Granny Flat NumbersCosts, rents and rules · Australia Price my flat
Our research

Methodology and data

Every number on this site comes from one of four places: our cost model, government bond records, the planning and tax rules themselves, or our reading of public forum posts. This page lists each input and where it comes from.

By Granny Flat Numbers editorial team · Updated

The cost model

The turnkey cost of a flat is the sum of seven lines. Construction is floor area times a rate per m² for the finish level ($2,450, $3,000 or $3,900 in Sydney, incl. GST), adjusted by a state factor, by 18% for regional areas and by a small-flat premium that tapers from 12% at 30 m² to zero at 50 m². Prefab uses 92% of the rate plus $12,000 for transport and crane; a kit with an owner-builder uses 62%. Site preparation is $12,000, $25,000 or $45,000; connections are $10,000 within 12 m plus $350 a metre; design and reports $8,000; then the state's approval route and, in NSW, a typical council contribution. The likely range shown is ±12%.

We set these values from published 2026 builder price lists and cost guides, and checked that the result matches published turnkey ranges: our Sydney 60 m² standard flat at $220,000 sits inside the $170,000–230,000 builders quote for the same size. The calculator and every table on the site use the same code, so they always agree.

Rents

Rents are medians of new bonds lodged with state governments, not asking rents. For NSW we downloaded the raw lodgement files for July–September 2026 (81,668 bonds) and computed medians ourselves by postcode group, dwelling type and bedrooms. For Tasmania we did the same with 8,219 record-level bonds from January–June 2026. In both states we used detached dwellings, the category granny flats are lodged under. Queensland (RTA, September quarter 2026), Victoria (Homes Victoria, September quarter 2025, the latest published) and South Australia (SA Housing Trust, April–June 2026) publish medians by area; there we used one-bedroom flats and two-bedroom houses. Groups with fewer than 10 bonds are left out.

Yield and payback

Rent is reduced by 2 vacant weeks a year. Running costs: agent 7.7% of rent, landlord insurance $650, maintenance 0.6% of the build cost, extra water and rates $700 a year. Tax is at the marginal rate you choose (30% by default), after the capital works deduction of 2.5% of 85% of the build cost. Payback is the build cost divided by the cash left after tax. On the "worth it" page we also charge for wear, spreading the construction cost over 40 years.

Rules and tax

Size limits, approval routes and dates are taken from the planning instruments and government pages, checked in October 2026: the NSW Housing SEPP 2021, Victoria's small second dwelling provisions as amended by VC306, Queensland's secondary dwelling guidance and council planning schemes, WA's R-Codes ancillary dwelling sheet, PlanSA's code amendment, Tasmania's State Planning Provisions and the ACT's missing middle reforms. Land tax uses Revenue NSW and SRO Victoria rates and RevenueSA's guide; capital gains tax and granny flat arrangements follow the ATO; pension rules follow Services Australia and the Social Security Guide.

Forum analysis

We searched the Whirlpool forums for threads about granny flats, secondary dwellings, ancillary dwellings and small second dwellings, and read 97 on-topic threads in full: 1,913 posts from 2011 to 2026, 481 of them from 2021 on. Each thread was assigned one main topic. From the recent posts we coded 39 reports (28 first-hand). We paraphrase them without usernames. This is an analysis of public posts, not a survey, and it over-represents problems.

Limits

  • Our costs are typical figures for a straightforward build. Your quotes can differ by more than our ±12% range on difficult sites.
  • Bond medians are for all dwellings of a type, not only granny flats. Victoria's data is a year older than the others.
  • Western Australia and the ACT do not publish comparable bond medians; for those we show the rent a flat needs rather than a payback.
  • Tax examples are illustrations, not advice; your accountant should run your own figures.

Input data

  • Construction rates: Standard = midpoint of Sydney metro granny flat build rate $2,500–3,500/m² (BRIbuild 'Granny flat costs by state', April 2026); budget and premium tiers scaled from giantA (July 2026) budget $1,800–2,200, mid $2,200–2,800, premium $2,800–3,500/m² and Word of Mouth (mid-2026) tiers.
  • Regional factor: Regional/metro ratio of BRIbuild midpoints, e.g. NSW $2,450 vs $3,000/m² (April 2026).
  • Small flats: Studio and 1-bed rates run 10–15% above 2-bed rates per m² (Duplex Building Design, July 2025: studio $2,100–3,000 vs 2-bed $1,800–2,600/m²). We taper the premium from +12% at 30 m² to zero at 50 m² so the price never jumps at one size.
  • Prefab: Prefab $90k–180k delivered and installed (ZoneScout, Aug 2026); delivery/offload $2k–10k plus crane (ArchiEng, May 2026). Kit: Steel kit 60 m² $41,200 supply only (Granny Flat Prices Australia, 2026); MaxTrans owner-builder total ~$70k for kit $25k + site, trades and fit-out (2025).
  • Site preparation: Site preparation $10k–30k (Amescorp, Oct 2026), $14k–41k (BlueGum, 2026), $15k–60k (The Quote Yard, 2026); sloping block +$5k–25k (giantA, July 2026).
  • Connections: Connections $10,900 for services within 12 m (Superior Granny Flats, 2025); $8k–15k (Amescorp, Oct 2026); $15k–30k when runs are long (Draftee, June 2026).
  • Design: Design and documentation $6,600–15,000 (giantA, July 2026); soil test and survey $2k–4k (Word of Mouth, mid-2026).
  • Extras: Landscaping & finishing $7k–15k (Innovista, Jan 2026); separate meters $3k–8k (Granny Flat NSW 2026).
  • New South Wales: CDC bundle $2,500–4,500 plus construction certificate (Duplex Building Design, July 2026); DA $8k–25k; s7.11 2-bed secondary dwelling: Blacktown $5,535, Canterbury-Bankstown $9,042 (March 2026 quarter).
  • Victoria: Melbourne metro $2,200–3,250/m² (BRIbuild, April 2026); council permits & compliance $6k–13k (Innovista, Jan 2026).
  • Queensland: Brisbane metro $2,300–3,100/m² (BRIbuild, April 2026); certifier $3k–6k, plumbing $500–1,500, QBCC warranty $1,500–3,500, town planner $3k–8k (Outhaus, Feb 2026). Infrastructure charges vary by council: Logan ~$22k net, Moreton Bay and Ipswich generally $0.
  • Western Australia: Perth metro $2,200–3,250/m² (BRIbuild, April 2026).
  • South Australia: Adelaide metro $2,000–3,100/m² (BRIbuild, April 2026); building consent $1,500–3,500, planning fee $500–1,500 (The Quote Yard, March 2026).
  • Tasmania: Hobart metro $2,100–3,200/m² (BRIbuild, April 2026).
  • Australian Capital Territory: Canberra 60 m² 2-bed $200k–260k vs Sydney metro $170k–230k (Word of Mouth, mid-2026), i.e. about 12% higher.
  • Rents, NSW: NSW Fair Trading rental bond lodgements, July–September 2026: medians computed by us from 81,668 raw bonds, detached 1- and 2-bedroom dwellings
  • Rents, QLD: Residential Tenancies Authority, median rents by LGA, new bonds, September quarter 2026
  • Rents, VIC: Homes Victoria Rental Report, median rents by LGA, September quarter 2025 (latest published)
  • Rents, TAS: Department of Justice (Tasmania) rental bond data, new bonds January–June 2026: medians computed by us from 8,219 bonds, detached dwellings
  • Rents, SA: SA Housing Trust Private Rental Report (CBS bond data), April–June 2026
  • Yield assumption (vacancy weeks): Our assumption: two weeks a year between tenants (one re-letting every 2 years at ~4 weeks). Rental vacancy in most capitals was near 1% in 2026, so this is conservative.
  • Yield assumption (management rate): Typical agent fee range quoted by AU property managers in 2026 was 5–10% of rent collected plus GST; 7% + GST = 7.7%.
  • Yield assumption (insurance): Landlord insurance quotes for a small detached dwelling were $400–$900 a year in 2026; midpoint used.
  • Yield assumption (maintenance rate): Our assumption for a new build: 0.6% of the build cost a year (appliances, painting, hot water unit over its life).
  • Yield assumption (rates water): Extra water/sewer usage and access charges that the owner cannot pass on without a separate meter, plus a small council-rate uplift in councils that rate secondary dwellings. Our estimate.
  • Yield assumption (div43 rate): ATO Division 43 capital works deduction: 2.5% a year for 40 years for residential construction started after 15 Sep 1987.
  • Yield assumption (construction share): Share of the total project cost that is capital works (construction, connections, design). Approval fees and some site costs are not deductible as capital works.
  • NSW bond data: NSW Government rental bond data
  • Queensland rents: Residential Tenancies Authority median rents
  • Victorian rents: Homes Victoria rental report
  • South Australian rents: SA private rental report data
  • Tasmanian bonds: Department of Justice (Tasmania), rental bond and rental data, January–June 2026, data.gov.au
  • Brisbane infrastructure charges: Brisbane Infrastructure Charges Resolution (No. 15) 2026
  • Pension conversion factors: Social Security Guide 4.6.4.60, reviewed 4 November 2024
  • Forum posts: Whirlpool forums, 97 threads, 2011–2026, read October 2026