What our analysis shows
Our calculations, data as of 23 October 2026
- On a flat block with the sewer within 12 m, site work, connections, design and approval are 18% of a Sydney flat's $220,000 cost. On a steep block with a 40 m service run they are 32%.
- Add a sewer main under the building footprint and a BAL-29 bushfire rating, and the same 60 m² flat reaches about $305,300, $85,000 more than the easy block.
- Every extra metre of trench to the sewer and water costs about $350 in our model. A flat at the back of a 50 m block can carry $13,000 more in connections than one beside the house.
- Where the flat sits on the block is the cheapest saving available. Moving it clear of a sewer main, closer to the house connections or onto the level part of the yard costs nothing on paper and can take thousands off the site work.
Site extras that cost the most, largest first
- BAL-40 bushfire construction$40,000–80,000
- Steep block, rock or retaining$45,000
- BAL-29 bushfire construction$20,000–45,000
- Service run of 40 m instead of 12 m$9,800
- Building over a sewer main (our allowance)$10,000
- Separate electricity meter$3,000
Our model values and allowances, and published 2026 ranges for bushfire construction. Excludes demolition and tree removal.
Three blocks, one flat
The same 60 m² two-bedroom flat, standard finish, built in Sydney on three different blocks:
| Line | Easy block | Typical block | Hard block |
|---|---|---|---|
| Construction | $180,000 | $180,000 | $180,000 |
| Site preparation | $12,000 | $25,000 | $45,000 |
| Sewer, water, power and stormwater connections | $10,000 | $14,600 | $19,800 |
| Design, engineering and BASIX certificate | $8,000 | $8,000 | $8,000 |
| Approval: Complying development certificate | $4,500 | $4,500 | $4,500 |
| Council s7.11 contribution | $5,500 | $5,500 | $5,500 |
| Sewer encasement allowance (main under the flat) | – | – | $10,000 |
| BAL-29 bushfire construction | – | – | $32,500 |
| Total | $220,000 | $237,600 | $305,300 |
Easy: flat, open access, services within 12 m. Typical: some slope or a tight side path, services 25 m away. Hard: steep with rock or retaining walls, services 40 m away, a sewer main under the footprint and a BAL-29 rating.
The building itself costs the same on all three blocks. What changes is the ground. This is why "from" prices on builders' websites mean little until someone has stood on your block with a soil test and the sewer diagram.
Distance to the sewer
Sewer, water, power and stormwater all run from the main house or the street to the flat. Our model allows $10,000 with the flat within 12 m, then $350 for each further metre of trench and pipe:
| Service run | Extra over 12 m |
|---|---|
| 5 m | – |
| 12 m | – |
| 20 m | +$2,800 |
| 30 m | +$6,300 |
| 40 m | +$9,800 |
| 60 m | +$16,800 |
If the flat sits below the sewer connection point, it may need a pump station, which adds both cost and a running part that can fail. Ask the plumber for the fall from the flat to the connection before you fix the flat's position.
Building near a sewer main
Many suburban backyards have a sewer main running across them. Water authorities control building near the pipe, and within its zone of influence, roughly its depth plus a margin either side, footings must go down past the pipe's level or be engineered so the building puts no load on it. An NSW approvals firm's guide, written in 2012, shows how the cost rises as the flat moves closer: a records search when the flat is well clear, peg-out and inspections plus a short engineering design when it is near, and concrete encasement at about $1,000 a metre when it sits on top of the pipe. Trade prices have risen since, but the order of magnitude has not changed: a few hundred dollars well clear of the main, several thousand on top of it.
Order the sewer service diagram (a few dollars from the water authority) before a designer draws anything. Moving a flat 2–3 m usually costs nothing on paper and can save thousands on site.
Sloping blocks and stormwater
On a slope, a slab means cutting and filling, and often retaining walls; that is the $45,000 in our steep-site figure. An engineered subfloor on piers, with bearers and joists, avoids most of the earthworks and lets overland water pass underneath. It costs more per m² of floor than a slab on level ground, but on a steep block it is often cheaper overall, so ask for both prices.
Stormwater is the other trap on a block that falls away from the street. Roof water must reach a legal discharge point. If the street is uphill and there is no drainage easement through a downhill neighbour, the options are a pump, an absorption trench where the soil allows it, or negotiating an easement, and councils can refuse an application that cannot drain. Check this before design.
Bushfire rating
In bushfire-prone areas the flat must be built to its Bushfire Attack Level. Published 2026 ranges for the extra construction cost over a standard build: BAL-12.5 $5,000–15,000, BAL-19 $10,000–25,000, BAL-29 $20,000–45,000, BAL-40 $40,000–80,000. In NSW a complying development certificate is not available at BAL-40 or in the flame zone, so the flat also needs a development application. A BAL assessment costs a few hundred dollars and should come before design.
Before you get quotes
- Sewer service diagram, and the depth of any main crossing the yard.
- Soil test (about $450–1,400) so the slab design and price are fixed, not an allowance.
- Contour survey if the block falls more than about 1 m across the flat's footprint.
- Access width: a 1.5 m side path rules out machinery and some prefab cranes, and adds labour.
- BAL assessment if the area is mapped bushfire-prone.
- Power: whether the house switchboard and supply can carry a second dwelling.
With these in hand, ask every builder for fixed site costs. A $9,000 variation for a thicker slab and extra piers after approval is a story owners still tell on forums; see what owners report and comparing builder quotes.
This week's worked example
Week of 19 October 2026. A new example, rule and owner report every Monday.
A 60 m² two-bedroom flat with a budget finish on a steep block, services 30 m from the house, in Greater Bendigo: our model puts it at $184,000 turnkey ($162,000–$206,000), or $3,070 per m². At the area's median 2-bedroom bond rent of $450 a week it would return 10.0% net and pay back in 13.1 years after tax.
Rule check: Builder deposits are capped by law: 10% in NSW, 5% in Victoria and Queensland for contracts of $20,000 or more, and 6.5% in Western Australia.
From an owner (NSW, 2023): A Mascot granny flat tenant could not order NBN because the flat's address was not registered; NBN offered a paid install, or council address registration first.