Reviewed 28 August 2026 · New Zealand homeowner guidance
An extension is new construction joined to an existing building. The junction, site and services often matter as much as the added floor area.
Why a square-metre rate is only a first filter
Published guides commonly express extension costs by square metre, but small additions may carry a higher rate because design, consent, foundations, site setup and the structural tie-in are spread over less new floor area.
A bedroom addition, a new bathroom and a two-storey extension can have similar footprints but completely different structure, services and finishing requirements.
The existing house controls part of the price
The builder must connect foundations, framing, roofing, cladding, insulation, services and internal finishes to what already exists. Matching discontinued materials or correcting undocumented work can add complexity.
Before pricing, establish the likely structural opening, roof connection, floor levels, drainage route and whether the switchboard or other services need upgrading.
Include the full project cost
An extension budget may need design, engineering, survey information, council fees, construction, utility changes, landscaping reinstatement, temporary accommodation and finance costs. A construction-only figure can therefore understate the funds needed to complete the project.
- Confirm whether GST is included.
- Separate professional and council costs from construction.
- Identify owner-supplied items and work outside the main contract.
- Hold contingency for ground and existing-building uncertainty.
The brief builders need
Provide the intended rooms and sizes, the reason for extending, finish expectations, site constraints and the parts of the existing house being altered. If concept plans exist, issue the same set to every tenderer.
Ask each builder to identify assumptions, exclusions and information still required. That makes the early pricing useful even before the design is fully resolved.
Frequently asked questions
Do house extensions require building consent?
Structural additions generally require building consent, and resource consent or other approvals may also apply. Confirm the actual proposal with the local council and project designers.
Is building up cheaper than building out?
Not automatically. Building up may avoid some land and foundation work but can require substantial strengthening, access and weather protection. Site-specific design is needed before comparing the options.
What should an extension contingency cover?
It should protect against ground conditions, undocumented existing work, service upgrades, tie-in changes and other genuine unknowns. It should not be used to hide selections already expected.
Sources and further reading
Turn the decisions into one renovation brief
Once the broad scope, location, budget and timing are clear, record them together before approaching builders.
Prepare a project brief