On an easy flat block, connections and site preparation are already inside the all-in price: about $176,000–$255,000 for a 60 m² granny flat in Melbourne. A harder block adds $10,000 to $50,000 or more. A slope adds $10,000–$35,000, a sewer pump $8,000–$16,000, reactive clay $12,000–$25,000 and tight access $5,000–$11,000.
Add these to the all-in price from the cost calculator. They are the same figures used across this site, checked 29 October 2026.
| Site issue | Adds about |
|---|---|
| Sloping block, more than about 1 m fall | $10,000–$35,000 |
| Sewer uphill or far away (pump needed) | $8,000–$16,000 |
| Tight side access (crane or hand work) | $5,000–$11,000 |
| Old shed or garage to remove | $4,000–$14,000 |
| Reactive clay soil (deeper footings) | $12,000–$25,000 |
| Second bathroom | $18,000–$28,000 |
Two or three of these on one block is common in older Melbourne suburbs. Ask every builder to price them as separate lines, so you can compare quotes like for like.
The flat’s drain joins the property’s existing sewer line, which runs to the water authority’s main at the legal point of discharge. If the flat’s floor sits lower than that point, or the run is long, waste has to be pumped, and that is where the $8,000–$16,000 comes from.
Melbourne is served by three retail water authorities: Greater Western Water, Yarra Valley Water and South East Water. Order the property’s sewerage plan from yours before a designer draws anything. If the flat would sit over or near a sewer easement, you will need the authority’s consent to build over or near it, and sometimes the pipe has to move.
Water usually tees off the house supply. Roof water from the flat has to go to the legal point of discharge for stormwater, often the kerb or an easement drain, or into a rainwater tank first. A long trench across a finished backyard is a cost people forget.
Footings are designed for the soil, which a geotechnical report sorts into site classes from A (sand and rock, little movement) through S, M, H1 and H2 to E (extremely reactive clay). Much of Melbourne’s west and north sits on reactive basalt clays, which is why deeper footings add $12,000–$25,000 there.
A flat block suits a concrete slab. On a slope of more than about 1 m, a raised floor on stumps or steel piers is often cheaper than cutting and retaining, but either way expect $10,000–$35,000 extra.
Modular and prefab flats arrive by truck and are lifted in by crane, so they need clear access or a crane that can reach over the house. With tight side access everything goes in by hand, which adds $5,000–$11,000. An old shed, garage or slab in the way costs $4,000–$14,000 to remove, more if it contains asbestos.
In Victoria a builder must be registered to take on domestic building work worth more than $10,000, and work over $16,000 needs domestic building insurance. The main house must already exist before a building permit can be issued for the flat. Read the Victorian rules for the planning side.
On an easy flat block with services close by, nothing extra: connections are already inside the all-in price, about $176,000–$255,000 for 60 m² in Melbourne. A hard block adds roughly $10,000 to $50,000 or more, mostly from slope, a sewer pump, reactive clay or tight access.
A short gravity run to the existing house drain is part of a normal build. If the flat sits lower than the sewer or far from it and needs a pump, budget about $8,000–$16,000 extra.
Sometimes. The Victorian building regulator says separate water or electricity meters may be possible depending on the utility provider. The cheaper option is a private sub-meter off the main house supply.
No, not if you want to skip the planning permit. A small second dwelling must not be connected to reticulated gas, so plan for induction cooking, a heat-pump hot water system and reverse-cycle heating.
1.8 m, unless the walls are fire-rated. The Victorian building regulator lists a 1.8 m separation from the main house for a small second dwelling without fire-rated construction.