House Extension Cost per square metre
Auckland Cost Guide
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If you are trying to budget a house extension in Auckland, the honest answer is that “cost per square metre” is only a starting point. The build type, site access, foundations, drainage, bathrooms, kitchen work, and how much of the existing house needs reworking all change the number quickly.
As a rough planning guide, small straightforward extensions often start around $90,000 to $140,000, while larger or more complex additions commonly land from $180,000 to $350,000+. Instead of relying on a single blended rate, I prefer to break the job down by extension type and the cost items that usually move the budget.
Typical Auckland extension types and starting cost ranges
Single Storey Extension
Usually the most cost-efficient option. Many Auckland single-storey additions start around $3,500 to $5,500+ per m2 before major kitchen, bathroom, or structural upgrades.
Second-Storey Extension
Higher because of structure, stairs, roof work, and cladding tie-ins. Many projects land around $4,000 to $6,500+ per m2.
Basement Conversion
Often the least predictable because excavation, retaining, drainage, and waterproofing can dominate the budget. Allow from roughly $4,500 to $8,000+ per m2 on complex sites.
Garage Conversion
Can be one of the lower-cost ways to add habitable space, but insulation, slab treatment, windows, and compliance work still matter. Many projects start around $2,500 to $4,500+ per m2.
What usually adds cost to an extension
The biggest budget movers are usually consultant and consent fees, foundation or structural strengthening, drainage and stormwater work, plumbing, electrical, roofing and cladding tie-ins, interior fit-out, and any bespoke joinery. Bathrooms and kitchens cost more per square metre than bedrooms or living rooms because they stack labour, waterproofing, tiling, fittings, and services into a smaller footprint.
Site access matters more than most owners expect. Narrow driveways, steep sites, retaining, restricted storage space, and difficult spoil removal can add real cost before the visible building work even starts. If the job touches the existing kitchen, roof, or load-bearing walls, you should also expect some temporary works and making-good costs around the original house.
For budgeting, I usually tell clients to hold a contingency of at least 10 to 15 percent. On renovation work, there is always a chance we uncover hidden framing issues, drainage problems, or scope changes once the house is opened up.
Talk through your extension budget before you commit
If you want a realistic extension budget, the best next step is to price your house, your site, and your brief together rather than lean on a generic online calculator. We can walk you through the likely cost drivers, the scope worth prioritising first, and whether a staged approach makes better financial sense.
Get in touch with Renovation Works and we’ll help you understand what is achievable before you spend money on the wrong design path.