The Blue Mountains has one of the strictest tree preservation regimes in NSW. BMCC’s tree management code protects a wide range of native trees, and some exotic trees near heritage items, from removal or damage without approval. Building a retaining wall near a protected tree requires careful planning to avoid root damage that can kill the tree, BMCC fines for tree damage without approval, and the liability that comes with harming a protected tree.
The bottom line: identify every tree within 5 metres of your proposed retaining wall before planning starts. Any tree over 4 metres in height, or any native tree over approximately 100mm trunk diameter, is likely protected. Get BMCC advice before excavating near it.
What Is a Tree Preservation Order in the Blue Mountains?
BMCC operates under its Development Control Plan (DCP) tree management code rather than individual tree preservation orders (TPOs) in most cases, the protection is area-wide rather than applied tree by tree. Under BMCC’s tree management code:
Protected trees include:
- Native trees over 4 metres in height (anywhere in the BMCC LGA)
- Native trees with a trunk circumference over 300mm (approximately 95mm diameter) at 1.0m above ground
- Trees with a canopy spread over 3 metres
- Trees listed as significant on BMCC’s significant tree register
- Trees within heritage conservation areas that contribute to the heritage character
- Some exotic trees in heritage conservation areas that are heritage-listed
What’s prohibited without approval:
- Removing a protected tree
- Damaging the roots, trunk, or canopy of a protected tree (this includes excavation near roots)
- Filling soil against the base of a protected tree
The relevant document is BMCC’s Tree Management Code, available on the BMCC website. Always check the current version, rules change.
The Root Protection Zone (RPZ)
The root protection zone is the area around a tree within which construction activity (particularly excavation, trenching, soil compaction, and fill placement) can cause root damage. Most councils define the RPZ based on trunk diameter and tree species.
BMCC’s general RPZ guidance:
- The structural roots of most trees extend laterally to approximately 5 times the trunk diameter at soil level, but many fine absorbing roots extend well beyond this
- For construction assessment, the commonly used RPZ is a radius equal to 12× the trunk diameter at breast height (1.3m above ground)
- Example: A tree with 300mm trunk diameter has an RPZ radius of 3.6 metres from the trunk centre
However, the actual RPZ varies by species. Shallow-rooted species (many eucalypts, some wattles) may have roots that extend to 15× the trunk diameter. Deep-rooted species may have less sensitive surface root zones. A qualified arborist can assess the specific tree.
How Retaining Walls Affect Tree Roots
Retaining wall construction near trees can damage roots in several ways:
Excavation (Cut Side)
Excavating for wall footings, drainage trenches, or the wall itself cuts through roots. Root damage from excavation creates entry points for pathogens, reduces the tree’s ability to take up water and nutrients, and can destabilise the root system (causing the tree to become unstable in wind). Cutting more than 20-30% of a tree’s root system can cause irreversible decline.
Compaction
Heavy machinery operating near a tree compacts the soil, reducing pore space and oxygen availability to roots. Root compaction kills roots slowly, the tree declines over 2-5 years after construction, and the cause is often not recognised.
Fill Placement (Fill Side)
Placing fill soil over an existing root zone, even clean, quality fill, changes the soil depth above the roots, affecting oxygen availability and root respiration. Raising the soil level by more than 100-200mm over the root zone of a sensitive tree can cause decline.
Changed Drainage
Retaining walls change drainage patterns. Concentrating water flow, preventing natural soil drying, or directing water onto roots can affect soil chemistry, oxygen levels, and pathogen growth in the root zone.
BMCC Approval for Work Near Protected Trees
Any excavation or fill within the RPZ of a protected tree, including excavation for a retaining wall footing, requires approval from BMCC. The approval pathway depends on what work is proposed:
If the tree is not being removed:
- A development application is typically required where work is proposed within the RPZ
- An arborist report (from a qualified arborist, typically AQF Level 5 or higher) assessing the impact on the tree is required
- BMCC may impose conditions including specific construction methods, a no-dig zone immediately around the trunk, and a construction monitoring role for the arborist
If the tree needs to be removed:
- A separate tree removal application is required
- BMCC approves tree removal for protected trees only in limited circumstances: the tree is dead or dying, it poses an imminent safety risk, or the development cannot reasonably be designed to retain the tree
- Approved tree removals typically require replanting equivalent canopy cover
For heritage-listed trees or trees within heritage conservation areas, the threshold for approval and conditions are more stringent. See [/guides/heritage-overlay-retaining-walls/].
Construction Methods That Protect Tree Roots
When a retaining wall must be built within the root protection zone of a protected tree, specific construction methods reduce root damage:
Hand Digging
For excavation within the RPZ, hand digging rather than machine excavation prevents root severance. Where roots are encountered, they should be pruned cleanly with secateurs or a pruning saw (not torn by a machine) and immediately treated with a wound sealant if specified by the arborist.
Root Bridging
For the wall footing closest to the tree, a root-bridging design can be used, the footing is deepened and narrowed, passing below the main root zone, rather than cutting through it. The wall panel bridges over the root zone at the surface. This requires engineering input to design the spanning element.
Air Spading
Air spading (using compressed air to excavate soil without cutting roots) is a technique used in high-value tree situations to locate and expose roots before any excavation, so the excavation can be designed to avoid major roots.
Soil Aeration and Decompaction
After construction adjacent to a tree, the root zone can be decompacted using an air spade or vertical mulching holes to restore soil oxygen levels. This can reduce post-construction decline.
Tree Protection Fencing
Tree protection fencing (1.8m orange plastic mesh fencing on steel star pickets) at the RPZ boundary must be installed before any construction activity starts and maintained throughout the project. This prevents machinery, materials stockpiles, and workers from encroaching on the root zone. BMCC typically requires this as a DA condition.
Practical Design Approach, Routing Around Trees
The most effective way to protect trees near a retaining wall is to route the wall around them. A wall that curves around the root protection zone of a significant tree costs more than a straight wall but avoids the approval complexity and root damage risk.
On Blue Mountains lots with multiple significant trees, this may mean a wall with several curves and changes in direction rather than a straight run. Good design works with the existing landscape rather than against it.
See [/guides/retaining-wall-garden-terrace-design/] for terracing approaches that can route retaining walls around existing trees by using multiple low walls instead of a single tall wall.
Arborist Reports, What’s Needed
For any retaining wall project within the RPZ of a protected tree, BMCC will require an arborist impact assessment report. The arborist report should:
- Identify and assess all trees within 5 metres of the proposed work
- Confirm tree species, health, structural condition, and RPZ dimensions
- Assess the likely impact of proposed excavation, fill, and construction on each tree
- Recommend construction methods to minimise impact
- Specify any monitoring requirements during and after construction
Cost of an arborist report: typically $600 to $1,500 for a residential retaining wall project with 2-4 trees assessed.
FAQ
Do I need BMCC approval to build a retaining wall within 3 metres of a large gum tree? Yes, if the eucalypt is over 4 metres in height or has a trunk diameter over 95mm, it is likely protected under BMCC’s tree management code. Construction within its RPZ requires a DA with an arborist report. Do not start work without this approval.
What happens if my contractor damages a protected tree? BMCC can issue fines for damage to protected trees, penalties under BMCC’s enforcement provisions can reach $1.1 million for serious breaches. You as the property owner may be liable even if a contractor caused the damage. Ensure your contractor is aware of the tree protection requirements and follows the BMCC-approved construction methods.
Can I remove a tree to make room for my retaining wall? Only if BMCC approves the removal, which requires demonstrating that the development cannot be designed to retain the tree. Tree removal approvals are not guaranteed, and BMCC may require replanting of equivalent canopy. For significant heritage trees, removal approval is rarely granted.
My tree is in a heritage conservation area, does that make it harder to do nearby work? Yes. Trees in heritage conservation areas have additional protection as contributors to the heritage character of the area. The threshold for approving nearby construction is higher, and the conditions BMCC imposes on approved works are more stringent.
What’s the difference between a TPO (tree preservation order) and the BMCC tree code? In the Blue Mountains, BMCC’s area-wide tree management code under the DCP provides equivalent protection to traditional individual TPOs. The effect is the same, trees meeting the protected criteria cannot be removed or damaged without approval. For properties with specific significant tree listings, check the BMCC significant tree register.
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