A well-planned wastewater budget starts with a suitable solution for the property and a clear understanding of what is included—from design and equipment through to installation and ongoing care.
For a granny flat or transportable home, the useful comparison is the complete arrangement, not just the price of a toilet or treatment unit.
This guide offers a practical way to compare proposals. It does not prescribe one budget for every property or assume that one technology is always the least expensive. Before comparing equipment, ask the project team to establish which arrangements are suitable for the proposed home. The starting point might be an available sewer connection, a suitable existing on-site system, an upgrade or a new on-site arrangement. Under the granny-flat exemption, the relevant servicing conditions must be satisfied; the chosen product does not determine the approval pathway by itself.[1] Our recommendation is to settle that starting point first. It gives suppliers a common basis for their proposals and helps you compare options that could genuinely work for the project. An equipment quotation and a complete installed proposal describe different scopes. Neither is inherently better: the important point is to know which one you are reviewing. A product supplier may be quoting the toilet, treatment equipment or related accessories. Other parts of the project may be supplied by a designer, builder, authorised plumber, drainlayer or earthworks contractor. Ask each supplier to identify the work included, the work excluded and the information still needed before a firm price can be given. Include freight, GST and commissioning in that conversation. The following comparison headings can help bring the separate proposals together.Begin with the servicing options available to the property
Separate equipment supply from the complete installation
| Part of the project | What to ask suppliers to identify |
|---|---|
| Assessment and design | Site investigation, design assumptions, drawings, professional input and the approval or exemption work included. |
| Equipment | The named models, rated application, chambers or tanks, ventilation, filters, pumps, controls and accessories included. |
| Site installation | Delivery, access, excavation, external drainage, connections, electrical work, land application and reinstatement, as applicable. |
| Commissioning and handover | Testing, adjustments, owner instruction, operating manuals, installation records and the party responsible for completing them. |
| Operation and maintenance | Manufacturer-specified tasks, consumables, servicing, residual handling, power use where relevant and replacement components. |
| Price basis and support | GST, freight, quotation validity, assumptions, allowances, warranty conditions and the support available after installation. |
Make useful existing infrastructure part of the assessment
An existing tank or pipe is a starting point for investigation, not a complete specification for the new dwelling.
Ask the wastewater professional to identify what is suitable to retain and what needs attention. MBIE’s on-site sewage guidance treats both the treatment equipment and the receiving land as parts of the system.[2]
For purchasing purposes, request a clear distinction between retained infrastructure and new work. This makes it easier to understand what the proposal provides and what the project team has relied on.
A new system should be compared against the assessed upgrade option—not against an assumption that the existing arrangement can simply take another household.
Compare waterless sanitation as a complete option
A waterless composting toilet can support a water-conscious design by removing toilet flushing demand. MBIE identifies waterless toilets as one option for reducing loading on an on-site sewage system.[3]
To assess its place in the budget, include the selected toilet’s installation requirements and the arrangement for the remaining household wastewater.
That means discussing the shower, basin, laundry and kitchen flows, rather than comparing a composting toilet price against the price of a complete all-wastewater system.
WCTNZ®’s greywater guidance distinguishes diversion, treatment and intended reuse or discharge. Ask the designer and supplier to explain which arrangement is proposed and why it suits the property.[4]
The useful question is not “Will this technology always be cheaper?” It is “What does this option provide for our property, and what is the complete scope?”
Match the specification to the home you intend to use
Give suppliers the same information about occupancy, regular visitors, full-time or occasional use and the fixtures included in the home.
Ask for the capacity and installation requirements of each named product. Have the project team confirm that the proposed layout provides the necessary access, ventilation and servicing space.
Where a supplier recommends additional capacity, treatment or controls, ask what requirement it addresses. That explanation allows you to distinguish an essential feature from an optional preference.
Our recommendation is to select the specification that fits the agreed design brief, with any future-use allowance clearly identified.
A useful value test: can you explain what the proposed system includes, why it suits the property and what the household will need to do to operate it?
Include the way you want to live with the system
Long-term value includes more than an initial purchase price. Ask suppliers to describe the actual owner tasks and the professional servicing requirements for the proposed equipment.
Request the manufacturer’s maintenance schedule and ask about consumables, access, power requirements where relevant, removal of residual material and replacement components. Use the product documentation and the project’s operating conditions rather than a general assumption about all composting or treatment systems.
MBIE’s maintenance guidance emphasises appropriate system use and care. That makes the handover information a useful part of the purchasing decision, not simply paperwork delivered at the end.[3]
A system with more owner involvement may suit one household. Another may prefer a different operating routine or an available service arrangement. Put those preferences into the brief before choosing.
Build a project-specific budget you can compare
Bring the equipment, site works, professional input and operating assumptions into one schedule. Mark each amount as a confirmed quotation, a stated allowance or an item awaiting assessment.
Ask whether prices include GST and freight, which tasks are assigned to other contractors, and how any proposed changes will be approved.
Where an estimate is used, identify its assumptions and have the responsible supplier explain what is needed to confirm it. That is more useful than applying a single national price range to properties with different servicing arrangements.
You can then compare the proposals on a consistent basis and make the final decision with a clearer view of both the installation and its ongoing use.
Start with a clear brief, then select the equipment
For an initial discussion, bring the site location, intended occupancy, available services, a basic floor plan and any existing wastewater information.
WCTNZ® can help with waterless composting toilet and greywater product selection, and the product information needed by your designer and installers. The project-specific design and installation scope should be established by the relevant professionals.
Good planning is about choosing a suitable system with confidence—not simply finding the lowest equipment price or specifying more treatment than the project needs.
Compare suitable options for your project
Share the property information and proposed household use with WCTNZ®. We can help you identify product options and clarify the equipment scope for discussion with your project team.
Discuss your project with WCTNZ®
