Secondary residences
Granny flat and dual occupancy engineering in Canberra
A granny flat or a second home on your block needs building approval, and it can't use the development approval exemptions. We design the slab, footings, frame and any retaining, and plan around a sewer main if one crosses the backyard.
- Fixed quote for your granny flat or second home, from your plans
- Fixed quote in under 20 minutes on average, during business hours
- Up to 90 m² for a secondary residence, one per block
Prefer to talk? Call 02 6189 9876
Or email info@canberrastructuralengineers.com.au
Monday to Friday, 8:00am to 5:00pm
- Engineering onlyWe never sell the repair or the build.
- Fixed quote per jobPriced from your photos or plans before we start.
- Fixed quote in under 20 minutes on averageDuring business hours, Monday to Friday.
- Canberra and the Queanbeyan sideEvery district, plus Queanbeyan, Jerrabomberra and Googong.
In short
Engineering for a granny flat or dual occupancy in Canberra is a fixed quote, priced from your plans. A secondary residence can be up to 90 m², one per block, alongside a single dwelling. A second dwelling rules out the development approval exemptions, so plan for DA and building approval. Footing and slab design starts from $1,290 + GST ($1,419 inc GST). Same prices across Canberra, Queanbeyan, Jerrabomberra and Googong.
- Fee
- Fixed quote, set from your plans before we start
- Quote
- Fixed, in under 20 minutes on average, during business hours
- Footings and site class
- From $1,290 + GST ($1,419 inc GST), soil test at cost
- Near a sewer main
- Footing design from $790 + GST ($869 inc GST)
- Size rule
- Up to 90 m², one per block, up to 2 storeys
- Area
- Every Canberra district, plus Queanbeyan, Jerrabomberra and Googong
Do I need approval for a granny flat in the ACT?
Yes, development approval and building approval. A granny flat adds a second dwelling, which rules out the DA exemptions, and as a house it needs building approval from a licensed certifier.
"Secondary residence" is the planning term for a granny flat. Under the Territory Plan from 1 July 2026, it can be up to 90 m², measured to the outside face of the external walls and excluding a garage. It's one per block, up to two storeys, and only alongside single dwelling housing. The same changes removed the old minimum block size thresholds.
Exempt development must not take a block to two or more dwellings. So a granny flat or a dual occupancy (two homes on one block) needs a development application (DA), as well as building approval (BA) with the engineering details the certifier must lodge.
One change is pending. A bill that would let secondary residences be built on single-dwelling leases without a lease variation passed in October 2026, but it wasn't in force when we wrote this. Check the current position on ACT Planning's Missing Middle page before relying on it. Our guide to granny flats and dual occupancy in the ACT covers the planning rules in full.
What engineering does a granny flat need?
The same structural parts as a small house: a site classification, footings and slab, the frame, and retaining where the block slopes. You get calculations and drawings your certifier and builder can use.
| Item | What we check or design |
|---|---|
| Site classification | How much the backyard's soil moves as it wets and dries, from a soil test we arrange at cost |
| Footings and slab | Founding depth and founding material, reinforcement, slab preparation and concrete strength |
| Frame | Steel or timber framing, bracing, tie-downs and fixings |
| Sewer and stormwater mains | Footings set out of the pipe protection envelope, with drawings for Icon Water |
| Slopes | Retaining walls, with drainage, founding levels and backfill |
| Two attached homes | The shared wall and the footings under it |
Your fixed quote lists the items your plans need and says if a site visit is needed.
A site classification done for the main house, if there is one, may not cover the backyard, especially where there's been cut or fill since. That's true whether your block is in an established suburb in Belconnen or a newer street in Gungahlin. More on that in footings, slabs and site classification.
What if a sewer main runs through the backyard?
Icon Water's requirements come first. Get its approval before building work starts, and keep the footings out of the pipe protection envelope. We design footings that do that and show it on the drawings.
Icon Water's published guidance indicates that its approval is needed before building work starts, even where no ACT planning approval is required. It treats the "pipe protection envelope" around a pipe as "the controlling requirement in all cases". Footings aren't permitted inside it, and "no permanent heavyweight structure is to be built within an easement or pipe protection envelope".
We don't quote clearance distances: they come from Icon Water's requirements for your pipe. Our footing design near sewer and stormwater mains starts $790 + GST ($869 inc GST), and the guide to building near a sewer in Canberra explains the terms.
- Find the main early. Mark where it runs before the granny flat is drawn, so the design can work around it.
- Deepen or set back. Footings can go deeper or sit further away so their load stays out of the envelope.
- Move the layout if needed. Shifting the granny flat a little can be simpler than deeper footings.
- One set of drawings for Icon Water and your certifier.
Can I build a dual occupancy on an RZ1 block now?
The rules now allow it, subject to the controls for your block. Since 1 July 2026, RZ1 allows duplexes, triplexes, terrace homes, townhouses and low-rise apartments of up to two storeys.
The Missing Middle Housing reforms started on 1 July 2026. RZ1 now allows those housing types up to two storeys, and RZ2 up to three. RZ1 blocks can also be subdivided without first building a dwelling, provided minimum block sizes are met. We don't quote those minimums here; your designer checks them for your block.
A dual occupancy is two full homes, so the engineering is two sets of footings, slabs and frames. Attached homes add a shared wall and the footings under it, and a sloping block adds retaining. For three homes or more, see townhouse and multi-unit engineering.
| Granny flat | Dual occupancy | |
|---|---|---|
| What it is | A secondary residence beside the main house | Two dwellings on one block |
| Size | Up to 90 m², excluding a garage | Set by the controls for your block |
| How many | One per block | Two homes |
| Storeys | Up to 2 | Up to 2 in RZ1 |
| Approvals | DA and BA | DA and BA |
What sets the price of granny flat engineering?
It's a fixed quote, set from your plans before we start. Size, storeys, slope, soil, a nearby main and how many drawings your certifier asks for all move it.
What should I send for a quote?
We're engineers only. We don't build granny flats, sell kits or recommend builders, so the design follows your block and your plans.
- Plans, yours or the builder's, or a sketch with sizes
- Photos of the spot where it will go, showing the slope and any trees
- Where the sewer runs, if you know, and any maintenance hole in the yard
- The suburb, and any soil report you already have for the block
What happens next?
Three steps, and you know the price before anything is booked.
Send your plans
Use the form above, or our full quote form. Add up to 5 photos or plans.
Get a fixed quote
In under 20 minutes on average, during business hours. It lists what we'll design and says if a site visit is needed.
Design and drawings
We classify the site, design the footings, slab and frame, and send drawings for your certifier and builder.
Often part of a granny flat or second home
Every service is quoted at a fixed price before we start. Send photos or plans and we reply in under 20 minutes on average, during business hours.
Building near sewer & stormwater mains
Footing design that keeps new work clear of sewer and stormwater mains, plus stormwater plans.
$790 + GST($869 inc GST) See the service
Footings, slabs & site classification
Site classification and footing or slab design for new homes, rebuilds, granny flats and extensions, with the soil test at cost.
$1,290 + GST($1,419 inc GST) See the service
Townhouses & multi-unit
Structural design for duplexes, townhouses and small multi-unit projects, including Missing Middle housing on RZ1 and RZ2 blocks.
Fixed quotefrom your photos or plans See the serviceGranny flat and dual occupancy questions
Something else on your mind? Call 02 6189 9876, email info@canberrastructuralengineers.com.au or send photos for a fixed quote.
How big can a granny flat be in Canberra?
Up to 90 m², measured to the outside face of the external walls and not counting a garage. It's one per block, up to two storeys, and only alongside a single dwelling.
Is there still a minimum block size for a granny flat in the ACT?
No. The Missing Middle reforms that started on 1 July 2026 removed the minimum block size thresholds that used to limit secondary residences.
How much does granny flat engineering cost?
It's a fixed quote from your plans. Footing and slab design with a site classification starts from $1,290 + GST ($1,419 inc GST), and footing design near a sewer main starts from $790 + GST ($869 inc GST).
Can a granny flat be built over a sewer main?
Not with footings inside the pipe protection envelope. Icon Water's guidance puts its approval before building work, and we design footings that keep the loads out of the envelope.
Can you check a granny flat builder's engineering?
Yes. A desktop review of plans or engineering you already have is from $390 + GST ($429 inc GST). We read what you send and give written advice, with no site visit.