What our analysis shows
Our calculations, data as of 23 October 2026
- Since 2021 the largest share of posts (183 of 481) is about internet, water meters and addresses, not about building. Getting a separate NBN connection to a rented flat is the most common practical problem owners describe.
- Approval questions come second (101 posts since 2021). The recurring story is not refusal but conflicting answers: certifiers and council officers disagreeing about the same lot.
- Unapproved flats are a buyer's problem more than a builder's. In the reports we coded, buyers learned of non-compliance from the contract or from council after settlement; one 2025 buyer faced about $15,000 of work before the flat could be rented.
- The owners' cost reports in 2025 ($250,000 all-in for 60 m² in Sydney, a $300,000 quote near Caboolture) sit at or above the top of our model's range. Builders' "from" prices are not what owners end up paying.
What the threads are about, all years
- Renting and tenants487 posts
- Internet, water and addresses288 posts
- Kits, prefab and owner-building278 posts
- Costs and builders219 posts
- Approval and council159 posts
- Unapproved flats135 posts
- Tax, pension and family113 posts
- Garage and under-house conversions78 posts
- Finance and loans60 posts
- Neighbours59 posts
- Resale value37 posts
97 threads, 1,913 posts, 2011–2026; 481 posts from 2021 onward. Our topic coding, one topic per thread.
What we read
We read every on-topic Whirlpool thread we could find about granny flats, secondary dwellings and small second dwellings, and gave each one main topic. From the posts since 2021, plus a few earlier ones that describe problems that still apply, we coded 37 reports: 27 first-hand accounts and 10 pieces of advice that several posters agreed on. We paraphrase them below without usernames. Older reports are dated so you can judge them.
Forum posters are not a random sample. People write when something is unclear or has gone wrong, so read this as a list of risks, not as odds.
Council and certifiers
A Logan owner's infrastructure charges notice: demand for the existing three-bedroom house plus a two-bedroom flat, minus a credit for the house, left $22,000 to pay.
QLD, 2024, first-hand
An owner of an R4-zoned lot in Cumberland got opposite answers on whether a granny flat was allowed: some certifiers and one council officer said yes, others said no.
NSW, 2023, first-hand
A follow-up two years later: of the certifiers a friend asked about the same zoning question, only one would issue a CDC.
NSW, 2025, first-hand
Ku-ring-gai council rejected an owner's granny flat; it was approved as a rumpus room and pool house once the stove was taken out.
NSW, 2021, first-hand
A heritage-listed family home cannot use the SEPP's complying development route for a granny flat; it has to go to council.
NSW, 2024, advice in thread
A design for a self-contained unit failed the building code because it used the kitchen sink as the laundry tub: the code requires a kitchen sink, a laundry tub and a basin as separate fixtures.
VIC, 2021, advice in thread
Council asked to inspect a garden studio for 'unauthorised use' after it began to be used for occasional overnight stays; the owner believed a neighbour had complained.
NSW, 2024, first-hand
Our reading: the Cumberland case shows why certifiers disagree. Section 50 of the NSW Housing SEPP applies the granny flat rules only where the local plan permits a dwelling house, and many R4 high-density zones prohibit dwelling houses. Get the answer in writing from the certifier who will sign the CDC, before you buy or design. In Queensland the infrastructure charge is the big surprise; Logan's $22,000 came with a development approval, which a compliant flat in Brisbane avoids. See Queensland costs.
Unapproved flats
A buyer of a brand-new house with a studio flat was given a rental appraisal by the selling agent, then told by council after settlement that it could not be rented until about $15,000 of compliance work was done.
NSW, 2025, first-hand
A buyer walked away from a Liverpool house whose granny flat had been built from an old carport and workshop on the boundary without approval.
NSW, 2023, first-hand
Several posters warned that asking council about legalising a flat, or a bank valuer's visit, can put an unapproved flat on the council's radar.
NSW, 2023, advice in thread
A family found through a council records search that their 1970s granny flat, rented out for decades, only ever had interim approval as a workshop.
NSW, 2025, first-hand
An owner seeking retrospective approval for a converted garage listed waterproofing the slab, a fire-rated wall and termite certificates; replies added a structural engineer's certificate and minimum ceiling height.
NSW, 2022, first-hand
Our reading: a selling agent's rental appraisal is not evidence that a flat may be rented. Ask for the occupation certificate or final inspection for the flat itself. Our guide to unapproved granny flats covers what legalising one involves.
Costs and variations
A poster's colleague paid $250,000 all-in for a 60 m² two-bedroom granny flat, doing only the cabinetry himself.
NSW, 2025, advice in thread
A family near Caboolture was quoted $300,000 for a small two-bedroom brick granny flat, which they called the price of an entry-level house.
QLD, 2025, first-hand
Another Brisbane-area owner found prefab units of any quality priced close to a small new three-bedroom house, and gave up.
QLD, 2025, first-hand
Converting a garage into a small second dwelling: the building surveyor required two replacement car spaces on the lot, even though the new rules need no parking for the flat itself.
VIC, 2024, first-hand
A Victorian owner-builder found the rules let them do framing, cladding and plastering, but not footings, subfloor, roofing, waterproofing, wiring or plumbing.
VIC, 2023, first-hand
One owner was quoted $30,000 just to turn half a double garage into a bedroom, with no structural work beyond a window and infilling the door.
NSW, 2023, first-hand
A quote to convert a garage into living space doubled within six months during the 2022–23 cost rises.
NSW, 2023, first-hand
A 7 x 4 m cedar studio cost $30,000 delivered and assembled to lock-up; with the family's own fit-out and bathroom it came to about $45,000.
NSW, 2021, first-hand
An owner who bought two transportables paid about $110,000 for 60 m² fully connected with council approval handled (2007), and $95,000 for 30 m² (2012).
QLD, 2021, first-hand
Replies warned that a $40,000 prefab budget ignores footings and electrical connection; $60,000 or more was a more realistic floor.
VIC, 2021, advice in thread
After design and approval, a builder added $9,000 for a thicker slab, edge beams, extra piers and sewer peg-out, which were not in the original price.
NSW, 2017, first-hand
Our reading: the 2017 variation of $9,000 for slab and piers is the classic site-cost gap; ask for fixed site costs after the soil test, not an allowance. Prefab and kit prices quoted "to lock-up" leave out footings, connections and fit-out, which owners put at $15,000–30,000 on top. Compare with our prefab versus custom and builder quote guides.
Internet, water and the address
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.
NSW, 2023, first-hand
An owner whose rented flat has its own address and meters obtained a second NBN location ID through a 'new development' application, but the conduit laid at build time had too many bends for the fibre pull.
Australia, 2026, first-hand
Wi-Fi from the main house did not reach a flat wrapped in foil insulation; a cable in conduit between the buildings did.
Australia, 2023, first-hand
After an owner asked Sydney Water about a separate meter for a rented granny flat, the property was reclassified and billed as dual occupancy.
NSW, 2012, first-hand
A common workaround: a private sub-meter on the flat's water line, with usage billed to the tenant each quarter.
NSW, 2023, advice in thread
Our reading: decide at design stage whether the flat will be rented separately. If it will, lay an NBN-spec lead-in conduit to the flat with as few bends as NBN's installation rules allow, register a separate address with council, and price a separate electricity meter. Each is cheap during the build and expensive afterwards.
Family, tax and pension
For a sister living rent-free in a flat on the owner's home, replies agreed there is no income tax or CGT effect; the CGT question arises only when rent is charged.
QLD, 2021, advice in thread
Adding a rented flat to a former home that is rented out under the six-year rule means apportioning CGT for the flat, because the owner never lived in that part.
Australia, 2023, advice in thread
Pensioner parents moving into a granny flat on a son's property were told that giving or lending their cash to the children would still count under the pension assets test for five years; paying to build the flat itself is assessed differently, as a granny flat interest.
QLD, 2026, first-hand
A family planning for a 77-year-old mother to downsize into a granny flat behind her own house struggled with how to protect the son's contribution, the siblings' inheritance and the pension at once.
NSW, 2025, first-hand
Our reading: a parent moving into a flat on a child's land is where most of the hard questions are. Who owns the flat, what happens to the money at death or entry into aged care, and how Centrelink sees it all need to be written down first. See a granny flat for your parents and capital gains tax.
The tenants' side
A renter counted about 10 new granny flats among 100 listings in one area, at $250–400 a week, against $300–450 for older standalone two-bedroom houses.
Australia, 2018, first-hand
Renters noted that older flats tend to include utilities in the rent while newer ones are separately metered.
Australia, 2018, advice in thread
A tenant whose landlord planned a granny flat in the yard was offered a $10 a week rent reduction during construction, for losing the garage and part of the yard.
NSW, 2018, first-hand
Our reading: tenants weigh a granny flat against a small house or apartment, and they mark it down for shared driveways, bins and yard. That matches what we found in NSW bond data: detached one-bedroom dwellings in Sydney rent about a quarter below one-bedroom flats. See rent by area.