AS 4970:2025 — What Changed, and What It Means for Your Project

When did AS 4970:2025 replace AS 4970-2009?

AS 4970:2025 superseded AS 4970-2009 when it was published on 30 May 2025. A correction amendment followed in December 2025, and the current version incorporates it. If you are holding an arboricultural report that cites the 2009 standard, it is citing a superseded document. That does not automatically make its conclusions wrong — the core arithmetic is unchanged — but the terminology it uses is out of date, and one of its judgements may now fall in a different category.

Working numbers? The tree protection zone calculator is updated for the 2025 standard, including the three encroachment bands.

The short version

The arithmetic did not change. The names did, the encroachment bands did, and the standard is now explicit about who sets the TPZ and which documents carry it.

What is the Notional Root Zone (NRZ)?

The area that used to be called the Tree Protection Zone is now the Notional Root Zone (NRZ). It is still trunk diameter multiplied by twelve, measured from the centre of the stem, and still never less than 2 metres or more than 15 metres in radius. Palms, cycads and tree ferns are not calculated at all — they get the 2 metre minimum.

The standard describes the NRZ as a primary trigger for arboricultural input on a development site. Any works inside it count as encroachment — above or below ground, temporary or permanent, whatever the construction method — and once works fall inside a tree’s NRZ, an Arboricultural Impact Assessment is required.

The rename is part of what the standard calls clarifying the protection zones. “Tree Protection Zone” used to mean both the calculated circle on a drawing and the area actually protected on site, and those are rarely the same shape. The 2025 standard splits them: NRZ is the calculation, and TPZ is the zone the project arborist sets aside to protect the roots and crown — starting from the NRZ, then adjusted for where the roots really are, the soil, the species and what is already built. On a real site the TPZ is often a different shape from the NRZ, and the report’s job is to justify the difference.

DBH became DSH (diameter at standard height)

Diameter at Breast Height is now Diameter at Standard Height. The measurement point is unchanged at 1.4 metres above ground, so every DBH figure you already have is a DSH figure — no remeasuring required. Only the name moved.

The standard’s appendix shows how to take it on awkward trunks. On sloping ground the 1.4 metres runs from the ground level at the centre of the trunk. A leaning tree is measured 1.4 metres along the trunk. Where the trunk forks or carries branch whorls at 1.4 metres, the measurement drops to the narrowest point below; a buttressed or deformed trunk is measured just above the buttress or the deformity; and a tree that forks below 1.4 metres has its stems combined into a single figure.

Encroachment: one threshold became three

This is the change most likely to affect a project already in design.

The 2009 standard had two categories. Encroachment of up to 10% of the protection zone, outside the SRZ, was minor. Anything over 10%, or inside the SRZ, was major.

AS 4970:2025 adds a middle band, with each measured as a share of the NRZ’s area:

In all three bands, an area equal to the encroachment is added to the TPZ elsewhere so there is no net loss of soil, unless the project arborist demonstrates the tree will stay viable without it.

The practical effect: a project taking 15% of the root zone was major under the old standard and is moderate under the new one. The tree still has to be shown to survive, but through design and construction controls rather than alternative designs and a full investigation — and that is the single most useful thing in the revision for anyone designing around a retained tree.

What has not changed is the SRZ. An encroachment inside the Structural Root Zone was major under the 2009 standard and is major under the 2025 one, however small the percentage. There is no such thing as a minor encroachment into the roots holding a tree upright.

The Structural Root Zone (SRZ)

The SRZ formula is unchanged: radius = (D × 50)^0.42 × 0.64, where D is the trunk diameter in metres measured just above the root buttress — not at 1.4 metres. A trunk under 0.15 m gets a fixed 1.5 m. The formula does not apply to palms, cycads or tree ferns, and for a multi-stemmed tree the project arborist chooses the measuring method and records why.

The standard requires the SRZ to be calculated whenever a major encroachment — more than 20% of the NRZ — is proposed. It also makes the point that the SRZ is about stability only. The root area a tree needs to stay healthy is much larger, which is what the NRZ is for.

Worked example: NRZ, SRZ and encroachment

Take a tree with a DSH of 0.5 m, and a trunk 0.6 m across just above the root buttress.

A driveway taking 15 m² of that tree’s root zone, outside the SRZ, is moderate under the 2025 standard, where the 2009 standard called it major. Run your own tree through the tree protection zone calculator, which shows its working.

TPS and TPP

The 2025 standard is explicit about the documents that carry the protection:

Unless the council requires otherwise, the impact assessment includes or comes with a site-specific TPS and TPP, and the current versions have to be on site for the whole of the works. Both are living documents: when approval conditions or the design change, they are updated under version control rather than left to drift from what is being built.

Who the project arborist has to be

The standard defines the project arborist as someone competent in arboriculture who will hold an AQF Level 5 Diploma of Arboriculture or equivalent. That person prepares the preliminary arboricultural report, the impact assessment and the TPS and TPP, then monitors and certifies the protection through the works to final sign-off.

What this means for you

If you have a report citing AS 4970-2009, it is not void. The NRZ figure in it is the same number the 2025 standard produces. But it is worth checking two things: whether an encroachment it called “major” now falls in the moderate band, and whether the council assessing it expects current references.

If you are commissioning a report now, it should cite AS 4970:2025 and use the current terminology. A report issued in 2026 against a superseded standard is the kind of thing an assessing officer notices.

If you are in design, the moderate band is worth knowing about before you finalise setbacks. A layout that was previously pushed out to stay under 10% may now have more room than you were told — as long as the project arborist can show the tree stays viable.