On the Sunshine Coast, the choice between a concrete slab and stumps is settled by your site, not your taste. Three things decide it: the site classification your soil test returns under AS 2870, how far the land falls across the building footprint, and whether the lot carries a flood hazard or steep land overlay. A slab-on-ground is the simpler, cheaper build on a flat, well-drained, low-reactivity block. Stumps — properly, a suspended floor on piers, posts or piles — win when the ground falls away, when an overlay pushes your floor level above natural ground, or when excavation has to stay off the site.
Start with the site classification, not the price
Before anyone quotes a footing system, the foundation has to be classified. The ABCB Housing Provisions under the National Construction Code state at clause 4.2.2 that the foundation on which footings and slabs are located must be classified in accordance with AS 2870. That returns a letter: A for sand and rock with negligible ground movement, S slightly reactive clay, M moderately reactive clay or silt, H highly reactive clay, E extremely reactive clay, and P for problem sites such as soft soils, loose sands, landslip or subsidence.
Where a site is classified H, E or P, the Housing Provisions state that because of the limitations of that Part, reference must be made to AS 2870 for design and construction information. In practice that means an engineered footing design rather than a standard detail. A Class A sand site at Marcus Beach and a reactive clay site in the Ninderry hinterland are not the same job, however identical the drawings look.
Concrete slab vs stumps: the direct comparison
| Consideration | Concrete slab-on-ground | Stumps (suspended floor) |
|---|---|---|
| Flat, well-drained lot | Usually the efficient answer: fewest trades, fastest to lock-up | Works, but you pay for clearance you may not need |
| Sloping site, modest fall | Viable while cut and fill stay inside the 2 m limits in Housing Provisions clause 3.2.1 | Often tidier: the subfloor absorbs the fall instead of the earthworks |
| Steep land, mapped as slopes of 15% or greater (Sunshine Coast Planning Scheme 2014, code 8.2.10) | Difficult: the first acceptable outcome steers you away from filling and excavation altogether | Named in the code: split level design that steps down the slope, or suspended floor construction that avoids filling and/or excavation |
| Flood hazard overlay | Only where the pad can be lifted with engineered fill and retaining without worsening flows | Strong fit: reaches the required floor level without importing fill |
| Reactive clay (Class M, H or E) | Common, but the slab must be stiffened and designed to AS 2870 | Piers can be founded below the active zone, reducing seasonal movement |
| Sand sites near the coast | Generally straightforward on Class A ground | Screw piles or bored piers where loose sand needs deeper founding |
| Acid sulfate soils, Area 1 land at or below 5 m AHD (Sunshine Coast code 8.2.1) | Excavating, dewatering or filling can trigger an acid sulfate soils investigation and management plan | Bored or driven piles disturb far less soil volume |
| Termite management (AS 3660.1) | Perimeter and penetration systems, installed before the pour | Shielding at each stump, plus a subfloor you can inspect |
| Ground clearance under the floor | Not applicable | Minimum 150 mm, or 400 mm where termite inspection is required (Housing Provisions Table 6.2.1b) |
| Services and later changes | Drainage set before the pour; changes mean cutting concrete | Services reachable under the floor |
How much your block falls changes the answer
Earthworks are where a slab quietly becomes expensive. The Housing Provisions cap un-retained bulk earthworks before you leave the standard details. Under clause 3.2.1, Un-retained bulk earthworks — site cut and fill, a site cut must be not deeper than 2 m from the natural ground level at any point, and fill must be placed and mechanically compacted in layers not more than 150 mm and be not more than 2 m in height from the natural ground level at any point. Past those limits you are into engineered retaining and engineered footings.
Sunshine Coast Council’s Planning Scheme 2014 pushes the same way. Its Landslide hazard and steep land overlay code (section 8.2.10) identifies steep land as slopes of 15% or greater on the overlay maps. Where a dwelling house sits on land having a slope exceeding 15%, the first acceptable outcome under AO3.1 is buildings of a split level design that steps down the slope, or that incorporate a suspended floor construction avoiding filling and/or excavation. The alternatives it offers are confining any filling or excavation to the driveway and the plan area of the dwelling house, or keeping any filling or excavation to not more than 2 m relative to ground level — or 1.0 m where within 1.5 m of any property boundary — without necessitating a retaining wall exceeding 2 m in height. Read that 1.0 m limit properly: it applies to fill as well as cut. On a steep block, stumps are the compliance path, not a styling decision — our guide to building on a sloping block in Queensland covers the rest.
If the lot sits in a flood hazard overlay
Under the Sunshine Coast Planning Scheme 2014 Flood hazard overlay code (section 8.2.7), Table 8.2.7.3.1 sets the requirements for accepted development. For a dwelling house, acceptable outcome AO1 is a finished floor level for all habitable rooms at least 500 mm above the defined flood event and the defined storm tide event. Where those events have not been modelled for the area, the code asks instead for all habitable rooms to be at least 600 mm above the highest recorded flood or storm tide inundation level. A separate outcome applies on sites in the drainage deficient areas identified on Figure 8.2.7.
In Noosa Shire the scheme is a different document, so do not carry Sunshine Coast numbers across the boundary. Noosa Shire Council states in Noosa Plan 2020 Fact Sheet 17: Flood Hazard, as amended 26 September 2025, that the mapped flooding and inundation extent behind its Flood Hazard Overlay is based on the 1% annual exceedance probability for the year 2100 — the mapping already looks ahead rather than at today’s conditions — and Council notes that mapping is indicative only and does not capture the full extent of potential flooding. The same fact sheet excludes a dwelling house from becoming assessable against the Flood Hazard Overlay Code, with minimum floor level provisions sitting in the Low Density Housing Code instead, and confirms Council can provide a Flood Search Certificate stating the minimum building height that new habitable rooms are required to build to.
Half a metre or more of required lift is where the arithmetic turns. You can get there with imported fill and a slab, but you are then compacting in 150 mm layers, retaining the perimeter and re-solving the drainage you have displaced onto someone else. A suspended floor reaches the same level without moving the ground. Confirm the overlays before you commission drawings — see building approvals on the Sunshine Coast and Noosa for the sequence.
Coastal ground: acid sulfate soils and termites
Within the Sunshine Coast Council area, the Acid sulfate soils overlay code (section 8.2.1) maps Area 1 as land at or below 5 metres AHD and Area 2 as land above 5 metres AHD and below 20 metres AHD. Much of the low-lying land around Cotton Tree and Maroochydore sits in Area 1. That code applies to assessable development, and its acceptable outcomes turn on avoiding disturbance: not excavating or otherwise removing soil or sediment containing acid sulfate soils, not extracting groundwater in a way that aerates previously saturated soils, and not filling land at or below 5 metres AHD in a way that moves actual acid sulfate soils below the water table or aerates them. Where disturbance is unavoidable you are into an investigation, and where acid sulfate soils are identified, a management plan — a real argument for a footing system that bores or drives rather than digs. Lots in Noosa Shire are assessed under the Noosa Plan 2020 instead, so confirm which scheme covers your address before assuming these figures apply.
Termites are the other coastal constant. Housing Provisions Part 3.4 requires a termite management system where a Class 1 or 10 building is constructed in an area where subterranean termites are known to present a potential risk of attack and a primary building element is susceptible to termite attack. The system must comply with AS 3660.1, or have been tested and passed the tests required by Section 5 of AS 3660.3. Elements consisting entirely of steel, aluminium or other metals, concrete, masonry, fibre-reinforced cement, naturally termite-resistant timber or preservative-treated timber are not treated as susceptible. Under a slab that usually means perimeter and penetration systems set before the pour. On stumps it means shielding at each stump plus a subfloor you can get under and inspect, which is why Table 6.2.1b lifts the minimum ground clearance from 150 mm to 400 mm where termite inspection is required.
Ground moisture is a separate requirement, not a termite one. For a slab-on-ground, Housing Provisions clause 4.2.8 calls for a vapour barrier of 0.2 mm nominal thickness polyethylene film, high impact resistant and branded “AS 2870 Concrete underlay, 0.2 mm High impact resistance”, lapped not less than 200 mm at all joints and sealed with sleeves at penetrations. A suspended floor deals with the same problem through subfloor ventilation: Table 6.2.1a sets the minimum aggregate ventilation openings, and those openings halve where an impervious membrane is laid over the ground.
The third option: a suspended concrete slab
Slab or stumps is not always a binary. A suspended concrete slab on piers or blade walls gives you the mass, acoustics and finish of concrete with the clearance of a stump build — useful on low-lying coastal sites where the floor level must come up but a lightweight subfloor is not the look. Our Memorial Avenue project at Cotton Tree, designed by K Architecture, is built on a suspended slab. Formwork, propping and reinforcement make it dearer per square metre, so it earns its place on merit, not as a default.
What drives the price gap
A foundation price is site-specific, and any figure quoted without a soil test and a contour survey is guesswork. The real drivers are the site classification, the volume of cut, fill and retaining, the founding depth your engineer specifies, concrete and reinforcement quantities, site access, and the termite system. On a flat Class A lot the slab usually wins; add two metres of fall, an overlay or highly reactive clay and the gap narrows or reverses.
The paperwork is the same either way. The QBCC requires a written contract for domestic building work priced over $3,300 including labour, materials and GST, with contracts priced at $3,301 to $19,999 under a Level 1 contract and $20,000 and over under a Level 2 contract. QBCC also requires Queensland Home Warranty Scheme insurance for most residential construction work valued at more than $3,300 including materials, labour and GST. Under the scheme’s time limits for cover and claims, you must first become aware of a structural defect within 6 years 6 months after the cover commencement day, and make the claim within 3 months after the day you first become aware of it. Before signing, confirm the builder is licensed for the class of work — see how to check a QBCC licence.
Common questions
Are stumps always dearer than a slab?
No. On a flat, easily accessed lot a slab is normally cheaper. Once a site needs more cut or fill than the 2 m the Housing Provisions allow as un-retained bulk earthworks, retaining walls, or a lifted floor level to satisfy a flood hazard overlay, the earthworks can push a slab past the cost of a suspended floor.
Can I still build a slab on a block with a flood overlay?
Often yes, provided the finished floor level of habitable rooms meets the overlay requirement — at least 500 mm above the defined flood event and the defined storm tide event under the Sunshine Coast Planning Scheme 2014 — and the fill does not make flooding worse next door. The practical question is whether the pad can be lifted that far without disproportionate fill and retaining. In Noosa Shire, get a Flood Search Certificate from Council first, because it states the minimum building height your new habitable rooms have to reach.
What is a Class P site, and does it rule out a slab?
Class P covers problem sites such as soft soils, loose sands, landslip and subsidence. It does not rule out a slab, but the Housing Provisions send you to AS 2870 for the design rather than a standard table, and that design often ends up as piers or piles to competent founding material with a suspended floor or slab over.
Who orders the soil test, and when?
The site classification should be done before working drawings are finalised, because it feeds the engineer’s footing design. On our jobs it runs alongside the contour and detail survey, so the design responds to real levels and real ground.