Guide · footings and site
Footings and site prep, without the mystery.
Soil tests, screw piles, slopes and access: what the ground under a granny flat costs and why.
Modular granny flats sit on engineered footings, usually screw piles at $4,500 to $9,000, sized from a soil test that costs a few hundred dollars. Site works all told ran $20,000 to $51,000 on our invoiced builds, with crane access the biggest swing.
The short version
A modular granny flat does not sit on a slab, it sits on engineered footings, usually screw piles, sized to a soil test. The soil test costs a few hundred dollars and drives everything: get it early. Budget $4,500 to $15,000 for footings on most SEQ blocks, plus site preparation that ranges from nothing on a flat mowed block to real money on slopes.
Soil first, always
The geotechnical report classifies your soil (sand, clay, reactivity) and the engineer sizes footings from it. Ours invoiced at $770. Skipping it means guessing, and footing guesses surface as door gaps and cracked linings two summers later. Flood-mapped sites also get a minimum floor height set by council; one of our builds carried a mandated finished floor level that set the whole subfloor design.
Footing types, priced
| System | Typical cost | Where it fits |
|---|---|---|
| Screw piles | $4,500–$9,000 | Most sites. Fast, no spoil, engineered per pile |
| Concrete piers | $6,000–$12,000 | Reactive clays, some slopes |
| Slab or strip | $10,000–$15,000+ | Rare for modular; specific engineering cases |
Site preparation that matters
- Access: the truck and crane need to reach the pad. Overhanging trees, tight gates and soft lawns are the usual surprises.
- Slope: modular handles slopes well, the module stays level while the piles change length, but crane time and pile depth both grow with the fall.
- Clearing: the pad plus working room. Council rules on vegetation clearing vary; small-lot dwellings are generally permitted to clear their footprint.
- Spoil and drainage: screw piles produce almost none, one reason we default to them. Stormwater must reach the legal discharge point, usually the kerb.
What we do versus what your installer does
On installed jobs inside our service area, footing design, supply and install are in our quoted site works: $20,000 to $33,000 typical all told, to $51,000 with hard crane access, invoiced on completed builds. On kit and ex-factory jobs anywhere in Australia, the engineered footing design ships with your drawings and any local installer or owner-builder works from it.
Straight answers
Why not a slab?
Modular dwellings arrive complete and get placed; piles suit that, cost less, produce no spoil, and handle slopes by varying pile length. Slabs make sense for site-built construction, not for craned modules.
What does a sloping block add?
Longer piles, more crane time, sometimes staged access. Slope is a footing and crane question, not a dealbreaker; it typically moves site works thousands, not tens of thousands.
Can my local builder do the footings?
Yes. The engineered footing design ships with every kit and ex-factory home, and any licensed builder or piling contractor installs to it.
Where this lands on price
Every price, every way to buy, on one page.
Kit $59,900 to finished and installed, with the all-in number printed.