What is a tree protection zone?
Hobsons Bay City Council describes a TPZ as a specified zone above and below ground, set aside to protect a tree's roots and crown from development damage. It also says the zone must be isolated from all construction activities.
It's usually bigger than people expect, because roots spread well beyond the trunk and often past the edge of the crown, mostly in the top layer of soil that machinery and trenches disturb.
Closer in is the structural root zone (SRZ). Hume City Council describes it as the roots and soil that keep the tree standing, and says the tree needs a larger area than that to stay healthy. Damage the SRZ and the tree becomes far more likely to fall.
What changed with AS 4970:2025?
The Australian Standard is AS 4970, Protection of trees on development sites. The 2025 edition was published on 30 May 2025 and replaced the 2009 edition, and Amendment 1 followed on 19 December 2025.
Its protection zones section covers the TPZ, calculating the Notional Root Zone (NRZ) and the structural root zone, and it grades encroachment into the NRZ as minor, moderate or major. It also covers arboricultural impact assessments, tree protection plans, protective fences and signs, and monitoring through construction to final sign-off.
Citing the 2025 standard, Hobsons Bay says the TPZ is the trunk diameter at standard height (DSH), measured 1.4m above ground, multiplied by 12. DSH is the new name for what older documents call DBH, diameter at breast height.
The rest of the 2025 figures sit in a paid standard we haven't been able to confirm, so we don't quote them. If an arborist or council gives you a minimum or maximum zone size, ask which edition it comes from.
Old edition or new: which numbers are you looking at?
| AS 4970-2009 (older documents) | AS 4970:2025 (current) | |
|---|---|---|
| Status | Replaced on 30 May 2025, but still quoted in older fact sheets and some local laws | Current, with Amendment 1 from 19 December 2025 |
| Root zone terms | TPZ and SRZ | TPZ, Notional Root Zone (NRZ) and SRZ |
| Trunk measure | DBH, diameter at breast height | DSH, diameter at standard height, 1.4m above ground (Hobsons Bay) |
| TPZ size | Radius of 12 times trunk diameter, no less than 2 m and no more than 15 m (as quoted by Wyndham and Hume) | 12 times DSH (Hobsons Bay); other limits not confirmed here |
| Palms, cycads and tree ferns | Crown plus at least 1m (Hume's procedures) | Not confirmed here |
What does that look like for a real tree?
Under the 2009 edition, Wyndham's 2019 fact sheet gave worked examples: a 20 cm trunk meant a 2.4 m TPZ radius, 50 cm meant 6 m, and 100 cm meant 12 m.
Councils can also set their own rule. Glen Eira's local law sets its TPZ using AS 4970-2009, and Boroondara sets a significant tree's TPZ at 12 times the trunk diameter at 1.4 metres, up to 15 metres. So the council's own rule is the one to design to.
What usually can't happen inside a TPZ
- Parking cars, trucks or machinery
- Refuelling
- Stockpiling soil or building materials
- Storing equipment
- Digging trenches or service pits
- Changing soil levels by cutting or filling
- Compacting the soil
That list draws on Wyndham's rules for work near its own trees, Banyule's measures for building sites and what Stonnington's local law counts as works in a TPZ. Banyule also asks for a circular fence around the tree and mulch inside the zone, and ACT City Services warns that vehicles in a TPZ can compact the soil around street trees.
Building or renovating near a tree: the order that works
Find out what's protected, early
Before the design is locked in, ask the council which trees on your block, the neighbour's and the street are protected. In Victoria, VicPlan shows the zones and overlays for any address.
Bring in an arborist at the design stage
A consulting arborist can measure the trees, map their TPZ and SRZ, and assess how the plans encroach. Yarra Ranges, for example, wants a report consistent with AS 4970:2025 that maps each tree's TPZ and SRZ.
Design around the zones
Footings, driveways, retaining walls, service trenches and changes in ground level are where trees get hurt. Moving them on paper is easy; moving a slab isn't.
Lodge the tree work with the build
Tree work tied to building often sits inside the development application rather than a separate tree permit, as Wollongong and Coffs Harbour both say. Ryde's form even asks for plans showing you considered designs that keep the tree.
Fence the zone before work starts
Protective fencing and signs go up first and stay up for the whole job.
Keep the arborist involved
Some councils want an arborist on site for certain works: Glen Eira requires works in a protected tree's TPZ to be done with a qualified arborist present.
What do councils ask for?
It differs a lot, so read your council's rules. In Hornsby, construction or earthworks in a protected tree's TPZ need a Tree Permit, covering things like driveways, slabs, retaining walls and trenching. Glen Eira won't grant a permit for works in a TPZ unless conditions are met, such as an arborist report showing the tree won't be unreasonably affected, and it may ask for root investigation.
Banyule treats encroaching on 10% or more of a TPZ as a major encroachment that may not be permitted, and warns that damaging a protected tree during construction could lead to a fine or prosecution.
In the ACT, the zone is set in law: the area under the canopy, a 2 metre strip around its edge, and a 4 metre area around the trunk measured at 1 metre high. Groundwork in it needs approval for a protected tree, and permission for work affecting a protected tree covers neighbouring land too.
Reports for these applications are consulting work. Wingecarribee and Blacktown, among others, want them from an arborist holding at least AQF Level 5. Our arborist reports page explains what goes in one.
Digging near trees and services
Lodge a Before You Dig Australia enquiry before any trench, footing or pit is dug, and remember its plans won't show private lines such as irrigation or a cable to the shed. Keep excavators and trenchers out of the SRZ unless the arborist has set out how the work can be done, because cutting the roots that hold a tree up can leave it unstable over a house or a site full of people.
What you can do, and what needs an arborist
You can
- Ask the council about protected trees before anything is designed
- Photograph the trees before work starts, as a record
- Write the TPZ fencing into the builder's contract
- Water the tree through dry spells during the build
- Keep your own car and trailer off the root zone
Leave to an arborist
- Measuring trees and working out the TPZ, NRZ and SRZ
- Arboricultural impact assessments and tree protection plans
- Root investigation when the council asks for it
- Root pruning to the standard where roots must be cut
- Site visits and sign-off as the build goes
Tree protection zones when you build or renovate: what to do next
Before the architect draws a line. We'd have a consulting arborist map the zones onto the site survey first, then design the house around them. It's the easiest point to change anything, and it keeps the tree, the permit and the build on the same page.
Want an arborist to look at it?
Tell us the job and your postcode, and a local arborist calls you to talk it through.