Averaged ground level (AS 4055)

Averaged ground level (AGL) is the reference datum introduced in AS 4055:2021, Wind loads for housing, against which the 6.0 m eaves and 8.5 m roof-peak scope-limit heights are measured. The 2021 revision moved away from “natural ground level at the front of the building” to an averaged datum so that sloping blocks are handled consistently. On a sloping block, AGL is not the same as natural ground level at any one corner of the building.

How AGL is calculated:

Identify the building footprint (the outer wall line of the dwelling).
Take the natural ground level (NGL) at each external corner of the footprint, including outer corners of articulations and bays.
Average those NGL values to produce the AGL datum.

For a square or rectangular dwelling, 4 corners are averaged. For an articulated plan with 10 corners, all 10 are averaged.

Why builders care:

AS 4055 scope gates: the 6.0 m eaves height and 8.5 m roof apex limits are measured from AGL. On a sloping block, getting AGL wrong can move the eaves measurement by 500-1,000 mm.
Wind classification: AS 4055 wind classes (N1, N2, N3) and corresponding bracing schedules in AS 1684 all depend on the build staying inside scope.
Truss and frame design: catalogue trusses are designed for the AS 4055 wind class; site-specific design (under AS/NZS 1170.2) only kicks in if the build is outside AS 4055 scope.

Common builder errors:

Using slab top as the datum: slab top is not AGL. AGL is measured before slab is poured, from natural ground.
Using one corner’s NGL: the front corner is often the convenient measurement point, but the Standard explicitly requires the average.
Ignoring articulation corners: every external corner counts, not just the four “main” corners.
Using finished ground level after cut-and-fill: AGL is based on natural ground level, not the post-earthworks profile. A cut-and-fill that lowers the downhill side doesn’t lower the AGL for AS 4055 purposes.

Sloping block worked example:

Imagine a four-corner building footprint on a block sloping from front (high) to rear (low):

Corner	Natural ground level (mm)
Front left	+200
Front right	+250
Rear left	-800
Rear right	-750

AGL = (200 + 250 - 800 - 750) / 4 = -275 mm relative to the front building datum.

Now: if the slab top is set at +400 mm (raised slab to handle the slope), and wall plate is 2,700 mm above slab, then:

Slab top above AGL: 400 - (-275) = 675 mm.
Wall plate above AGL: 675 + 2,700 = 3,375 mm.
Eaves height above AGL (if eaves is at wall plate): 3,375 mm.

A two-storey on the same footprint would add another floor (~2,700-3,000 mm), pushing eaves to ~6,075-6,375 mm above AGL, over the 6.0 m AS 4055 limit. The same build on a flat block (AGL near front NGL) would be under the limit.

Who measures AGL:

Surveyor: ideally, on a contour-surveyed block with corner pegs.
Designer / draftsman: from the survey contour map.
Builder verification: should confirm AGL during set-out, especially on builds that are marginal under the AS 4055 scope gates.

For builders:

On a sloping block, ask the engineer or designer for the calculated AGL early in design. Don’t assume AGL is the front NGL.
Brief the surveyor to provide all building-corner natural ground levels, not just a single benchmark.
Marginal eaves-height projects: keep eaves height comfortably below the 6.0 m line. 5.6-5.8 m allows for construction tolerance and roof variations; 5.9 m is asking for a re-engineer mid-build.

Also known as: AGL, averaged natural ground level, AS 4055 datum.

Category: Structural / wind design / AS 4055.

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