The "$1,800–$2,800 per square metre" advertised by volume project-home builders, and the "$2,800–$4,500 per square metre" quoted for custom builds, is the base build price only — what the builder commits to for slab, frame, roof, cladding, internal linings, plumbing, electrical, cabinetry, flooring, painting and a standard inclusions list. It is not the total project cost. On a flat suburban block with confirmed inclusions, the total typically runs 20–25% above the base build figure. On a sloping block, that gap stretches to 50%, 80%, or more.
Headline base build excludes most of what makes a vacant lot habitable. The list includes: the land itself and stamp duty; site costs (excavation, retaining, drainage, footings beyond a standard slab); service connections (water, sewer, stormwater, power, NBN, gas); council and DA fees, including BASIX, NatHERS and statutory levies; design and consultant fees; selections and upgrades beyond the standard schedule; driveways, fencing, landscaping; bushfire (BAL) compliance; flood-overlay engineering; finance costs during the build; and contingency. None of these are optional. Together they typically add $150,000–$500,000+ to a $500,000 base build, depending on the lot.
Site costs by slope category (typical premiums above a flat block):
| Slope | Approximate added site cost |
|---|---|
| Gentle (under 5%) | $5,000–$15,000 |
| Moderate (5–15%) | $20,000–$50,000 |
| Steep (15–25%) | $50,000–$120,000 |
| Severe (over 25%) | $100,000–$250,000+ |
| BAL rating | Typical added construction cost |
|---|---|
| BAL-LOW or BAL-12.5 | $0–$10,000 |
| BAL-19 | $10,000–$25,000 |
| BAL-29 | $20,000–$45,000 |
| BAL-40 | $40,000–$80,000 |
| BAL-FZ (Flame Zone) | $80,000–$200,000+ |
- Class S (slightly reactive): $2,000–$5,000
- Class M / M-D (moderate): $5,000–$18,000
- Class H1 / H1-D (highly reactive): $12,000–$30,000
- Class H2 / H2-D (very highly reactive): $20,000–$55,000
- Class E / E-D (extremely reactive): $40,000–$90,000
- Class P (problem site): $30,000–$200,000+
Other line items that catch buyers off-guard: retaining walls at $250–$2,500 per square metre depending on material and engineering; sloped driveways with retaining at $200–$400 per square metre; charged stormwater at $3,000–$15,000 (Post 18); on-site detention tanks at $6,000–$25,000; long rural power connections at $10,000–$80,000+ (Post 19); septic or AWTS for unsewered lots at $8,000–$30,000; statutory fees (DA, building permit, long service levy, infrastructure contributions) at $5,000–$60,000+ depending on state and council; and construction-loan interest of $15,000–$20,000 on a $500,000 loan during a typical 12-month build, more for longer builds.
Design and consultant fees add up. On a $600,000–$900,000 custom build, fees typically include architect (registered, AIA) at 8–15% of construction cost; or building designer (BDAA) at 3–7%; town planner $3,000–$15,000; registered surveyor $1,500–$5,000; geotechnical investigation $800–$15,000 depending on depth; structural engineer $3,500–$12,000; civil and hydraulic engineer $2,500–$8,000; BAL assessment $400–$1,500; BASIX or NatHERS assessor $400–$1,200; arborist report $1,000–$4,000; quantity surveyor pre-contract feasibility $2,500–$8,000. Total professional fees on a typical custom build commonly fall between $30,000 and $130,000 before the builder's contract.
Stamp duty is a state tax based on land value (when bought separately from a build contract). Several states reformed first-home buyer concessions in 2024–25 — Queensland removed the value cap on new builds for first-home buyers from 1 May 2025; SA, WA and ACT all liberalised concessions over the same period — so verify current entitlements at the time of purchase. Where land and house are bought under a single combined "house and land package" contract, duty may be calculated on the combined value, which can be materially more.
Contingency. Industry guidance is to add 10% on flat in-estate builds with confirmed inclusions, 15% on infill knockdown-rebuilds, 20% on sloping or bushfire-affected blocks, and 25%+ on rural-residential or Class P/E sites. A contingency is not an "if" allowance; it's a "when" allowance.
A worked example. A 200 m² home on an infill block with a moderate 10–15% slope, BAL-12.5 and Class H1 soil, in a metropolitan area:
- Base build at $2,400/m² × 200 m² = $480,000
- Site costs (moderate slope premium) = $35,000
- H1 footing premium = $18,000
- Sloped driveway = $13,000
- OSD and stormwater = $14,000
- Service connections (water, sewer, power, NBN) = $14,000
- BAL-12.5 = $3,000
- Design fees (designer at 5% + structural, hydraulic, BASIX, survey, geotech) = $42,000
- DA, building permit, contributions = $18,000
- Long service / BCITF levy = $1,800
- Stamp duty on $700,000 land (NSW non-FHB, indicative) = $26,800
- Conveyancing and legal = $2,500
- Construction loan interest (12 months) = $15,000
- Contingency (15% on construction) = $95,800
- Total project cost (excluding land purchase price): ~$779,000
That's approximately 60% above the $480,000 base-build headline. A steeper rural-residential block with Class P soils, BAL-29 and a long power run can run 100–120% above its equivalent headline.
The provisional-sum trap. The single most common cause of budget blow-out is a fixed-price contract loaded with provisional sums (PS — estimates for work not yet defined) and prime cost items (PC — allowances for products not yet selected). If a builder quotes a $15,000 PS for earthworks on a clearly sloping block with no geotech, the cost is hidden in the variation register — and variations during build are typically 15–25% above market rate. Get the geotech done before contract. Demand PC allowances at the level you'd actually select, not at builder-default. And take any standard form contract — particularly an ABIC, HIA or Master Builders form — to a property solicitor for $400–$1,200 of review time before you sign. That review almost always pays for itself many times over.
Questions worth asking the builder and quantity surveyor:
- Is this a fully fixed-price contract or are there provisional sums and PC items? Can I see the full schedule?
- What does your site-cost allowance assume about soil class, slope and rock?
- Have you priced the BAL rating into the contract or is it a provisional sum?
- Who carries the risk if geotech comes back worse than expected?
- What is your variation rate on your last ten similar projects (as a percentage of contract value)?
- Are PC allowances at standard or "builder default" levels — and what is the realistic upgrade cost?
- Can I get a quantity surveyor's independent benchmark before contract?
- What contingency are you advising me to hold separately from the contract sum?
Who can help. A Quantity Surveyor (AIQS member, MAIQS or FAIQS) gives an independent cost estimate against the builder's quote and benchmarks provisional sums — typically $2,500–$8,000. An independent project manager or owner's representative provides oversight during tender and contract administration (typically 3–6% of construction value). A construction-savvy mortgage broker structures progress payments and forecasts cashflow including the rent-plus-interest overlap. A property solicitor reviews the building contract before signing — $400–$1,200 for a contract review and one of the highest-value spends in the project.
A useful planning lens: budget the total project for the home you actually want, then back-solve to the maximum land price that works. The other way round — buying the land first and discovering the build is unaffordable — is one of the most common patterns of distress in the Australian residential market.
This article is general information only — a starting point for your own questions, not financial, construction-cost or quantity-surveying advice. Construction costs are volatile and vary widely by region, builder, market conditions and site complexity. All figures here are 2025–26 indicative ranges and will date. Always engage a Quantity Surveyor and a property solicitor before signing a building contract, and request from the builder a full schedule of provisional sums, prime cost items and exclusions. Independent advice should be obtained before making any property decision.