Tree assessment, retention and when you need an arborist
Mature trees are the most valuable thing on most blocks and the easiest to kill accidentally. Root zones, structural roots, council tree protection, and the works that quietly finish off a tree two years later.
A twenty-year-old jacaranda or a remnant tuart is the one thing on your block you cannot buy. Paving, a pergola, turf, even a semi-mature tree in a 1000-litre bag — all purchasable. Twenty years of root system in Perth sand is not.
That is what makes tree assessment different from the rest of site analysis. Everything else you record is a fact you can check later. A tree has a delay built into it: the works happen in autumn, it looks fine through winter and spring, and it starts thinning out over the second summer after that. By then the bobcat is long gone and nobody connects the two.
What a professional does at this stage
Before any concept sketch, a landscaper or landscape architect numbers every tree worth keeping, measures it, calculates its root zones, and draws those zones on the base plan as circles at scale. Then the design gets drawn around them.
That order is the whole point. Amateurs design the garden, then discover the retaining wall runs through a root plate and hope for the best. Professionals treat a retained tree as a fixed obstruction — like a sewer line or a boundary — and shape the design to it. If the design genuinely cannot work around the tree, that becomes a decision to remove it, made deliberately in Phase 3 with the cost and the approval understood, rather than an accident discovered in Phase 8.
The second thing a professional does is decide, tree by tree, whether they are qualified to make the call at all.
Measuring the tree
Take a tape around the trunk at 1.4 m above ground and divide the circumference by 3.14 to get the diameter. If the trunk forks below 1.4 m, measure each stem — multi-stem trees are handled differently, and that is where a calculator earns its keep. AS 4970:2025 calls this measurement DSH; the 2009 edition and most council paperwork still call it DBH. Same thing.
The two circles that matter
Notional root zone (NRZ). The formula-derived circle — radius in metres is roughly twelve times the trunk diameter in metres, with a minimum of 2 m and a cap at 15 m. This is the area you plan around. In the 2025 standard the tree protection zone is the physical fenced area implemented on site, which an arborist sets using the NRZ as the starting point.
| Trunk diameter at 1.4 m | Notional root zone radius | Roughly |
|---|---|---|
| 150 mm | 2.0 m (minimum applies) | A young street tree |
| 300 mm | 3.6 m | A 15-year-old jacaranda |
| 500 mm | 6.0 m | A solid established shade tree |
| 800 mm | 9.6 m | A mature marri or tuart |
| 1250 mm and above | 15 m (cap applies) | A significant remnant tree |
Structural root zone (SRZ). A much smaller circle, calculated from trunk diameter at ground level, minimum 1.5 m radius. These are the roots holding the tree upright. Cutting into the NRZ risks the tree’s health; cutting into the SRZ risks the tree falling on your house.
Encroachment into the NRZ is classified as minor at 10% or less, moderate between 10% and 20%, and major above 20% or anywhere it touches the SRZ. Anything past minor needs an arborist to sign off on how it is done.
The works that kill a tree two years later
Almost none of these look destructive on the day.
- Trenching for retic, power or drainage. A 300 mm trench across the root zone can sever a large share of a shallow-rooted tree’s absorbing roots. A directional bore or a hand-dug radial trench costs an hour more and does a fraction of the damage.
- Raising the soil level. Two hundred millimetres of imported loam and new turf over the root zone is one of the most reliable tree killers going. Roots need air; low-permeability soil over the top suffocates them slowly.
- Compaction. A bobcat doing three-point turns over the root zone, a sand stockpile, a brick pallet, a site shed. Compacted sand does not recover on its own.
- Sealing the surface. Liquid limestone is very common in Perth and goes down as a continuous slab — a driveway widened to the fence line can cover most of a verge tree’s root zone in one pour.
- Washing out. Concrete slurry, render slop and brick-cleaning acid tipped onto the ground near the trunk — a site-management failure, covered in site establishment and safety.
- Changing the water. A tree that has spent fifteen years drinking spray overrun from a lawn will not survive that lawn becoming dripline natives on a two-day roster unless it gets a zone of its own.
The WA specifics
Council rules genuinely vary — do not take a rule of thumb
There is no state-wide rule about removing a tree on your own land in WA. Three Perth councils, three different answers, all verified:
| Council | Approval needed to remove a tree on private land? |
|---|---|
| City of South Perth | Yes, for a Regulated Tree — 8.0 m or more high, and/or 6.0 m average canopy, and/or 1.5 m trunk circumference at 1.4 m |
| City of Stirling | Only if the tree is on the Significant Tree Register, which the owner has to nominate it for |
| City of Vincent | No — the City treats it as a civil matter between neighbours |
So the only correct answer for your block is the one from your own council. Ring the planning counter, give them the address and the trunk circumference, and get it in writing. While you are there, ask whether the property carries a section 70A notification — in Stirling a registered tree binds the title of every lot containing its structural root zone, so a neighbour’s tree can obligate you.
Verge trees are separate and simpler: they belong to the council everywhere in Perth. Do not prune, poison or remove one, and check before you propose a new crossover. That intersects with approvals.
Polyphagous shot-hole borer
Since 2021 the metropolitan area has been under a Quarantine Area Notice for this beetle. Practically, that governs where prunings, stumps and mulch can go, what size material has to be chipped to, and disinfecting pruning tools between trees. If you are removing a host tree, the disposal path is part of the job now, and a contractor who is casual about it is telling you something. See also WA pests and diseases.
Perth sand changes the root map
The textbook circle assumes roots spread evenly. On the Swan Coastal Plain they follow water and air — shallow, wide, and concentrated wherever the ground stays cool and damp: under paving, along old retic lines, under the mulched bed you were about to excavate. On a Karrakatta profile with limestone at 400 mm the root plate is flatter and wider than the radius suggests, because there is nowhere to go but sideways. Check what you found in Perth soils against the trees you are keeping.
When to get an arborist
Pay for a consulting arborist if the council protects the tree and you need a report to support an application; if any excavation, footing, wall or paving falls more than about 10% inside the NRZ or touches the SRZ at all; if the tree overhangs a structure and you are unsure it is sound; if a neighbour’s tree is involved and the conversation is heading somewhere legal; or if you are weighing up removing something large and want an independent opinion on whether it can be kept.
Ask for AQF Level 5. That is the consulting qualification and it is what WA council policies specify. AQF Level 3 is the climbing and pruning ticket — excellent for doing the work, not the person to tell you whether the tree survives your driveway. Prefer a consultant who does not also sell removals.
A useful report gives you the tree’s health, structure and useful life expectancy, the SRZ and NRZ dimensions, and a method for protecting it through the works: fencing position, ground protection, supervision points, and what is not allowed inside the fence.
Next
Transfer each retained tree onto your base plan as two circles at scale, with a retain, remove or arborist-report note against it. The constraints and opportunities plan turns those circles into no-go areas that the concept design has to respect.
Checklist
- Measure trunk diameter and calculate the structural root zone for each retained tree
- Check whether your council protects any tree on or near your block
- Identify works planned inside a root zone and find alternatives
- Decide which trees need an arborist report before you touch them
Where to go deeper
Every link below was fetched and checked before it shipped. Nothing here is a guess.
- Paid · $154.97 incl GST for a single-user PDFstandardAustralian
AS 4970:2025 Protection of trees on development sites
Standards Australia (via Intertek Inform)
Why this oneThis is the document every arborist report, council tree policy and development condition in Australia points at. It covers how to decide which trees are worth retaining and how to protect them through construction, and it defines the root-zone calculations and the minor/moderate/major encroachment categories that decide whether your design is a problem or not. The 2025 edition supersedes AS 4970-2009 and renames things — the formula-derived circle is now the notional root zone and the tree protection zone is the physical fenced area on site.
Change this for PerthIntertek Inform is an authorised reseller; Standards Australia's own store sells the same document. You almost certainly do not need to buy it for one back yard — an arborist applies it for you, and the calculator below does the arithmetic free. Note also that most WA council policies still name AS 4970-2009, so expect the older wording in council paperwork for some time yet.
Link checked 2 Aug 2026
- Freecalculator5 min per treeAustralian
TPZ calculator (NRZ, SRZ and encroachment) for AS 4970:2025
Assurance Trees
Why this oneFree, in the browser, and built on the current 2025 edition rather than the 2009 one. You enter trunk diameter at 1.4 m and trunk diameter at ground level, single or multi-stem, then define the encroachment as a building offset, a corner or an area; it returns the notional root zone and structural root zone radii and areas, classifies the encroachment as minor, moderate or major, and exports a PDF. It is the fastest way to find out whether the paving you sketched is a problem.
Change this for PerthRun by a NSW consulting arborist firm as a lead generator. The maths is the standard's rather than theirs, so the numbers are fine, but the output is not an arborist report and no WA council will accept it as one.
Link checked 2 Aug 2026
- Freegovernment10 minPerth / WA
Trees on private property — City of South Perth
City of South Perth
Why this oneA worked example of the strictest end of the WA range, and worth reading even if you live elsewhere because it shows you the shape of the rule. South Perth requires development approval before removing or damaging a Regulated Tree on any zoned land including private residential, defines Regulated by height, canopy diameter or trunk circumference, charges an application fee, and states the maximum penalties under the Planning and Development Act 2005.
Change this for PerthApplies only inside the City of South Perth. Do not assume your council does the same — two of the three councils compared in this lesson do something completely different.
Link checked 2 Aug 2026
- Freegovernment20 minPerth / WA
Local Planning Policy 6.17 — Significant Tree Register
City of Stirling
Why this oneThe clearest WA-written definition of the terms you need. It defines an Arboriculturist as someone holding a minimum of AQF Level 5 in arboriculture, spells out what an arborist report must contain (health, value and general state of the tree, the structural root zone, the tree protection zone, and a tree management plan setting out the methodology to preserve it), and sets the nomination thresholds for the register. It also shows how a voluntary register actually binds land — through a section 70A notification on the certificate of title of every property containing the structural root zone, including a neighbour's.
Change this for PerthA City of Stirling policy adopted 15 October 2024, so the thresholds and the panel process are Stirling's alone. The definitions of structural root zone, tree protection zone and arborist report are generic and transfer anywhere in Perth.
Link checked 2 Aug 2026
- Freegovernment5 minPerth / WA
Trees located on private property — City of Vincent
City of Vincent
Why this oneThe other end of the range, in about four sentences. Vincent states plainly that a tree on private property is a civil matter between adjoining owners and does not generally involve local government, and only steps in where a tree is dead, dying, diseased or structurally unsound. It also notes that a tree dropping leaf litter or fruit is not considered dangerous, which settles a great many fence-line arguments.
Change this for PerthVincent's position can change — several inner Perth councils have moved toward tree registers in recent years. Confirm the current position with the planning counter before you rely on it.
Link checked 2 Aug 2026
- Freegovernment20 minPerth / WA
Polyphagous shot-hole borer
Department of Primary Industries and Regional Development (DPIRD)
Why this oneThe official page for the pest that has changed how tree work is done in Perth. First detected here in August 2021, no longer considered feasible to eradicate, and now managed through a Quarantine Area Notice covering the metropolitan area with restrictions on moving untreated wood and green plant material out of the zone. It carries the host list, the reporting channels, and the chipping and transport rules that govern what happens to your prunings and stumps.
Change this for PerthThe zone boundaries and the movement conditions have been revised repeatedly since 2021. Check the current Quarantine Area Notice on this page before you book a removal or take green waste anywhere, rather than relying on what a contractor told you last year.
Link checked 2 Aug 2026
- Freegovernment30 minPerth / WA
Management of significant trees
Heritage Council of Western Australia / Department of Planning, Lands and Heritage
Why this oneState guidance, published January 2026, on when a tree carries cultural heritage significance under the Heritage Act 2018 and when it is better handled through local government tools instead. The useful part for a homeowner is that it explains the whole WA protection landscape in one place — heritage list, local heritage survey, significant tree register, local planning policy — and confirms that the tree protection zone determined under AS 4970 is what defines the protected area around a listed tree.
Change this for PerthWritten for local government heritage officers, not homeowners. Read Part One and the section on defining a curtilage; the assessment criteria and worked examples are for people preparing listings.
Link checked 2 Aug 2026
- FreedirectoryAustralian
Find an arborist — IACA member directory
Institute of Australian Consulting Arboriculturists
Why this oneIACA members are AQF Level 5 or equivalent consulting arborists, and membership requires no involvement in tree removal or pruning work. That last condition is the whole point: you are buying an opinion on whether a tree can be kept, and it should not come from someone whose other revenue is taking trees down. The directory filters by state, including WA.
Change this for PerthIACA is headquartered in NSW and the WA member list is short. Arboriculture Australia also runs a national directory, and several Perth consultancies hold AQF5 without IACA membership — treat this as one filter, not the only one, and always ask to see the qualification.
Link checked 2 Aug 2026
- Freearticle15 minPerth / WA
AS 4970:2025 tree protection guide
Paperbark Technologies
Why this oneA plain-language summary of what changed in the 2025 edition, written by a Perth arboricultural consultancy. It covers the rename of the calculated zone to notional root zone, the shift from DBH to DSH terminology, the minor/moderate/major encroachment bands, and the split of the old tree management plan into a tree protection plan and a written tree protection specification. Useful for understanding what your arborist will hand you and why it now comes as two documents.
Change this for PerthThis is a consultancy's marketing blog, based in Karrinyup, and it exists to sell arborist reports. The technical summary matches an independent NSW source checked alongside it, but treat the framing as advertising and confirm anything load-bearing against the standard itself or your council.
Link checked 2 Aug 2026
The rules, with sources
WA regulation changes. Each point below names its primary source and the date it was last checked — always confirm with your own council before you build.
In the City of South Perth, development approval is required before removing or damaging a Regulated Tree on any zoned land, including private residential property. A tree is Regulated if it is 8.0 m or more high, and/or has an average canopy diameter of at least 6.0 m, and/or has a trunk circumference of at least 1.5 m measured 1.4 m above the ground. The application fee is $147, and maximum penalties under the Planning and Development Act 2005 are $200,000 for a natural person and $1,000,000 for a body corporate.
City of South Perth — Trees on private property ↗ · checked 2 Aug 2026
In the City of Vincent, no council approval is required to remove a tree on your own private property. The City states that trees on private property are a civil matter between adjoining owners and do not generally involve local government, and it intervenes only where a tree is dead, dying, diseased or structurally unsound.
City of Vincent — Trees located on private property ↗ · checked 2 Aug 2026
In the City of Stirling, inclusion on the Significant Tree Register is voluntary and requires the landowner's consent plus a section 70A notification on the certificate of title of every property containing the tree's structural root zone. Once a tree is on the register, development approval is required for its removal, destruction or interference.
City of Stirling Local Planning Policy 6.17 — Significant Tree Register, adopted 15 October 2024 ↗ · checked 2 Aug 2026
Polyphagous shot-hole borer is a declared pest and prohibited organism in Western Australia. The Perth metropolitan area is covered by a Quarantine Area Notice that restricts moving untreated wood and green plant material out of the zone, and suspected infestations must be reported to DPIRD.
DPIRD — Polyphagous shot-hole borer ↗ · checked 2 Aug 2026