When does a retaining wall need a permit and an engineer in Tasmania?

Hobart's slopes mean a lot of retaining walls, and most owners want two answers: does this wall need approval, and does it need an engineer? In Tasmania both come from the Director's Determination: Categories of Building and Demolition Work (version 1.4, applying since 12 April 2021). Height decides most of it, but where the wall sits can matter just as much. Your building surveyor confirms the category for your wall, and whether a newer version of the Determination applies.

Retaining wall rules in Tasmania: quick facts

Low risk near a boundary

Up to 0.5 m retained within 1.5 m

Low risk elsewhere

Up to 1.2 m, set back its own height from buildings

Engineer designed

Up to 1.8 m, built by a licensed builder

Notifiable

Over 1.8 m, or in an easement or landslip band

Planning

May apply as well (City of Hobart: near a boundary, or 1 m or more)

Our design

From $750 + GST

What height of retaining wall needs a permit in Tasmania?

A retaining wall needs a building surveyor once it retains more than 1.8 m, or sits in an easement or a landslip hazard band; between the low-risk limits and 1.8 m it needs an engineer's design and a licensed builder, but no surveyor. In everyday speech "permit" means approval. For most walls above the limits that approval is notifiable work, with a building surveyor's Certificate of Likely Compliance, rather than a council building permit.

Source: Director's Determination v1.4. Every low-risk wall must also meet the Standard Limitations. Confirm the category for your wall with your building surveyor.

Two details catch people out. The 1.2 m wall must sit at least its own height away from any building, on your block or the neighbour's. And the Standard Limitations cap cut or fill at 500 mm within 1.5 m of a boundary, even for a licensed builder, so a deep cut or fill near a boundary can take a wall out of low risk. Category 2 walls also must not let the filling or excavation affect sewer, stormwater or water infrastructure.

How is the height of a retaining wall measured?

The Determination measures a retaining wall's height as the distance between the ground level in front of the wall and the level of the ground it retains. It is the step in the ground that counts, not the size of the blocks or the depth of the footing. Terraced walls are worth raising with your building surveyor early, because an upper wall close to a lower one adds load to it, and the engineer will design the two together.

Does my retaining wall need an engineer or a building surveyor?

Work through these questions in order, and stop at the first answer that settles it.

1. Is the wall in an easement, within 1 m of a council drain, within 2 m of TasWater infrastructure, or in a landslip hazard band?

   Check your title and ListMap.

   Yes: not low risk. A building surveyor, consents and a designNo: go to the next question

2. Does it retain no more than 0.5 m within 1.5 m of a boundary, road or right of way, or no more than 1.2 m elsewhere, set back its height from buildings?

   Cut and fill near the boundary must also stay within 500 mm.

   Yes: Category 1 low riskNo: go to the next question

3. Does it retain 1.8 m or less?

   Category 2 is for a licensed builder only.

   Yes: an engineer's design and a licensed builderNo: notifiable work, with an engineer's design and a building surveyor

4. Then check planning separately

   A wall can need a planning permit whatever its building category. See the planning section below.

What changes in an easement or a landslip band?

An easement or a landslip hazard band takes even a small wall out of low risk; West Tamar Council's example is that a 1 m high retaining wall in an easement, or on land in a landslip hazard band, is not low-risk work. Under the Standard Limitations, no building work may go over an existing drain or within 1 m of its edge without the council general manager's written consent, or into a service easement without the consent of the person it benefits, and work must stay outside TasWater easements and at least 2 m from TasWater infrastructure.

The Landslide Planning Map on ListMap has five bands. In the Acceptable band there are no landslip controls; the Landslip Hazard Code applies in the low, medium, medium-active and high bands. Low-risk work in one of those bands becomes notifiable, and a landslip hazard report is prepared by a geotechnical practitioner to the AGS 2007 landslide risk guidelines, with an AS 1726-2017 site investigation. Rosetta, in Glenorchy, and Taroona are documented landslide areas. Read building in a landslip area, building over a stormwater easement and our landslip reports page.

Does a retaining wall need a planning permit?

Sometimes, and separately from the building rules: the Determination says there is no connection between planning approval and the building category. The City of Hobart's fences page says a retaining wall within 1.5 m of any boundary, or one retaining a difference in ground level of 1 m or more, may require planning approval. It also notes that retaining walls are very likely to need building assessment. Heritage-listed places and heritage precincts add their own planning controls. Other councils publish their own advice, so ask the duty planner at yours before you commit to a design.

Can you put a fence on top of a retaining wall in Hobart?

Yes, but the fence is assessed as a fence, with its own height limits, and the wall must be designed for the extra load the fence puts on it. Here is what the published rules say.

• Building rules. A masonry or concrete fence up to 1.2 m, another fence up to 2.1 m, or a chain link fence up to 3 m is Category 1 low-risk work (clauses 1.3.6 to 1.3.8). Bigger fences, screens and boundary walls are notifiable (3.1.4).
• City of Hobart planning. A front fence, meaning within 4.5 m of a frontage, needs no permit up to 1.2 m above existing ground level, or up to 1.8 m if the part above 1.2 m is at least 30% see-through. Side and rear fences need a permit above 2.1 m. Heritage-listed places need a permit for any front fence, unless it matches the original.
• Beside a road. The City's safety fence policy makes the owner responsible for the first safety fence, and its upkeep, where the owner's own works create a level difference between their land and the road.

Structurally, a fence on top acts like a sail: wind on the fence pushes on the top of the wall. Tell the engineer about any fence, pool barrier or balustrade at the design stage, so the wall and the post footings are designed together.

What does an engineer provide for a retaining wall?

An engineer's retaining wall design gives your builder drawings and calculations to build from, and gives the building surveyor a Form 35, with the wall checked for the soil, the slope, drainage and anything loading the top.

• Wall type to suit the site: sandstone, concrete block, concrete, timber sleepers or gabions
• Footing and base sized for the ground, using a site classification where needed
• Drainage behind the wall: gravel, an ag pipe and where the water goes
• Loads on top: driveways, buildings, pools, fences and sloping ground above
• Form 35 for the building surveyor, and a Form 55 to certify a wall already built

Our retaining wall design starts from $750 + GST. Building a pool into a slope? Read putting a pool on a sloping block. Need a wall that's already built signed off? See Form 55 certificates.

When to call a structural engineer

Call an engineer before the wall is ordered or the ground is cut, while changing the design is still free.

• The wall will retain more than 0.5 m near a boundary or 1.2 m elsewhere
• The site is in a landslip band, or the wall is near an easement or drain
• A driveway, house, pool or fence will sit above or close to the wall
• An existing wall is leaning, cracking or bulging
• Your building surveyor has asked for a design or a Form 55