 |
| Dishwashing machine (domestic) | 3 | 40 |
| Water closet pan | 4 | 80 |
| Shower | 2 | 40 |
| Bath (with or without overhead shower) | 4 | 40 |
| Basin | 1 | 32 |
| Laundry (single or double tub, with or without a clothes washing machine) | 5 | 40 |
| Clothes washing machine (domestic) | 5 | 40 |
| Bathroom group (WC pan, bath and shower, basin and bidet in one compartment) | 6 | Sized per fixture |
Two rows deserve a stare. The laundry is the most expensive thing in the building at 5 units — more than the WC pan, more than sink and dishwasher combined. And the bathroom group rating of 6 is far cheaper than itemising the same fixtures, which comes to 12. Whether your bathroom qualifies as a group is your drainlayer's call, not a kitchen company's, but ask it if you are near the line.
The double bowl is free, and so is the insinkerator
Here is where most granny flat advice goes wrong. Table 2 rates a domestic kitchen sink at 3 discharge units and spells out the scope in the description itself: "single or double, with or without waste disposal unit". Single and double are the same number. A waste disposal unit is the same number again.
So the advice that you must drop to a single bowl to protect your fixture unit budget is not what the table says. Specify the double if the kitchen wants a double. In 70m² the honest argument against one is bench real estate, not drainage — a double eats most of a 900mm cabinet run you cannot spare. The logic behind standard cabinet sizing in a townhouse kitchen bites long before the drainage rules do.
Where a kitchen does add load is the second sink. A scullery sink is another 3 units, because Table 2 sees another kitchen sink. Put a scullery behind the main bench in a 70m² flat and you are spending floor area and fixture units on it. At this size that is usually the wrong trade.
Doing the sums on a real 70m² flat
Numbers beat opinions. Here is a straightforward one-bedroom flat, every fixture itemised off Table 2.
| Fixture | Discharge units | Running total |
|---|---|---|
| Kitchen sink (single or double) | 3 | 3 |
| Dishwasher (domestic) | 3 | 6 |
| Water closet pan | 4 | 10 |
| Shower | 2 | 12 |
| Basin | 1 | 13 |
| Laundry tub with clothes washing machine | 5 | 18 |
| Remaining headroom to the cap | — | 12 spare |
Eighteen. A complete, sensible dwelling sits at a bit over half the allowance. Now try to break it. Add a second bathroom — WC pan, shower, basin — and you are at 25. Add a scullery sink, 28. Swap the main shower for a bath and you hit 30, exactly on the line. That is 70m² with two bathrooms, a bath, a scullery and a laundry, still not over.
In a dwelling capped at 70m², floor area runs out long before fixture units do. The 30 unit limit is a backstop against someone registering a boarding house as a granny flat. If a supplier tells you they have value-engineered your sink to protect your fixture unit budget, they are solving a problem you do not have.
The rule that actually breaks kitchens
In the same list of conditions, with no more emphasis than the cap, sits this: no pumped systems inside the building. Read plainly, everything leaves under gravity. No macerator behind the WC. No waste pump under the sink. No greywater pump in the cabinet. If waste needs help getting out, the exemption is gone and you are applying for a consent.
For a kitchen that points at one thing: the island. An island sink on a slab-on-ground has to get waste out horizontally, under the floor, with fall the whole way to the main. That pipework is set out and cast in before the pour. If the island moved 600mm during design, or arrived after the slab went down, the conventional fix is a pump — and the pump is exactly what you cannot have. Sizing and placing a kitchen island gains a hard constraint here, and you cannot design around it afterwards.
One nuance trips people up. The guidance is explicit that these additional requirements apply to the dwelling's water supply, sanitary plumbing and drainage but do not apply to on-site systems. A pump forming part of a septic tank out in the yard is a different question from a pump inside the dwelling. That distinction is your drainlayer's to apply — it is not an invitation to sneak a macerator under the sink.
Grades, falls, and why the slab decides your layout
Geometry is pinned down too. The main drain must be no less than DN100 at 1:60. Branch drains no less than DN65 at 1:40. Upstream vents connecting to any branch or main drain no less than DN65. These are conditions, not recommendations you can trade against each other.
Convert the grades to millimetres and the design problem appears. A 1:40 branch falls 25mm per metre. A 1:60 main falls about 16.7mm per metre. An island sink four and a half metres from the nearest wall needs roughly 113mm of fall on its branch alone — before it reaches the main, before trap depth, before the main starts falling to the boundary. On a floor level capped at one metre above supporting ground, that vertical budget is not generous.
Here is the irony. Under G13/AS2 a DN100 drain at 1:60 carries 205 discharge units. Your dwelling is capped at 30. The pipe you must lay has nearly seven times the capacity you are allowed to use. Hydraulically nothing here is close to marginal. The geometry is — getting fall from the middle of a slab out to a boundary — and that is decided by the set-out, not the pipe.
So sequencing is the whole game. The kitchen layout has to be fixed before the drainage set-out, and the set-out before the pour. That is far earlier than most people expect to commit to a kitchen. Same discipline as measuring off the slab rather than the plan, except the plan has to be right first, because concrete records your decision permanently.
Fixture units have never once stopped a job for me. The pump is what stops it — and you find out the day the drainlayer walks the slab, with the cabinets already loaded in the van.
A pump inside the building ends the exemption.
What goes wrong
Most failures here are not rule breaches at all — they are sequencing. The kitchen gets treated as a late decision and the slab goes down set out for a wall sink. Then the owner falls in love with an island. On a consented build that is a variation. Here it is a pump, and a pump is a consent. Get the drainlayer and the kitchen designer talking in the same week — the coordination that keeps trades from colliding on an install.
Imported advice is the second trap. Much of what circulates about granny flat plumbing is Australian, or written against the old minor dwelling rules, or a guess in confident language. The double bowl myth is the tell — repeated everywhere, flatly contradicted by Table 2. Check anything you read, including this, against the current MBIE guidance and checklist.
Third, the cap is a whole-of-building test and people forget it. Nobody counts the laundry, and the laundry is 5 units. Nobody counts the second bathroom until it is drawn. The kitchen gets blamed for the total because it is specified last, when sink and dishwasher together are 6 of the 30 and never moved.
The last one catches people at the end. This exemption is not finished when the building is. Apply for a PIM before you start; complete within two years of it being issued or it lapses; submit final plans, Records of Work, Certificates of Work and electrical and gas safety certificates within 20 working days of completion. Your plumber or drainlayer must give you a Record of Work under section 27A of the Plumbers, Gasfitters, and Drainlayers Act 2006. Until every record is in hand, the building is not complete. Development contributions still apply. If it will be rented, Healthy Homes obligations on kitchen ventilation are a separate test on top.
What to ask before you sign
- Ask your drainlayer to total the fixtures off G13/AS1 Table 2 and write the number on the concept plan, before anything else is decided.
- Ask whether the kitchen sink runs to the main entirely under gravity, at DN65 and 1:40 minimum, with no pump anywhere inside the building.
- If an island sink is drawn, ask how much fall is available to the main, and confirm the branch is in the slab set-out before the pour date.
- Ask whether the bathroom qualifies for the Table 2 bathroom group rating or must be itemised, if you are near the cap.
- Confirm your plumber and drainlayer are registered and will issue a Record of Work — without it the building is not complete.
- Ask who produces the as-built sanitary plumbing and drainage plans that go to council with your final plans.
- Check no level entry or wet-floor shower has crept in, because it voids the exemption regardless of your fixture count.
- Ask your kitchen supplier to commit to a layout before the slab, and to quote supply and install so nobody argues about the sink cut-out later.
Frequently asked questions
Does a double bowl sink use more fixture units than a single bowl in a granny flat?
No. Table 2 of G13/AS1 rates a domestic kitchen sink at 3 discharge units and defines it as "single or double, with or without waste disposal unit". A double and a single are identical against the 30 fixture unit cap. The real argument against a double in a 70m² flat is the bench and cabinet space it eats, not drainage.
Can I have an island sink in a granny flat under the building consent exemption?
Yes, provided the waste reaches the main entirely by gravity at the required minimum sizes and grades, and the branch is set out and cast into the slab before the pour. What you cannot do is use a macerator or waste pump to make it work, because the exemption prohibits pumped systems inside the building. That makes the island a decision you take before the concrete, not after.
How many fixture units is a dishwasher in New Zealand?
A domestic dishwashing machine rates 3 discharge units under Table 2 of G13/AS1, with a minimum trap and discharge pipe diameter of 40mm. With the kitchen sink's 3, a standard granny flat kitchen accounts for 6 of your 30 units. The bathroom, laundry and any second sink make up the rest, and a typical one-bedroom flat totals around 18.
Does a waste disposal unit affect the fixture unit count or count as a pumped system?
It does not change the count — Table 2 rates a domestic kitchen sink at 3 units "with or without waste disposal unit". A disposal unit grinds food into a gravity drain rather than pumping it, so it differs from the macerator pumps the exemption prohibits inside the building. If you have doubts about a specific product, put it to your registered drainlayer before ordering.
What happens if my granny flat kitchen needs a pump or exceeds 30 fixture units?
Nothing illegal — you simply fall outside the exemption and the work needs a building consent like any other dwelling. That is a cost in time and fees rather than a disaster, and it is avoidable if the layout is settled early. Redesigning so the waste runs under gravity is almost always cheaper, since exceeding 30 units in 70m² takes real effort and a pump usually means the sink is in the wrong place.
Getting a number without the guesswork
If you are building one of these, the kitchen is your smallest compliance problem and the easiest to get wrong through bad timing. The fixture units will be fine. The sink position needs settling before the drainlayer sets out the slab, and that is weeks earlier than anyone plans. Send us the floor plan — even a rough one with the sink roughly where you want it — and we will tell you what it means for the cabinetry and where the layout will fight the drainage.
We manufacture in our own East Tamaki workshop and supply and install under one contract and one invoice, so there is nobody to point at when the sink cut-out misses the waste. Twenty-three years, ten-plus kitchens a week: we can price off a rough scope and have a trade-priced number back inside 24 hours, plus GST. A compact kitchen at this size sits in the lower five figures for a sensible melteca specification and climbs from there for stone and soft-close everything — the ranges are in our Auckland kitchen cost breakdown. Send the plan before the slab, and the drainage question answers itself.