House Extension Footings in Victoria: Cost, Pad Footings and Steel Alternatives

Short answer: house extension footings in Victoria are the new supports under a rear, side or upper-floor addition. They are designed to AS 2870 from a soil test on your block, and the hard part is not the footing itself but the join to the existing house: the addition has to move with the old building on Melbourne’s reactive clay, not against it. For a stumped timber-frame extension, engineer-certified concrete-free steel is usually the cleanest fit — it goes in through the side gate, founds below the active clay zone and, with an adjustable head, lands the new bearer line exactly on the old floor level. Easy Footings Pty Ltd supplies and installs BMSA RapidStump, StumpRite and SurePile across Melbourne metro and regional Victoria. Phone 0419 994 555.

The longer answer covers what makes an extension different from a new build; the realistic footing options (slab, strip, pad, bored pier, driven steel, screw pile, adjustable stump) and where pad footings genuinely belong; how AS 2870 clay and the existing house drive the design; the backyard access problem; the Victorian permit path; what moves the cost; and when adjustable or concrete-free footings are the right call. Sister pages already exist for granny flats, tiny houses and decks — this page is the extension version, where an existing house is always in the way.

What Makes Extension Footings Different

A new house gets a clean site, a soil report, a slab or stump layout drawn on a blank page, and a concrete truck parked at the kerb. An extension gets none of that. The footings for a rear or side addition have to work around five things a new build never sees:

• An existing house that has already moved. A 1950s weatherboard or a 1980s brick veneer has settled onto its footings over decades. Whatever you add now starts from zero and has its own settlement ahead of it.
• A floor level that is already fixed. The new bearers have to land on the old floor line, or the new slab on the old slab, to the millimetre. There is no tolerance to “pack it later”.
• Footings you cannot see. Very few older Victorian homes come with a soil report or footing drawings. The existing footing depth is often a guess until someone digs beside it.
• A backyard, not a site. Access is a side gate, a lawn, established trees, a sewer line and the neighbour’s fence. The house you are extending blocks the obvious path in.
• A household still living there. Wet trades, spoil piles, a pump line through the kitchen and a week of curing all land on people who are trying to cook dinner.

Those constraints, not raw load, decide the footing system on most Melbourne extensions. A single-storey timber-frame addition is a light structure; any compliant footing carries it. The question is which footing can be installed through a 1 m gate, found below reactive clay, meet an existing floor level exactly and not crack the house in two the first summer.

Rear, Side and Upper-Floor Extensions

The footing job changes with the kind of addition:

• Rear ground-floor extension (the common one). A kitchen, living or family room off the back of the house, usually timber frame with a Colorbond or tiled roof. If the existing house is stumped, the extension is almost always stumped too so the floor runs through. If the house is on a slab, the extension is usually a slab, or a stumped floor stepped up to the old level.
• Side extension or infill. Bedrooms or a garage along one boundary. The tightest access of all: often the only way in is through the house or over the front fence. Boundary setbacks, neighbour footings and the existing side wall footing all sit in the same trench.
• Raised or deck-level extension over a falling yard. A rear room that steps out over a slope with a deck or verandah beyond it. This is a sloping-block footing problem: stepped concrete pads and retaining, or piles cut to one level datum.
• Upper-floor (second-storey) addition. The new floor usually loads the existing walls and therefore the existing footings. The engineer assesses whether those footings can carry the extra load; on reactive clay or shallow old footings the answer can be new piers, underpinning or a separate steel frame with its own footings. That is a structural-engineering job first and a footing job second.
• Extension plus restump. Many Melbourne extension jobs bundle a restump of the old house because the floor is already out of level. Doing both at once with the same adjustable system gives one continuous level bearer line. See the Melbourne restumping guide.

The Junction: Differential Movement

Ask any Victorian building inspector where extensions fail and the answer is the same: the line where new meets old. A crack opens in the render or plaster at the junction, a door binds, a floor develops a step. The cause is differential movement. The two parts of the house are moving by different amounts, and the join is where the difference shows.

It happens because the old footings and the new footings are rarely founded the same way. The 1950s stumps might sit 500 mm into clay that has been heaving and shrinking around them for seventy years and reached a kind of equilibrium. New pads or piers go in deeper, or shallower, or into a corner of the block with a different moisture history, a tree or old fill. On Class M to H2 clay the new footing then has its own seasonal cycle, out of step with the old one.

The engineer manages it with design, not hope:

• Found below the active zone. A pile or pier taken below the depth of seasonal moisture change stops moving with the seasons. If the new footings do not heave, half the differential problem is gone.
• Compatible depth. Where the addition ties directly to the old footing, the engineer specifies a founding depth and detail that behaves like the existing one, or isolates the two.
• Articulation. A designed movement joint at the junction (in the brickwork, the slab or the lining) lets the two parts move a little without tearing the finish.
• Drainage and site moisture. New downpipes, a changed roof catchment and a paved area can wet one side of the house and dry the other. Getting stormwater away from both old and new footings matters as much as the footing itself.
• Adjustability. On a stumped extension, an adjustable steel stump gives a second chance. If the new floor settles a few millimetres in its first two years, the bearer line can be wound back to level instead of packed or jacked.

None of this is exotic. It is standard AS 2870 practice for additions. The point is that a footing quote for an extension that does not mention the existing house is a quote for the wrong job.

Footing Options Compared

The pattern matches the rest of this site. On an open, flat, Class A or M block with a slab house, a slab extension is often the natural answer and concrete does the job well. The moment the house is stumped, the yard is tight, the soil is reactive or the block falls away, the conversation moves to founded steel. The full trade-off between the two families is on the concrete-free vs traditional concrete page.

Pad Footings vs the Alternatives

“Pad footings for an extension” is one of the most common searches that lands on this site, so it deserves a straight answer. A pad footing is an isolated square or rectangle of concrete under a post or stump. Typical Victorian residential pads run from about 450×450 mm to 900×900 mm and 600 to 900 mm deep on Class A, S and M sites, sized by the engineer for the point load and the soil class. The pad-versus-strip logic is laid out on our pad vs strip footings page.

Where pads are a legitimate extension footing: a light timber-frame stumped addition on Class A, S or stable Class M ground, with open access for a mini-excavator and a barrow, in a dry season, where the existing house is on similar shallow footings and the engineer is happy the two will behave alike. In that case a pad under each stump, or a small bored pier, is a perfectly sound and often cheap answer.

Where pads are the wrong tool:

• Class H1, H2 or P clay. A 600–900 mm pad sits inside the seasonally active zone, which on Melbourne’s west and north can be well over a metre deep. The pad heaves in winter and drops in summer, and the extension goes with it while the old house does not.
• Old fill. Backyards in older suburbs often carry 0.5–1.5 m of uncompacted fill from a removed pool, an old shed slab or past landscaping. A pad bears on the fill. A pile goes through it to native ground.
• A sloping yard. Pads on a fall become a staircase of over-height holes and formwork. Each one is a different depth; the spoil goes uphill.
• Trees. Every pad is an excavation inside the root zone. A row of pads under a rear extension beside a mature gum is a lot of root disturbance and, on clay, a lot of moisture change.
• No access and a wet program. Twelve to twenty hand-dug or augered pad holes in a Melbourne winter, with the spoil barrowed out through the side gate, is the part of the job nobody quotes honestly.

Against pads, the alternatives split cleanly. A bored concrete pier is the deeper concrete option: it founds below the active zone but still needs a rig, spoil removal and a cure. Driven steel and screw piles do the same founding job with no pour and no hole; the difference is speed, weather and mess rather than structural capacity. A precast footing block sitting on the surface is not an extension footing on any Victorian clay site, whatever the hardware store says.

AS 2870 Clay and the Soil Report

In Victoria, residential footings — including extension footings — are designed to AS 2870-2011 Residential slabs and footings from a geotechnical site classification. The classes run A, S, M, H1, H2, E and P; the higher the class, the more seasonal surface movement the engineer has to design for. Full explanation: AS 2870 explained.

Much of Melbourne’s west and north — Werribee, Melton, Tarneit, Point Cook and similar basaltic clay corridors — classifies M to H2. Class M is the most common suburban result; H1 and H2 are routine on the western plains. Regional Victoria does the same: Geelong, Ballarat, Bendigo, the Mornington Peninsula, Gippsland, the Yarra Ranges, Sunbury and Macedon all see reactive clay. Trees, fill or abnormal moisture push a site to Class P.

Two extension-specific points about the report:

• Test where the extension is going, not where the house is. The borehole belongs in the backyard, near the proposed footing line and near any tree. A classification lifted from a neighbour’s report or the original 1970s house file is not a classification of the ground the new footings will sit in.
• Find out what the existing house sits on. A test pit beside an existing stump or footing tells the engineer the old founding depth and material. That single hole is what lets them design the junction rather than guess at it.

Get the report before the engineer drafts the footing schedule. A redesign because the report came back H2 instead of M is the most common cause of program delay on a Victorian extension, and the second most common cause of a footing quote doubling.

Backyard Access

The extension sits behind the house. The footing crew gets whatever is left after the existing building, the deck, the carport, the side gate and the trees. In practice:

• Side gates are typically 900–1,100 mm wide. A concrete pump line fits. A concrete truck does not. A 1.7 t mini-excavator on rubber tracks usually fits; a pier-drilling rig often does not.
• Lawns turn to mud. Two wet days in a Melbourne winter and the side passage is a track. Every barrow of spoil compounds it.
• Services run through the backyard. Sewer and stormwater frequently cross exactly where the extension goes. A 450 mm bored pier finds a clay sewer line more often than anyone likes. A steel pile can be shifted a couple of hundred millimetres on the engineer’s say-so and driven again.
• Spoil has to leave the way it came in. A slab or a set of piers for a 50–60 m² extension can generate several cubic metres of spoil. Displacement piles produce none.

On a flat block with a wide side driveway, concrete is competitive and the access cost is small. On the standard Melbourne backyard the access cost is what quietly pushes a slab or pier quote past a steel one before any timber is cut.

When Adjustable or Concrete-Free Footings Fit

Concrete-free steel is not the answer to every extension. It is the answer to a very specific and very common set of them. It fits when:

• The existing house is stumped. A weatherboard, Federation or post-war timber-frame house on stumps wants a stumped extension so the floor runs through. New steel stumps under the addition are the natural continuation of the old floor structure.
• The floor level has to match exactly. An adjustable head — StumpRite gives up to 120 mm of screw height adjustment — means the new bearer line is wound to the old floor level on the day, and can be wound again if the addition settles.
• The soil is Class M to H2 or P. Driven and helical steel found below the active zone, which is the whole answer to differential movement on reactive clay.
• Access is a side gate. The kit walks in. No truck, no pump line through the house, no spoil back out.
• The block falls away. Each pile finishes at its own cut-off; one level bearer line sits over the slope with no stepped pads and no retaining. See footings for sloping blocks.
• There is a tree. A screw pile inside a tree protection zone is a small-footprint installation compared with a row of bored holes. See footings near trees in Melbourne.
• The program cannot wait on weather. Steel goes in on a wet day and the frame starts the next morning. There is no cure window and no second mobilisation.
• You are restumping at the same time. One adjustable system under old house and new addition gives a single continuous datum and one crew.

It fits less well when the existing house is on a slab and the extension floor is tiled straight through at ground level (a slab extension is usually the honest answer there); when a heavy two-storey masonry addition is designed on strip footings the engineer wants matched in kind; or when shallow rock stops a driven footing short of design depth, in which case the engineer moves to a helical pile or a pier. The concrete-free system still has to be engineered, certified and inspected like any other footing.

Victorian Permit Path (High Level)

A house extension is structural building work and needs a building permit from a registered building surveyor (council or private) before the footings go in. The surveyor will expect an AS 2870 site classification, an engineer’s footing design and certification for the system used, and a footing inspection before the frame proceeds. For concrete-free systems the inspection record is the installer’s drive or torque log for each footing plus the engineer’s compliance certificate.

A planning permit may also be needed depending on setbacks, site coverage, overlays (heritage, bushfire, flooding, vegetation) and your council’s scheme. The surveyor or council decides; this page does not invent thresholds. If the extension is close to a boundary, protection of adjoining property can add its own steps. Easy Footings does not issue permits. For the plain-English footing-and-permit picture see building footing regulations in Victoria, and for the bundled restump case do you need a permit to restump. This is a practical overview, not legal advice.

What Extension Footings Cost in Victoria

Easy Footings does not publish a fixed “extension footing price”. The number moves with floor area and layout, construction weight (timber frame vs brick veneer vs masonry), soil class and founding depth, slope, access, the number and type of supports, and how much work the junction with the existing house needs. What we can do is show the drivers and point to indicative figures already published on this site, all ex GST and all site-specific.

Read those figures the right way. A stumped 50–60 m² timber-frame extension typically needs somewhere in the order of 12–20 supports on a 1.2–1.8 m grid, and a larger or heavier addition more; our screw pile guide puts a house extension at 20–40 piles over one to two days on site. The concrete-free quote is mostly the supports themselves and one mobilisation. The concrete quote is the pads or piers plus the rig, the pump, the spoil, the cure and the second visit, and every one of those items grows with worse access, worse soil and more slope. That is why the gap between the two is small on an open flat block and large on a typical Melbourne backyard.

Two cautions. The house restump bands published elsewhere on this site ($500–$1,000+ per stump; $5,000–$15,000 partial; $20,000–$50,000 full) are for restumping an existing house — jacking, removal and relevelling — not a price for new extension footings, although a bundled extension-plus-restump job draws on both. And supply-only StumpRite components from $10.35 ex GST are parts, not an installed footing. Call 0419 994 555 or send the plans, the soil report and a photo of the side gate through the contact form for a real number.

Choosing RapidStump, StumpRite or SurePile

Easy Footings Pty Ltd (ABN 44 689 644 175) installs BMSA concrete-free systems across Melbourne metro and regional Victoria — Geelong, Ballarat, Bendigo, the Mornington Peninsula, Gippsland, the Yarra Ranges, Sunbury and Macedon. We do not manufacture the steel. For extensions the three lines split like this:

• RapidStump is the usual pick for a stumped timber-frame rear extension on stiff Class M or better ground with side-gate access: a hybrid driven steel footing and stump in one, installed without a pour and ready for bearers the same visit.
• StumpRite is the adjustable steel house-stump range, and the natural choice when the new floor has to meet an existing stumped floor exactly, or when the extension and a restump of the old house are done together on one continuous datum. Up to 120 mm of screw height adjustment; supply only or installed. See also adjustable stumps installed in Victoria.
• SurePile steps in for Class H1 and H2 clay, old fill, a sloping backyard, a tree protection zone, or a heavier addition. The helical pile reaches competent ground several metres down and its installation torque confirms the engineer’s capacity on the day.

For a side-by-side of the three systems see RapidStump vs StumpRite vs SurePile. If you searched for extension footings because an addition is going onto a stumped Melbourne house on clay, the next step is a site look. Call 0419 994 555 or use the contact form.

Extension Footing FAQs

The Bottom Line

An extension footing is judged at the junction with the old house. Test the ground where the addition is going, find out what the existing house sits on, and let the engineer design the two to move together on Melbourne’s reactive clay. Pads are fine for a light addition on easy ground with open access. On Class H clay, fill, a slope, a tree or a side-gate-only backyard, found the new floor below the active zone: bored piers if a rig can reach and the program can wait, or engineer-certified driven, adjustable or helical steel if it cannot. On a stumped house, adjustable steel lands the new floor on the old one to the millimetre and keeps a second chance in hand. Call 0419 994 555 for a quote.