A house extension is one of the most common — and most worthwhile — residential projects. Done well, it transforms how you live in your home and adds significant value. Done poorly, it blows your budget and creates problems for years. Here's what to expect on cost in Auckland and Northland in 2025.
The quick numbers
| Extension Type | Cost Range (construction only) |
|---|---|
| Basic addition (bedroom, bathroom) | $3,500 – $5,000 per m² |
| Standard living extension (open plan) | $4,000 – $6,500 per m² |
| High-spec or complex extension | $6,500 – $10,000+ per m² |
| Second storey addition | $5,000 – $8,500 per m² |
| Garage conversion (to habitable) | $1,800 – $3,500 per m² |
These are construction costs only — they don't include architecture fees, engineering, council consent, or site-specific costs like demolition, drainage, or retaining.
A useful rule of thumb: budget your total project cost (including design, consent, and construction) at roughly 20–30% more than the raw build cost. So a 40m² extension with a $5,000/m² build rate would be $200,000 in construction — and realistically $240,000–$260,000 all in.
Why is the range so wide?
Build costs in New Zealand vary enormously based on:
Complexity of the work
A simple rectangular addition bolted onto an existing house is straightforward. An addition that requires underpinning the existing foundation, removing load-bearing walls, or building on a sloped site will cost significantly more — both in construction and in the engineering required.
Specification
A standard kitchen in a family extension might cost $25,000. A high-spec kitchen in the same space can cost $80,000–$120,000. The bones of the building are similar — it's the finishes that swing the budget dramatically.
Connection to the existing house
Every connection between old and new is a cost — weatherproofing, matching levels, tying in the roof, removing walls. Projects that connect at multiple points or require extensive work to the existing building cost more than a simple lean-to addition.
Site conditions
Ground conditions, access, and slope all affect cost. An extension on a flat section with easy truck access is simpler than the same project on a steep Auckland hillside where a crane is required.
What about the interior — does renovation cost more?
Renovating an existing part of the house while you're extending is extremely common — and often makes financial sense because you're already disrupting the house anyway. Internal renovation costs vary widely:
- Kitchen renovation: $25,000 – $100,000+
- Bathroom renovation: $18,000 – $50,000
- New internal layout (wall removal, new flooring): $15,000 – $40,000
Is an extension worth it?
Usually, yes — but it depends on your property. In most Auckland suburbs, a well-designed extension that adds usable living space or a bedroom adds more value than it costs. In Northland, the economics are slightly different because property values are lower — so it's worth thinking carefully about whether the money is better spent on the extension or on something else.
The key word is well-designed. A badly designed extension that makes the house feel chopped up or dark, or that ignores the relationship between old and new, doesn't add value and can actually make the property harder to sell.
How to get the most from your budget
- Design first, price second. Don't ask a builder to price before you have a design. You'll get wildly different numbers that are impossible to compare, and you'll make decisions based on inaccurate information.
- Be specific about your brief. "I want to extend the kitchen" is not a brief. "I want a 30m² open-plan kitchen and dining addition with a north-facing deck" is a brief. The more specific your brief, the more accurate your price.
- Get three builder quotes from the same set of drawings. Tendering off the same documentation gives you genuine price competition and apples-to-apples comparison.
- Budget a contingency. 10–15% of the build cost is sensible for a renovation project. Hidden issues in existing buildings are common.
- Decide your priorities. If budget is tight, identify which things are non-negotiable (the extra bedroom, the indoor-outdoor flow) and which can be deferred or simplified (the high-spec kitchen, the fancy cladding).
We've done dozens of extensions across Auckland and Northland. Our first consultation is free — we'll give you a realistic picture of what your specific project might cost before you commit to anything. Get in touch here.