Buying a water tank for a unit or duplex isn’t the same job as buying one for a standalone house. Space is tighter, approvals can involve more than one party, and the usual sizing rules don’t always translate cleanly. This guide covers what actually changes, so you’re not stuck applying house-sized advice to a block that doesn’t fit it.
The two property types aren’t identical either. A duplex is usually two separate titles, so each side can often make its own tank decision. A unit block typically means shared common property, which means body corporate involvement before anything gets installed. Sorting out which situation you’re in is the first step, well before you start comparing tank sizes.
Duplexes: One Site, Two Decisions
Because a duplex is usually built as two separate titles, each owner can typically choose their own tank independently. That said, plenty of duplex builds share a single roof structure or a common driveway, and both of those affect where a tank can physically go.
If your duplex was part of a BASIX-assessed build, there’s a detail worth knowing. BASIX allows a single rainwater tank to be nominated as a shared facility across multiple dwellings on one project, rather than requiring a separate tank for each. For the full detail on how that’s calculated, see our BASIX water tank sizing guide.
Units: Check With Body Corporate First
For a unit in a strata scheme, the tank conversation usually starts with body corporate, not with tank size. Common property, shared roof catchment and any changes to a building’s exterior generally need approval before installation. Getting that sign-off early saves reworking a plan later, so it’s worth raising before you’ve settled on a specific tank.
Once approval is in hand, the practical constraint is almost always space. Most units don’t have a backyard to work with, which is exactly why the tank types below tend to dominate this end of the market.
What Size Tank Actually Fits a Unit or Duplex
Standalone-house sizing guidance assumes a full roof catchment and open garden space, and that assumption doesn’t hold for most units and duplexes. As a general guide:
- A unit with a small shared catchment or courtyard: A 1,000–2,000 litre slimline or underdeck tank is a realistic starting point for supplementary garden or outdoor tap use.
- A duplex with its own dedicated roof area: 2,000–5,000 litres, depending on the block’s catchment and whether toilets or laundry are connected.
- A duplex meeting a BASIX shared-tank requirement: Sizing is calculated per project rather than by these general ranges – see the BASIX guide linked above for the full method.
In every case, treat these as a starting point rather than a final figure. Roof catchment, household size and what the tank connects to all shift the right size in either direction.
The Tank Types That Actually Work Here
Three tank types cover almost every unit and duplex scenario, and each solves a different version of the same space problem.
Slimline tanks are the most common choice. A narrow side passage is often the only spot available on a unit or duplex block, and slimline models are built specifically to fit that gap without eating into usable outdoor space.
Underdeck tanks suit a duplex or townhouse with a raised deck or entertaining area. Fitting the tank into that dead space underneath keeps the yard itself completely usable.
Underground tanks work well where a unit or duplex has no visible space to spare at all, and preserving the look of a small courtyard or garden matters more than easy access to the tank itself.
Underdeck tanks suit a duplex or townhouse with a raised deck or entertaining area. Fitting the tank into that dead space underneath keeps the yard itself completely usable.
Underground tanks work well where a unit or duplex has no visible space to spare at all, and preserving the look of a small courtyard or garden matters more than easy access to the tank itself.
If you already know which tank size and colour you want, our slimline tank pricing guide has the full model range and current pricing. We have also written an Underdeck Water Tank Pricing Guide and an Underground Water Tank Pricing Guide.
Plumbing and Shared Wall Considerations
A shared wall or an adjoining roofline changes where pipework can run, and that’s worth checking before ordering rather than after delivery. For duplexes with a shared roof, confirming which side’s guttering feeds which tank avoids a messy retrofit later. For units, any plumbing that touches common property typically needs the same body corporate sign-off as the tank installation itself.
Frequently Asked Questions: Water Tanks for Units and Duplexes
Do I need body corporate approval for a water tank in a unit?
Usually, yes. Common property and shared roof catchment mean most strata schemes require sign-off before installation. Raise it early, before you’ve settled on a specific tank.
Can one water tank serve both sides of a duplex?
Sometimes. If the duplex was part of a BASIX-assessed build, a single shared tank can often be nominated to meet both dwellings’ requirements. See our BASIX sizing guide for how that’s calculated.
What size water tank suits a unit with a small courtyard?
A 1,000–2,000 litre slimline or underdeck tank is a realistic starting point for supplementary use, though the right figure depends on your available catchment.
What’s the best tank type for a narrow duplex side passage?
A slimline tank is the most common fit, since it’s built to fit narrow spaces without reducing usable yard area. Underdeck and underground options suit blocks with even less room to spare.
Where can I see pricing for slimline tanks?
Our slimline tank pricing guide covers the full model range and current pricing once you’ve settled on a size.
Buying a water tank for a unit or duplex? Browse the full tank range or call 1300 826 532 to talk through sizing and approvals for your property.