Speciality · Basement excavation
Going under an existing Sydney home, safely.
Under-house excavation, party-wall underpinning, contiguous piling, and the waterproofing detail that decides whether a basement is a usable room or a permanent damp problem.
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In short
Basement excavation: the short answer
Yes — a basement can usually be dug under an existing Sydney house. It's done in stages: the house and any shared party wall are underpinned, the ground is retained with piling or staged underpinning, and the excavation is waterproofed to a defined Type A, B or C strategy before it becomes a room. Ground conditions and the waterproofing detail decide the cost — and whether the room stays dry.
Key facts · Approvals
Reports usually needed
A geotechnical report (bore log, water table depth, expansive-soil assessment), structural engineering for underpinning and retention, and dilapidation reports on neighbouring properties before excavation starts.
Key facts · Waterproofing
Waterproofing
A defined Type A, B or C strategy (BS 8102) decided before the slab is poured — most Sydney basements end up Type A with a Type C drained perimeter.
Key facts · Who it suits
Who it suits
Owners of existing Sydney homes, including terraces and semis with party walls, who want a cellar, wine room, gym, garage or extra level beneath the house.
Key facts · The team
Who's on the team
The director walks the dig at every stage change; an independent waterproofing consultant signs off the waterproofing detail; third-party dilapidation reporting on neighbours.
What's involved
Under house excavation — what a basement project involves
Excavating below an existing Sydney home is a structural-engineering project before it's a building project. The original footings need to be retained or underpinned. The party walls of any neighbour need to be protected — and in semi-detached and terrace contexts, that means independent geotechnical advice and usually a pre-condition dilapidation report served on the adjoining owners under the Dividing Fences Act and common-law nuisance principles. Excavation is staged so the ground above the dig is always supported. Waterproofing is detailed before the slab is poured, not after. Drainage is engineered to AS 3500 with a sump-and-pump strategy where gravity discharge isn't available. None of this is improvised on site.
What's involved
Excavating under a house — how the dig actually runs
Excavating under a house is done from the inside out and in stages, never as one open hole. The original footings are underpinned first, so the house is carried on new concrete before any bulk earth leaves. Access then decides the method: where a small machine can get under or beside the house it digs and loads spoil onto a conveyor or into bins; where it can't, spoil is moved out by hand through a side path, which is slower and costs more. In sandstone the dig is usually rock-sawing or hammering rather than digging, and council noise rules limit the hours it can run. Neighbours' properties are photographed in a dilapidation report before the first bay is opened, and the engineer inspects each underpin before it is poured. The house stays standing and supported at every step.
What's involved
Sydney ground conditions and what they mean for your project
Sydney sits on Hawkesbury sandstone in most of the inner ring (Eastern Suburbs, Lower North Shore, Inner West), Ashfield shale through parts of the Inner West and lower Northern Beaches, and alluvial fill closer to the harbour and rivers. Sandstone excavation is the cheap end — typically rock-bearing pads, dry digs, minimal dewatering — but rock-hammering hours are restricted by council noise regulations and neighbour-impact is high. Shale is mixed: faster to dig but weaker structurally, often needing more substantial retention. Alluvial and fill sites are the expensive end: dewatering required, perched water tables common, contiguous piling or sheet piling for retention, and structural slabs designed for hydrostatic uplift. The geotechnical report — bore log, water table depth, expansive-soil assessment — is the single most important pre-design document.
What's involved
Underpinning, retention, and the staged excavation method
Underpinning original footings is rarely a single-step operation. Standard practice is the 'pit-and-pin' method: the new excavation is broken into 1–1.5m wide bays, each bay is excavated to the new basement depth one at a time (typically every third bay, then the alternating bays), the underpin mass concrete pour is placed, and the original footing is dry-packed onto the new underpin. Bays are sequenced so the building above is always supported by at least 70% of its original bearing area. For deeper excavations adjacent to neighbouring structures, contiguous piling (steel-cased CFA piles drilled to rock) is often used as a permanent retention wall. Soldier piles with timber lagging is the cheaper option for non-permanent retention but requires backfill against the new basement wall.
Where it goes wrong
Why basement projects blow out
The single most expensive failure mode is discovering — mid-dig — that the geotechnical report didn't catch a rock shelf, a perched water table, or a service main running under the easement (Sydney Water trunk mains and Ausgrid HV cables are the usual surprises). The second is doing the structural retention on the cheap and ending up with differential settlement and cracking in the house above. The third is treating waterproofing as a finish rather than as a structural decision — a Type A tanked membrane system designed during the slab pour is fundamentally different from a Type C cavity-drain system designed after the fact, and choosing wrong locks you in for the life of the building. The fourth is neighbour disputes: unresolved party-wall, vibration, or noise complaints have stopped more Sydney basement excavations than ground conditions ever have.
What's involved
Waterproofing strategies — Type A, B, and C
BS 8102 (the British standard most Sydney engineers reference) defines three approaches. Type A: barrier protection — a continuous waterproof membrane (sheet, liquid, or bentonite) applied to the structure, designed to keep water out entirely. Best when designed and installed during construction; difficult and expensive to retrofit. Type B: integrally protected structure — the concrete itself is the waterproof barrier (water-stop joints, hydrophilic seals, low-permeability mix). Used for basement walls where Type A access is constrained. Type C: drained protection — a cavity-drain system inside the structure that collects any water that does get through and discharges it to a sump and pump. Most resilient when groundwater is unpredictable; requires ongoing pump maintenance. Most Sydney basements end up as Type A with a Type C drained perimeter for redundancy.
What's involved
Concrete basement construction — walls, slab and structure
Once the excavation is retained, a basement is built as a reinforced-concrete box. A structural engineer designs the retaining walls for the earth and water pressure behind them and the slab for any groundwater uplift beneath it, and the walls are tied into the underpinning above. Waterproofing is part of the structure, not a coating added later: the strategy (Type A membrane, Type B waterproof concrete, or a Type C drained cavity) is chosen before the slab is poured, and construction joints, penetrations and the slab edge are detailed so water has no easy path in. Concrete is poured and cured in a set sequence, then inspected before the room is lined and fitted out. The result is a structural floor of the house, not an add-on.
How we run it
How Varloch approaches basements
We treat the excavation, retention, and waterproofing as one structural sub-project — engineered, sequenced, and signed off — before any internal fit-out scope is finalised. The director walks the dig at every stage change. We use third-party dilapidation reporting on neighbouring properties before any excavation starts. Neighbour relations are managed actively because basement excavations in dense Sydney suburbs upset neighbours faster than anything else, and we'd rather have the awkward conversation about working hours and dust before the rock-hammer arrives than during the first complaint to council. And the waterproofing detail is signed off by an independent waterproofing consultant on every project — not left to the membrane installer to interpret the architect's drawings.
What's involved
Under house excavation — making use of the subfloor
Most Sydney houses sit on a subfloor: brick piers, a crawl space, sometimes a half-height storeroom at the low side of a sloping block. Under house excavation means removing that ground — in controlled stages — so the space becomes a usable room with proper head height. It isn't always a full basement. On a lot of jobs the brief is smaller: lowering the subfloor at the low side of a sloping lot to get a garage in, turning a 1.2 m crawl space into a 2.4 m storeroom, or digging out half the footprint for a rumpus and laundry.
What's involved
When it makes sense — and when it doesn't
Under house excavation works best on three kinds of site. Sloping blocks, where one side of the house is already well out of the ground and you're only digging a wedge, not a box. Sandstone sites, where rock sits shallow and the dig is dry and self-supporting. And streets where land is so expensive that every square metre gained without buying more land is cheap by comparison — Mosman, Kirribilli, the Eastern Suburbs, Manly and the Pittwater peninsula. It stops making sense when the water table is high (dewatering and a fully tanked structural box change the numbers completely), when the house is on reactive clay with tired footings that would need full replacement anyway, when a sewer or stormwater main runs underneath, or when the attached neighbour's footings are shallow enough that the party wall becomes the project. A geotechnical report — two to four boreholes, roughly $6,000 to $14,000 — answers most of this before you spend anything serious.
Cost & timing
What it costs in Sydney
The honest range is wide because the ground decides it. As a guide from our own pricing: underpinning runs about $3,500 to $7,000 per linear metre of wall; bulk excavation $180 to $450 per cubic metre in sandstone, depending on access and whether the rock can be ripped or has to be sawn; tanking and drainage $180 to $350 per square metre of below-ground envelope; engineering and inspections $15,000 to $40,000. A partial dig-out for a garage or storeroom on a sloping lot can land well under the full-basement figure. A full habitable lower level under an existing house is typically $550,000 to $950,000 all in for around 100 m² — call it $6,000 to $9,000 per square metre. That sounds like a lot until you compare it with land at $12,000 to $20,000 per square metre in the suburbs where people actually do this.
What's involved
Northern Beaches: slopes, sandstone and the geotech policy
A lot of our under house work is on the Northern Beaches — Manly Vale, Newport, Bilgola, Avalon and Whale Beach — because the topography suits it. Many houses there are built into the slope with a tall, unused undercroft at the low side, which is the cheapest under house excavation there is. The catch is approvals. Northern Beaches Council applies geotechnical risk management controls across the old Pittwater area and other mapped hillside land, which means a geotechnical engineer's report and sign-off forms part of the DA, and their inspections become hold points during the dig. Sandstone is common and shallow, which helps the structure but means rock-sawing and hammering under restricted council hours, and real neighbour management. Coastal sites can also carry perched water from uphill properties and street runoff, so drainage and tanking get designed for water pressure, not just damp.
What's involved
Approvals for under house excavation
Most under house excavation in Sydney needs a development application, because you're changing the structure, often the floor area, and sometimes the building envelope or the driveway. A small number of lower-ground alterations can go through as complying development; your certifier confirms which path applies before design is locked. A dilapidation survey of the neighbouring houses costs $2,000 to $5,000.
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