Quick answer
Under the granny flats exemption, hot water must come from a controlled heat source. That rules out a wetback off a wood burner and a solar thermosiphon, and no pumped systems are allowed inside the building. Three options remain: a mains-pressure electric cylinder indoors, a gas continuous flow unit on an outside wall, or a heat pump cylinder outdoors. A 180-litre mains-pressure unit from a mainstream New Zealand range runs roughly 490mm across and 1.7m tall, or 580mm across and 1.16m tall. It will not go under a benchtop, and it will not fit inside a standard 600mm cabinet. It needs a full-height cupboard on the floor, decided before the joinery is drawn: in 70 square metres, the only spare bay is usually the pantry.
Key points
- The exemption bans uncontrolled water heating, so no wetback and no solar thermosiphon, and a solid fuel heater needs a consent anyway.
- It also bans pumped systems inside the building on network-connected sites, so an internal macerator takes the build out of the exemption.
- A mains-pressure cylinder is the default, and at 490–580mm across and up to 1.8m tall it needs a full-height cupboard.
- Putting the heat outside, as gas continuous flow or an outdoor heat pump cylinder, is the only way to get that bay back.
- Boiling taps are not named either way, so run the model past your plumber and get it in writing before you order.
The exemption picks your hot water system for you.
A client rings with a 70 square metre plan for the back of a section off Great South Road. Two bedrooms, a bathroom, a laundry cupboard, a galley kitchen along the north wall. Three and a half linear metres, tall pantry at the blank end. The builder is an LBP, the PIM is lodged, and it goes up under the granny flats exemption with no consent of any kind. Then the plumber asks what nobody has asked: where is the cylinder going? There is one place left. It is the pantry.
That keeps happening because the exemption is quietly prescriptive about hot water, and people read the headline (70 square metres, no consent, in force since 15 January 2026) without the plumbing conditions underneath it. General information follows, not a design: your registered plumber and the supervising LBP make the calls that count, and MBIE's guidance and conditions checklist are the documents to read before you order anything.
What the exemption actually says about water heating
MBIE publishes a building design conditions checklist. Most of it is predictable: single storey with no mezzanine, detached, self-contained, 70 square metres or less, four metres maximum height, at least two metres off other buildings and boundaries, light framing, caps on cladding weight. Then comes a section headed additional plumbing and drainage criteria. That is the one that reaches into your kitchen.
- Total fixture units must not exceed 30.
- No pumped systems inside the building.
- The main drain must be at least DN100 at a minimum 1:60 grade.
- Branch drains must be at least DN65 at a minimum 1:40 grade.
- Upstream vents must be at least DN65.
- No uncontrolled water heating.
Two things there matter. The water supply must be designed to G12/AS1 or G12/AS3, the only pathways named. And MBIE's guidance notes these additional requirements apply to systems connected to a network utility operator, not to on-site systems. For a back-section granny flat in urban Auckland you are on council water and sewer, so they bite. Out at Kumeu on tank and septic, ask your plumber where you sit.
"Uncontrolled" is a Building Code term, not a vibe
This is where copied content falls over. Uncontrolled sounds like a description; it is a definition. Under G12, a controlled heat source has controls keeping the water in the storage tank no greater than 90 degrees. Uncontrolled is the opposite, and the guidance names two examples every time: a wetback on a solid fuel appliance, and a solar thermosiphon. G12/AS1 states the consequence plainly: untempered hot water must not be provided to fixtures or appliances from a storage water heater connected to an uncontrolled heat source, such as a wet-back or solar water heater.
So the wetback dies twice. It is an uncontrolled heat source, and MBIE's guidance separately states that installing a solid fuel heater requires a building consent. Put a wood burner with a wetback on the plan and you do not have an exempt build. You have a consent application nobody has filled in.
Then the cylinder has to go somewhere
Rule out the wetback and the thermosiphon and the Auckland default is a mains-pressure electric cylinder. Thermostatically controlled, boring, compliant. Also physically large, which is the part that gets discovered late. Published dimensions from a mainstream New Zealand mains-pressure range make the point better than arguing does.
| Storage | Diameter | Height | Empty | Fits a 600mm cabinet? |
|---|---|---|---|---|
| 135 L (tall) | 490 mm | 1,325 mm | 49 kg | Width yes, height no. Tall cupboard. |
| 135 L (stubby) | 580 mm | 935 mm | 54 kg | No. Wider than a 600 carcass. |
| 180 L (tall) | 490 mm | 1,710 mm | 60–62 kg | Width yes, height no. Full-height run. |
| 180 L (stubby) | 580 mm | 1,165 mm | 64 kg | No. Wider than a 600 carcass. |
| 250 L | 580 mm | 1,555 mm | 80–82 kg | No, on both counts. |
Run that against the joinery. A standard base cabinet is typically a 720mm carcass on a 150mm kick, which is where the 870mm bench height comes from, and the rest of the standard NZ cabinet dimensions do not help here. Every cylinder above is taller than 870mm. None go under a bench. A 600mm carcass with 18mm sides leaves about 564mm clear, so the 580mm stubbies do not fit at all, and the 490mm units leave roughly 37mm each side before a single fitting. Then weight: 64kg empty, 244kg full. It sits on the slab. Not a cabinet with a cylinder in it. A cylinder with a cupboard built round it, and no shelf underneath.
A cylinder will not fit under a benchtop.
The cupboard holds more than the tank. G12/AS1's mains-pressure detail notes a minimum one metre of copper pipe from the storage water heater to the temperature control device, so the tempering valve sits out on a pipe run and must stay reachable. The relief valve needs a drain to a safe discharge point, which argues for an external wall. Add an access panel: a sealed cupboard is a callback waiting to happen.
The cheapest way to get the bay back is to put the heat outside
If a full-height cupboard is the price of an indoor cylinder, the move is to not have one. Two routes do that. Gas continuous flow units are made to be mounted on, or recessed into, an exterior wall, and the compact models in the Rheem range are rated for roughly one to three people. That is granny-flat shaped. The exemption permits an independent gas supply, so ask your gasfitter early about reticulated gas versus bottles. The other route is a heat pump cylinder, listed in the published New Zealand ranges for outdoor installation and sized for four to six people. That gets the tank outside entirely, though it is a compressor: think about noise near a bedroom window.
| Option | Allowed? | Costs you inside | Watch out for |
|---|---|---|---|
| Mains-pressure cylinder, indoors | Yes. Controlled source. | A full-height 600mm bay | Relief drain, valve access, floor loading |
| Gas continuous flow, external wall | Yes, with independent gas | Nothing | Gas to the site, bottle location, flue |
| Heat pump cylinder, outdoors | Yes. Thermostatically controlled. | Nothing | Noise, siting, cold-morning recovery |
| Gas storage cylinder, indoors | Yes, with independent gas | A bay, plus a flue | Some models natural gas only |
| Wetback off a wood burner | No, twice over | n/a | Uncontrolled source; burner needs consent |
| Solar thermosiphon | No | n/a | Uncontrolled source; G12/AS2 not a pathway |
| Macerator or booster pump inside | No | n/a | Sloping sites, marginal gravity fall |
Does a heat pump water heater count as a pumped system? A heat pump is a refrigeration circuit, not a plumbing pump, and the outdoor units sit outside anyway, so the condition reads as aimed elsewhere. Sensible interpretation, not a ruling, and your plumber carries it. Trivial to settle at design stage, expensive once the slab is down, which is the argument for coordinating the trades properly.
Can you actually have a boiling tap?
Probably. Nobody has published a ruling either way and I am not inventing one. The condition says no uncontrolled water heating. A boiling tap's under-bench tank runs near boiling by design. G12 defines a controlled heat source as one whose controls keep the storage tank no greater than 90 degrees. Read that literally and out of context and it looks awkward.
Read it in context and it looks fine. The condition sits in a list about water supply, sanitary plumbing and drainage, and it targets the wetback and thermosiphon feeding a storage water heater. A boiling tap is a thermostatically controlled drinking-water appliance on its own cold feed. Most plumbers read it that way. But the record of work is signed by your plumber and the supervising LBP. If they will not sign, you do not have an exemption. You have a consent application. Ask about the model and get it in writing before the order goes in. The fallback is a mixer and a kettle.
The physical side needs no hedging: it is just measuring. The under-bench unit wants a ventilated cupboard, and Zip specifies an ambient operating range inside the cabinet plus air gaps around it. It needs a power point and filter access. That is the sink bay, which already holds a waste trap, isolating valves and the bin. A real trade against kitchen storage that actually gets used.
Every granny flat I've plumbed this year, the cylinder cupboard was an argument. Not because it's hard. Because nobody drew it, and by the time I'm on site the joinery's already made.
What goes wrong
None of these are hypothetical. Same handful of mistakes, roughly the same order, job after job.
- The cylinder is found after the joinery is ordered. Cabinets are cut to a schedule, so a 600 bay that becomes a cylinder cupboard is a remake, not an adjustment.
- It gets shoved into a bedroom wardrobe instead. Legal, common, quietly hated by the tenant. Still a quarter of a tonne full, still needs a relief drain.
- Someone specs a wetback because the plan has a wood burner. Two conditions gone at once, unnoticed until the completion notification will not add up.
- The site falls the wrong way and someone reaches for a macerator. On a Titirangi slope, DN100 at 1:60 constrains where the building sits.
- The cupboard falls between contracts. Builder thinks it is kitchen joinery, joiner thinks it is a builder's box, priced by nobody. Worth knowing what a quote actually includes.
What to ask before you sign
- Where is the cylinder on the plan, at scale, with the valves and relief drain?
- Is the water heater a controlled heat source, confirmed in writing?
- Can the heat go outside instead, and what does that free up?
- Does gravity drainage work at DN100 and 1:60 with no pump inside?
- What is the fixture unit count, and who counted it?
- Boiling tap: which model, cupboard size, ventilation, power point, plumber sign-off?
- Who builds the cylinder cupboard, and which contract is it in?
- If it will be rented, does the extraction spec stack up against Healthy Homes too?
Send us the plan with the cylinder already on it
If you have a 70 square metre plan with a kitchen on it, send it through with the cylinder question already asked. Or send it and say you do not know yet, which is more useful, because that is the conversation worth having while the plan is still a drawing. Better now than on the day the tall unit arrives 600mm too narrow. We are a manufacturer, not a showroom: our own workshop in East Tamaki, 23 years, 2,000-plus kitchens, ten-plus a week out the door.
Supply and install under one contract and one invoice, trade pricing, no showroom overhead in the number. The person pricing your job is the person whose team installs it over five to seven days. A rough scope is enough to start, drawings sharpen it, and the quote comes back inside 24 hours. Getting a compact layout right is mostly deciding what the small kitchen will not have, and the cylinder is the one item you do not get a vote on.
Frequently asked questions
Can I put a hot water cylinder under the kitchen bench in a granny flat?
Practically, no. Mains-pressure cylinders in a mainstream New Zealand range start around 935mm tall for a 135-litre stubby and run past 1.7m for a 180-litre tall unit, while a standard base cabinet gives about 720mm of carcass under an 870mm benchtop. The 580mm models are also wider than the clear width inside a 600mm cabinet. It needs a full-height cupboard on the floor: full, it weighs around 244kg.
Does the granny flat exemption ban solar hot water?
Not in those words, but it effectively rules it out. The exemption bans uncontrolled water heating, and a solar thermosiphon is an uncontrolled heat source under G12. The only water supply pathways it names are G12/AS1 and G12/AS3, while the acceptable solution for solar water heaters is G12/AS2. Solar hot water is still legal in New Zealand: wanting it just means applying for a building consent.
Can I have a boiling water tap in a granny flat kitchen?
Most likely yes, but nobody has published a ruling either way. A boiling tap is a thermostatically controlled drinking-water appliance on its own cold feed, rather than a storage water heater on an uncontrolled heat source, which is what the condition targets. Your plumber and the supervising LBP sign the paperwork that goes to council, so ask about the specific model before you order and get it in writing.
Why can't I use a macerator or a pump if the site falls away from the sewer?
The exemption's plumbing criteria for network-connected systems include no pumped systems inside the building, alongside a main drain of at least DN100 at 1:60. On a sloping Auckland site that constrains where the building goes, rather than being a detail you solve at the end. If gravity fall does not work, you regrade, move the building, or get a consent. MBIE's guidance says these criteria do not apply to on-site systems, so check which you are in.
Does the kitchen tap have to be tempered to 50 degrees like the shower?
G12/AS1 caps delivered hot water at 50 degrees at the outlet of sanitary fixtures used for personal hygiene: showers, baths, hand basins and bidets. The kitchen sink is not on that list, which is why the kitchen tap runs hotter. Separately, the storage cylinder's thermostat must be set at not less than 60 degrees to control Legionella. That is why a tempering valve sits between the tank and the bathroom.