In short
In Tasmania a granny flat is a "secondary residence": up to 60 m² of gross floor area, measured to the outside of the walls, sharing the house's access, parking and service connections. Check with your council whether it needs a planning permit. It is generally notifiable building work, and the engineering is an AS 2870 site classification, footing or slab design, framing, stormwater and a Form 35 for the building surveyor.
Key takeaways
- The current planning limit is 60 m² gross floor area, measured to the outside of the walls. A proposed rise to 90 m² had not been approved when we checked.
- Planning and building approval are separate. Your council deals with planning; a building surveyor issues the Certificate of Likely Compliance.
- Every granny flat needs a site classification and footings designed for it, whether it's built on site or delivered as a transportable.
- Steep blocks add retaining walls, suspended floors and sometimes a landslip report. Price them in early.
Granny flats are one of the simplest ways to add a home to an existing block in Greater Hobart, for family, carers or a tenant. The planning rules decide how big it can be and where it can go. The engineering decides what it sits on. This guide covers both, with the Tasmanian forms and limits that apply.
Granny flats in Tasmania: quick facts
- Planning name
- Secondary residence (formerly ancillary dwelling)
- Size limit
- 60 m² gross floor area, outside of walls, decks excluded
- Building approval
- Generally notifiable work: building surveyor's CLC
- Engineering
- AS 2870 site class, footings or slab, framing, stormwater
- Certificate
- Form 35 from each designer
- Our fee
- Fixed quote per job, within 1–2 business hours
What counts as a granny flat in Tasmania?
A granny flat in Tasmania is a "secondary residence": an extra self-contained home on the same lot as a single dwelling, with a gross floor area of no more than 60 m². The planning scheme also calls it an ancillary dwelling in older documents. To qualify, it must:
- have a gross floor area of 60 m² or less (decks are excluded)
- be appurtenant to the single dwelling, meaning it belongs with the main house
- share the house's vehicle access and parking
- share water, sewer, electricity and telecommunications connections and meters, where the house is connected to them
Tasmania measures gross floor area to the outside of the walls, so 60 m² gives less internal space than the same figure in some other states. It can be attached to the house, inside it, or a separate building. If neither dwelling meets the definition, the pair is assessed as multiple dwellings, which have tighter rules.
90 m² is proposed, not in force
Draft Amendment 01-2026 of the State Planning Provisions proposes raising the limit to 90 m². It was exhibited from 10 June 2026, with representations closing on 22 July 2026. We found no decision approving it when we checked in November 2026. Kingborough was still to move to the Tasmanian Planning Scheme in May 2026. Check the current rule with your council.
Does a granny flat need a planning permit and building approval?
A granny flat needs building approval, and often a planning permit as well: they are two separate steps. A secondary residence is allowed wherever a single dwelling is allowed, but setbacks, height, site coverage and overlays such as heritage, landslip or bushfire can still require a permit. Some councils, such as Sorell, say planning approval is needed in every case.
| Planning approval | Building approval | |
|---|---|---|
| Law | Land Use Planning and Approvals Act 1993, Tasmanian Planning Scheme | Building Act 2016 and the NCC (NCC 2022 until 30 April 2027) |
| Who decides | Your council, as planning authority | A building surveyor issues a Certificate of Likely Compliance; council is notified |
| What it checks | Use, size, siting, setbacks, height, parking, overlays | Structure, footings, fire, energy, wet areas, stormwater |
| Engineering input | Sometimes: a landslip report or stormwater concept | Always: site classification, footing or slab design, Form 35 |
| Typical category | Depends on zone, design and overlays: ask your council | Generally notifiable; permit work in some hazard areas |
Our building approvals guide explains low-risk, notifiable and permit work.
What engineering does a granny flat need?
A granny flat needs the same structural engineering as a small house: a site classification, footings or a slab designed for that class, framing that suits the site's wind conditions, and a Form 35 from each designer for the building surveyor.
Site classification
A soil investigation to AS 2870 gives the site class, from A to E or P. Clays weathered from dolerite are documented as expansive in Hobart. See site classification and footings.
Footings or slab
A slab, strip footings or piers sized for the site class and loads. On reactive clay, the design limits how much the floor moves.
Framing and bracing
Beams, lintels and bracing for the site's wind classification, which depends on terrain, shielding and topography, not the suburb.
Form 35
The Certificate of the Responsible Designer that goes with the Certificate of Likely Compliance application.
Decks and verandahs attached to the granny flat count too. A deck that is roofed, or 1 m or more above the ground below, is notifiable work and usually needs a deck design (from $450 + GST).
What changes on a steep block?
On a steep Hobart block, the engineering grows: the choice between cutting a level pad and suspending the floor, the retaining walls that come with it, and sometimes a landslip report. These items often cost more than the footings themselves, so design them first.
- Retaining walls: over 1.2 m, or over 0.5 m within 1.5 m of a boundary, a wall is no longer low risk. See retaining wall design (from $750 + GST).
- Suspended floors: piers or stumps down the slope can avoid big cuts, but need bracing and footings into firm ground.
- Landslip bands: in any band, low-risk work becomes notifiable and a landslip report may be needed. See building in a landslip area.
- Access: a narrow or steep driveway limits machines and deliveries, which shapes the footing type.
Steep, vegetated blocks are often bushfire-prone too. We don't prepare bushfire assessments: a bushfire hazard practitioner prepares that.
What about stormwater and services?
A granny flat shares the property's stormwater connection, and the extra roof and paving can trigger on-site detention. Simple stormwater work, up to a 290 m² catchment with a 100 mm connection and no detention tank, can be low-risk plumbing work. Anything beyond that, including any detention, is notifiable plumbing work designed by a licensed designer with a Form 35.
- City of Hobart: development adding less than 100 m² of impervious area, with no change of use, is exempt from its detention requirements. Above that, standard tanks apply up to 300 m², and an engineer's design beyond.
- Council pipes: building within 1 m of a council stormwater pipe, or on a stormwater easement, needs the council's consent. See building over a stormwater easement.
- Other councils such as Clarence use their own detention tables. Our stormwater and drainage service covers the design.
Can you convert a garage or use a transportable?
Yes, but both still need engineering. Converting a garage changes it from a Class 10a building to a Class 1a dwelling, so it needs building approval. The existing slab, footings and framing were designed for a car, not a home, so an engineer checks what they can carry and designs any upgrades, such as new footings under added walls or a beam where a door is filled in.
A transportable or modular granny flat arrives finished, but the site still needs a site classification, footings or stumps designed for that soil and slope, and tie-downs for the site's wind conditions. The building surveyor will ask for those site-specific details, not just the factory's drawings.
When should you call a structural engineer for a granny flat?
- Once you have a concept plan, before the design is fixed, so footings and walls are priced early
- When the building surveyor asks for a site classification, footing design or Form 35
- On a sloping block, where cut, fill and retaining walls drive the cost
- Before converting a garage or removing walls in the main house to link the two
- Near a council stormwater pipe or easement shown on your title
One fixed quote for the engineering
Our extensions and renovations service covers granny flats: site classification, footings, framing and the Form 35, priced as one fixed quote. Send your plans for a quote within 1–2 business hours. Get a fixed quote.
Need an engineer?
Planning a granny flat?
Send your concept plans and the address. We reply within 1–2 business hours with a fixed quote for the engineering.
Sources
- State Planning Office, Draft Amendment 01-2026 of the State Planning Provisions: Secondary residences, explanatory document (May 2026). Current 60 m² definition, proposed 90 m², gross floor area measured to the outside of walls
- Sorell Council, Info Sheet: Secondary Residences (July 2025). Planning approval and "generally notifiable building works"
- City of Hobart, Stormwater Management Policy for Development (adopted 27 May 2024). On-site detention thresholds and consent within 1 m of a council pipe
- Director of Building Control, Determination: Categories of Plumbing Work v1.4 (18 August 2022). Low-risk stormwater limits (2.3.4) and notifiable stormwater work (3.0.1)
- Urban Drainage (General) Regulations 2026 (Tas). Structures that need council consent near public stormwater