In short
Two tests apply. For building approval, a retaining wall is exempt up to 1.2 m high, measured from the top of the wall to the lowest ground beside it; up to 1.5 m it's only partly exempt and must meet the building code. For development approval, a wall in the front zone or within 1.5 m of a side or rear boundary is only exempt up to 0.4 m.
Key takeaways
- Building approval: exempt up to 1.2 m, measured from the top of the wall to the lowest adjacent ground. Only partly exempt up to 1.5 m.
- Planning: 0.4 m in the front zone or within 1.5 m of a side or rear boundary. Elsewhere, cut up to 1.2 m and fill up to 1.0 m.
- Height means the actual earth cut being retained, and engineer plans must show it.
- A fence that's part of a retaining wall loses the fence exemption from building approval.
- No ACT law sets how neighbours share the cost of a retaining wall. The Common Boundaries Act covers fences and party walls only.
When does a retaining wall need building approval in the ACT?
Once it's taller than the exemptions allow. A retaining wall needs no building approval up to 1.2 m high. Between 1.2 m and 1.5 m it's only partly exempt: it must comply with the exempt building code, or the BCA deemed-to-satisfy provisions where there's no code. Beyond that, it needs building approval from a licensed certifier.
| Wall height, top of wall to lowest adjacent ground | Building approval |
|---|---|
| Up to 1.2 m | Exempt, if the general tests are met |
| Over 1.2 m, up to 1.5 m | Partly exempt: it must comply with the exempt building code (or the BCA deemed-to-satisfy provisions). In ACT Planning's words, "if it's more than 1.2m high, design and construction standards will apply" |
| Over 1.5 m | Outside the exemptions: building approval needed |
Sources: Building (General) Regulation 2008, Schedule 1 Part 1.3, items 4 and 5; ACT Planning BA page. Your certifier confirms which applies.
The general tests apply at any height. Exempt work must not affect "the structural integrity of any part of a building", and must not involve asbestos. Ask your certifier about any wall that holds up the ground under a house or its footings, even a low one.
How is retaining wall height measured?
For building approval, it's measured from any point at the top of the wall to the lowest adjacent ground level. Access Canberra's Construction Note 005/2021 adds that the height means the actual earth cut being retained, and that "all engineer plans must clearly show the actual height dimension of the earth cut to be retained".
For planning, heights are measured above datum ground level (DGL), the reference ground level the planning rules use. So the same wall is measured two ways for the two approvals, and it can pass one test and fail the other.
When does a retaining wall need development approval?
When it's taller than the planning limit for where it sits. Near boundaries, that limit is far lower than the building approval limit.
| Where the wall is | Exempt from development approval up to |
|---|---|
| Between the front boundary and the building line | 0.4 m above DGL |
| Within 1.5 m of a side or rear boundary | 0.4 m above DGL |
| Anywhere else on the block | Cut up to 1.2 m, fill up to 1.0 m, or a combination up to 1.2 m, measured on the lowest side |
Source: Planning (Exempt Development) Regulation 2023, Schedule 1, section 1.50.
A cut wall holds back natural ground you've dug into. A fill wall holds back soil brought in to build the ground up. Two more conditions apply near boundaries:
- Within 1.5 m of the boundary, the wall must be the only class 10 structure there, or meet the rules for a second structure (s 1.41).
- For a single dwelling built under the exempt controls, total cut or fill within 1.5 m of a side or rear boundary must not exceed 1.5 m (NI2025-492, control 18).
Illustrative example
A 0.9 m wall within 1.5 m of the rear boundary needs no building approval, because it's under 1.2 m. But it's over the 0.4 m planning limit for that spot, so it isn't exempt from development approval. The same 0.9 m wall as a cut further into the block could be exempt from both, if it meets the general tests.
Can I put a fence on top of a retaining wall?
You can, but it changes the approvals. The building approval exemption for fences, up to 1.8 m, doesn't apply to a fence that's part of a retaining wall.
The planning limits for fences are measured above datum ground level: panels up to 2.3 m and posts up to 2.5 m, with none between the front boundary and the building line. So a fence on top of filled ground is likely to be measured from datum ground level, not from the top of the fill. We found no single ACT rule for the combined height of a wall and fence: your certifier confirms.
Exempt fences and walls must also not divert or concentrate surface water so that it ponds or flows onto other land.
Who pays for a retaining wall between neighbours?
No ACT law sets it. The Common Boundaries Act 1981 covers dividing fences and party walls only, and doesn't mention retaining walls.
For fences, the Act sets a "basic urban fence": a 1.5 m timber paling fence. Neighbours each pay half its cost and maintenance, and the ACT Civil and Administrative Tribunal (ACAT) decides fence disputes. Retaining walls have no equivalent rule, so neighbours have to work out cost and responsibility between themselves. An engineer's design, with the wall's height and drainage set out, gives both sides the same facts to work from.
Across the border the rules differ. In Queanbeyan, Jerrabomberra and Googong, NSW rules apply, and we work with Queanbeyan-Palerang Regional Council and your certifier: see our Queanbeyan page.
When is a low retaining wall still not exempt?
When it fails one of the general tests. A wall well under the height limits still needs approval if:
- it's in an easement or utility protection space without the utility's written permission, or interferes with plumbing and drainage clearances;
- it's in a protected tree's protection zone, where excavating deeper than 100 mm, or raising soil more than 100 mm, over 4 m² or more is prohibited groundwork under the Urban Forest Act 2023 (see building near protected trees);
- the work would breach the Heritage Act 2004;
- it affects the structural integrity of a building, or involves asbestos.
On sloping new-estate blocks, check the cut and fill near side and rear boundaries early, before the house design is fixed. Hilly estates such as those in the Molonglo Valley are where this comes up, and our guide to building on new-estate blocks covers fill, rock and slopes.
When do you need an engineer for a retaining wall?
Whenever the wall needs building approval, and for any wall where a failure would affect a house, a driveway, a pool or the neighbour's land.
Over 1.2 m, design and construction standards apply. For a wall that needs building approval, the certifier lodges retaining wall details under form AF2025-18, including drainage, founding levels and backfill, and those usually come from an engineer. Ground matters too: a wall in reactive clay, on fill or near a sewer main needs footings designed for that ground.
Our retaining wall engineering starts at $890 + GST ($979 inc GST), with calculations and drawings your certifier and builder can use. Your fixed quote says if a site visit is needed. If you're building a house or extension on the same block, our footings, slabs and site classification work covers the ground under both. We design walls; we don't build them or recommend builders.
Planning a retaining wall?
Send a photo of the slope and the height you need for a fixed quote in under 20 minutes on average, during business hours.
Sources
- Building (General) Regulation 2008 (ACT), Schedule 1 Part 1.3. Items 1 to 5: fences and retaining walls
- ACT Planning: Check if you need a building approval (BA)
- Access Canberra Construction Note 005/2021: Retaining walls. Height means the actual earth cut retained
- Planning (Exempt Development) Regulation 2023 (ACT), Schedule 1. Sections 1.48 (fences) and 1.50 (retaining walls)
- Single dwelling development controls, NI2025-492. Control 18: cut and fill near boundaries
- ACT Planning: Check if you need a development approval (DA). General criteria for exempt work
- Common Boundaries Act 1981 (ACT). Dividing fences and party walls
- ACT City Services: Tree protection laws
