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Foundations for modular and prefab homes
| Site classification report | About A$400 – A$800[1] |
|---|---|
| Slab, two-bedroom modular | A$12,000 – A$20,000, one builder[2] |
| Piers or stumps, sloping site | A$10,000 – A$18,000, one builder[2] |
| Screw pile, installed | About A$900 – A$2,500 each, Adelaide[3] |
| Reactive soil uplift | A$8,000 – A$35,000 added, per published guides[4][5] |
| *Footings are excluded from most modular base prices and are set by the engineer for each site. | |
A modular or prefab home sits on footings built on site, and the footing is almost always priced outside the home. Published guidance puts a slab for a typical two-bedroom modular home at A$12,000–A$20,000 and pier or stump foundations on a sloping site at A$10,000–A$18,000 with one builder, while reactive soils are reported to add A$8,000–A$35,000.[2][4][5] The figures are commercial guidance, not independent survey data, and the footing for any site is decided by the engineer once the soil class is known.
Off-site construction moves the house into a factory, but not the ground under it. The footings are designed for the particular block and built while the modules are being manufactured — which is why builders list them as a separate cost.[6][7] This page covers the footing systems used under modular and prefab homes, what decides between them, and what each is published to cost.
The soil decides first §
Before footings can be designed, the site needs a classification. A site classification report is put at roughly A$400 to A$800, takes one to two weeks, and is required by the building surveyor before footings are designed or a permit is issued.[1]
The classes run from A (sand or rock, little or no movement) through S, M, H1, H2 and E as soil reactivity increases, with P for problem sites such as uncontrolled fill or abnormal moisture.[8] Under the building code, sites classified H, E or P must be designed with reference to AS 2870 rather than the code's simpler deemed-to-satisfy provisions.[9] Piled footings are among the systems used on H, E and P sites.[1] The full explanation is in Site classification and soil reactivity.
The main options §
- Slab on ground
- A concrete slab poured on the prepared site, with the modules set on top. Published at A$12,000 to A$20,000 for a typical two-bedroom modular layout by one builder.[2] A slab adds thermal mass, which suits passive solar design.[10] On sloping or uneven ground a slab needs piers, stiffening beams or a suspended design.[10]
- Stumps or piers with a suspended floor
- Short columns of steel, concrete or timber set into footings, carrying the floor above the ground. One builder publishes pier or stump foundations at A$10,000 to A$18,000 for uneven or sloping sites.[2] Stump lengths can be adjusted to bring the floor to a common level, which is why stumps suit sloping blocks, and a bearer-and-joist floor on stumps is usually lower in cost and embodied energy than a slab — at the price of very little thermal mass.[10]
- Bored concrete piers
- Holes drilled to a design depth and filled with reinforced concrete, often joined by a cap beam. A March 2026 Adelaide trade-marketplace guide gives A$280 to A$600 for a 300 mm pier and A$420 to A$900 for a 450 mm pier, installed, with a cap beam at A$180 to A$380 per linear metre.[3]
- Screw piles
- Steel shafts with helical plates, screwed into the ground by machine. The same guide gives A$900 to A$2,500 per pile installed, depending on diameter and length.[3] Screw piles can be cost-effective where excavation and spoil disposal would be expensive, and many can carry load immediately; but buried rubble, old footings, rock, corrosive or coastal soils and high lateral or uplift loads can make them a poor fit or need extra engineering.[11] Piled footings are designed under AS 2159.[11]
What a footing system is published to cost §
| Item | Published figure | Source type |
|---|---|---|
| Site classification report | A$400 – 800 | Geotechnical service[1] |
| Geotechnical investigation, residential | A$400 – 900 | Trade marketplace, Adelaide[3] |
| Structural engineer design and certification | A$800 – 3,000 | Trade marketplace, Adelaide[3] |
| Slab, typical two-bedroom modular | A$12,000 – 20,000 | Builder[2] |
| Piers or stumps, sloping site | A$10,000 – 18,000 | Builder[2] |
| Six-pier residential system, installed | A$4,500 – 14,000 | Trade marketplace, Adelaide[3] |
| Piering on H2 or P sites, added | A$8,000 – 25,000 | Cost guide[4] |
| Waffle slab or piers on reactive soil, added | A$15,000 – 35,000 | Cost guide, NSW[5] |
The six-pier figure from the Adelaide guide describes a small structure; a modular home needs many more support points, so the per-pier prices are the more useful numbers to multiply.[3] The reactive-soil figures come from cost guides for conventional houses, but the soil reacts the same way whether the house above it was built on site or in a factory.[4][5]
Why this matters more for modular §
Three features of off-site construction raise the stakes on footings.
The footings are built before the house arrives. The footings are prepared while the modules are manufactured, and the modules arrive finished, so level and position have to be right before delivery day. How handover is checked at both the factory and the site is covered in Handover inspections for modular homes.
Sloping and remote blocks favour stumps and piers. Adjustable stumps suit sloping ground,[10] and screw piles avoid excavation and spoil removal,[11] which is valuable where concrete trucks and tip runs are expensive. The broader cost picture for rural sites is in Building on rural land.
Raised floors answer flood levels. Where a site is flood-affected, the floor may have to sit above a defined flood level, which changes the footing design. See Flood-prone land and prefab homes.
What is not covered here §
No independent Australian survey of footing costs for modular homes has been located. The pier and pile prices are from a single trade marketplace for metropolitan Adelaide and may not reflect other cities or regional areas.[3] Costs for slab and stump systems by state, and costs for termite management systems and subfloor ventilation, are not covered by the sources compiled here. The engineering of any particular footing — depth, spacing, pile type, corrosion protection — is a matter for the site's engineer. Excavation and retaining walls are covered in Site works costs.
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References §
- Easy Footings (commercial geotechnical and footings service) — Commercial guidance. States that for a residential block a site classification report costs roughly A$400–$800 and takes one to two weeks, that the building surveyor requires it before footings can be designed or a building permit issued, and describes the footing systems used across the classes including piled footings on H, E and P sites. Commercial guidance, not independent survey data. easyfootings.com.au/blog/as-2870-residential-slabs-footings-explained.html
- Livit Constructions — Modular house cost guide — site preparation and utility connections reported to add 10–20 per cent to base module costs depending on terrain, access and local service availability; earthworks and site clearing A$5,000–$15,000; standard slab foundation A$12,000–$20,000 for a typical two-bedroom layout; pier or stump foundations A$10,000–$18,000 for uneven or sloping sites. Builder-published commercial guidance. livitconstructions.com.au/post/cost-of-building-moduar-house-brisbane
- The Quote Yard — Commercial trade marketplace cost guide for pier foundations, March 2026, stated to reflect Adelaide metropolitan rates. Gives installed prices per unit of A$280–$600 for a 300 mm bored concrete pier, A$420–$900 for a 450 mm bored pier, A$400–$1,200 for a driven precast concrete pile and A$900–$2,500 for a screw pile or helical pier (A$900–$1,400 for a 76 mm pile 3.0 m long and A$1,400–$2,500 for an 89 mm pile 4.5 m long); a cap beam at A$180–$380 per linear metre; structural engineer design and certification at A$800–$3,000; a residential geotechnical investigation at A$400–$900; and a typical six-pier residential system installed at A$4,500–$14,000. Commercial guidance, not independent survey data. thequoteyard.com.au/services/pier-foundation-sa
- BuiltSimple — Construction cost calculator Australia 2026 — site preparation, excavation, retaining walls, drainage and piering reported at about A$18,000 for a flat suburban Brisbane block and A$60,000–$90,000 for a sloping Sydney North Shore block; H2 or P-class sites adding about A$8,000–$25,000 in piering. Commercial guidance. builtsimple.com.au/blog/construction-cost-calculator-australia
- HomeChoice Australia — Cost of building a home in NSW 2026 price guide — foundation work A$55,000–$95,000 for a 200 m² home; reactive soil conditions requiring reinforced waffle slabs or pier systems reported to add A$15,000–$35,000 to basic slab costs. Notes that a quoted rate per square metre typically refers to the building envelope only. Commercial guidance. homechoiceaustralia.com.au/cost-of-building-a-home-in-nsw-australia-a-2026-price-guide/
- Anchor Homes — Victorian modular builder, August 2026 Victorian cost article. Lists pre-construction fees, council approvals, service connections, structural engineering, re-establishment survey, excavation or elevated footings, crane hire and energy, bushfire and wind rating upgrades as costs outside its base price. Builder-published commercial guidance. anchorhomes.com.au/blog/how-much-does-a-prefab-modular-home-cost-in-victoria-in-2026
- Built by Modify — 2026 modular cost guide — lists soil testing, engineering, site feature survey and energy rating as separate budget items, with footing design dependent on soil classification. Builder-published commercial guidance, not independent survey data. builtbymodify.com.au/blogs/modular-home-cost
- Intertek Inform (standards publisher) — Commentary on AS 2870:2011. Describes the site classifications by soil reactivity — Class A sand or rock with little to no movement, Class S slightly reactive, Class M moderately reactive, H1 and H2 highly reactive, Class E extremely reactive, and Class P problematic sites including filled material that can vary widely across a site or sites with abnormal water conditions. intertekinform.com/en-us/resources/blog/as-2870-2011-residential-slabs-and-footings-standard/
- Australian Building Codes Board — National Construction Code — ABCB Housing Provisions, Part 4.2 Footings, slabs and associated elements. Includes Table 4.2.2, the general definition of site classes; states that where a site is classified H, E or P, reference must be made to AS 2870 for design and construction information because of the limitations of the Part; and sets out the M-D sub-classification for deep-seated movement at a design depth of suction change of 3 m or more, which is also beyond the scope of the Housing Provisions. ncc.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions/4-footings-and-slabs/part-42-footings-slabs-and-associated-elements
- Renew — Magazine article, April 2018, on choosing a floor system. States stump-supported floors are particularly useful on uneven or sloping ground because stump lengths can be adjusted to a common level, while slabs on such ground need piers, stiffening beams or suspended designs; that bearer-and-joist floors are usually lower in cost and embodied energy than a slab; that slabs provide thermal mass while lightweight stump-supported floors provide little; that stumps can last 50 to 100 years or more; and that prefabricated floor cassettes and pre-cut bearer and joist systems can shorten on-site time. Independent magazine; figures are qualitative and dated. renew.org.au/renew-magazine/building-materials/the-right-floor-for-your-build/
- Unsustainable Magazine — Article, February 2026, on screw pile foundations. States AS 2159 sets minimum requirements for designing, constructing and testing piled footings in Australia; that screw piles can be cost-effective where excavation and spoil disposal would be expensive and many can carry load immediately; and that obstructions such as rubble, boulders or old footings, corrosive or coastal soils, significant lateral or uplift loads and reactive clay without drainage control can make them a poor fit or require more engineering. Gives no Australian prices. Independent publication, not an official source. unsustainablemagazine.com/construction-screw-pile-foundations/
Figures are as published by the sources above at their respective dates and are paraphrased summaries, not verbatim reproductions. Verify current pricing and rules directly with builders, lenders and planning authorities.