Trees on a vacant lot — and on neighbouring lots within reach of the build envelope — are protected by a layered system that catches many buyers off-guard. The technical anchor is AS 4970 "Protection of trees on development sites" (the 2009 edition is still widely referenced, with a revised AS 4970:2025 published in May 2025 introducing the "Notional Root Zone" terminology — many councils are still transitioning). On top of that, local councils overlay their own Tree Preservation Orders (TPOs) and significant tree registers, often with substantial penalties for unauthorised removal.

The core concept is the Tree Protection Zone (TPZ) — the area around a tree that must be respected during design and construction. The formula is simple: TPZ radius (in metres) equals the trunk diameter at breast height (DBH, measured at 1.4 m) multiplied by 12, with a minimum of 2 m and a maximum of 15 m for most species. A tree with a 50 cm trunk has a 6 m TPZ — a circle 12 m across in which no excavation, no fill, no compaction and no significant disturbance should occur. Inside that radius, footings need engineering attention; outside, the tree is usually unaffected by ordinary building work.

The TPZ has a smaller sibling — the Structural Root Zone (SRZ) — calculated by a separate formula and representing the area needed simply to keep the tree standing. The SRZ is non-negotiable; any encroachment is treated as a serious impact. Encroachments into the TPZ are classified as minor (≤10%), moderate (10–20%) or major (>20%), with progressively more stringent justification and arboricultural oversight required.

Councils enforce. Multiple Sydney councils (Hornsby, Ku-ring-gai, City of Sydney), Melbourne local authorities, and significant-tree-listed lots in Adelaide and Perth carry on-the-spot fines of $1,500–$5,000 per tree, and prosecutions for major significant-tree removals have produced fines exceeding $100,000 per tree in Land and Environment Court and VCAT proceedings. "We'll just take it out after settlement" is almost always unlawful, and historic aerial photographs make it easy to detect.

Practically, three scenarios drive cost. Designing around trees typically means screw piles or bored piers in place of strip footings ($8,000–$25,000 added) or a suspended slab ($25,000–$80,000). Applying to remove a tree costs $100–$500 per tree in council fees, plus removal cost of $1,500–$8,000 per mature tree, and often a replanting requirement. Construction-stage supervision by an arborist adds $300–$600 per inspection, usually 4–10 inspections across a build.

Questions worth asking the seller:

  • How many trees are on the lot and within 15 m of the boundary? What are their species, DBH and health?
  • Is any tree on a significant tree register, heritage tree register, or under a specific Tree Preservation Order?
  • Has an arboricultural impact assessment been done for any previous DA?
  • Are any trees on neighbouring lots whose TPZs reach into this lot?
  • Are there council street trees with TPZs that would affect the driveway crossover?
  • Have trees been removed from the lot recently — and was that under a permit?
  • Does the lot have any positive covenant requiring tree retention (cross-reference with the 88B or restriction document)?

Who can help. A consulting arborist at AQF Level 5 (Diploma of Arboriculture) minimum, ideally a member of the Institute of Australian Consulting Arboriculturists or Arboriculture Australia. For court-quality or major-project work, AQF Level 8 (Graduate Diploma). Indicative 2025–26 cost: tree survey and arboricultural impact assessment on a typical residential lot $1,200–$3,500; complex multi-tree assessments $3,500–$10,000+.

A practical tip: pull the most recent aerial photo of the lot from a service like Nearmap or Google Earth historical imagery before viewing. If there are mature trees visible in the image that aren't on the ground, ask why. Council records will usually tell you whether a removal permit was issued. Buying a lot with unlawful removal history can transfer liability to the new owner in some jurisdictions, and it almost always means a closer look from council on any future DA.

Trees are often the deciding factor in whether a lot's "advertised buildable area" is the same as the actual buildable area. A mature gum or fig with a 12 m TPZ in the middle of a 600 m² block doesn't just shrink the design — it changes the cost of every footing decision the architect and builder make.

This article is general information only — a starting point for your own questions, not arboricultural, planning or engineering advice. Tree protection laws, council Tree Preservation Orders, significant-tree registers and AS 4970 application vary by state, territory and council, and recent standards updates may not yet be reflected in every jurisdiction. Always engage a qualified consulting arborist, and request from the seller any prior arboricultural assessment and council tree-removal correspondence. Independent advice should be obtained before making any property decision.