NGL stands for Natural Ground Level: the ground surface as it existed before any cut, fill or earthworks, used as the baseline for cut, fill and height.
What NGL means
NGL is Natural Ground Level: the ground surface as it was before anyone disturbed it. It is the baseline the design is measured against – how much cut, how much fill, how deep the excavation goes, and in parts of Australia, how tall the building is allowed to be.
The word doing the work is “natural”. NGL is not the same as the surface you would survey today. If the site has been filled, benched, terraced, or had a building and a driveway on it, today’s surface can sit well above or below natural ground. Treating the two as interchangeable is one of the more expensive mistakes available on a sloping or previously developed site.
Why it becomes a legal number
In parts of Australia this is not just a drafting nicety. In Victoria and Western Australia, planning and building height controls are commonly written as a vertical distance measured from natural ground level to the top of the roof or parapet, which turns NGL into a legal number rather than a drawing convention.
Do not assume that carries nationally. In New South Wales the standard instrument LEPs measure building height from “ground level (existing)”, which is a different defined term – and one this page distinguishes from NGL below. Check the scheme that governs your site before you measure anything.
What “natural” means has been fought over. The older working assumption in Victoria was the ground level existing when the application was lodged; tribunal decisions since 2016 have pushed the other way, treating natural ground level as a question of fact about how the land sat before human intervention, and accepting interpolated levels where the original surface has been disturbed. If a height limit is tight, have the surveyor establish both the pre-disturbance surface and the present one, and design to the lower of them.
How NGL appears on a drawing
On elevations and sections, natural ground level is drawn as a dashed or chain-dotted line running behind the building and labelled. On plan it appears as surveyed spot levels and contours. Between NGL and the finished ground line, cut and fill are usually hatched or annotated. Levels are in metres to three decimal places against the project datum:
NGL RL 23.950
Because NGL is evidence rather than design, good sets tie it back to its source:
NGL RL 23.950 - REFER FEATURE SURVEY DATED 12.03.26, AHD
And where it is being compared to the design surface:
NGL 23.950 / FGL 24.100 - 150 FILL
Terms it gets confused with
- EGL – existing ground level: the surface as it is now, including any earlier fill or site clearing. On an undisturbed site NGL and EGL are the same; on a disturbed one they are not, and that difference is often the whole argument. In New South Wales the equivalent defined term, “ground level (existing)”, is the one height is measured from, so on a NSW job the distinction is not academic.
- NS – natural surface, the civil engineer’s term for the same thing, common on earthworks and road drawings.
- OGL – original ground level, used the same way as NGL and very common on earthworks drawings and on sets produced in India and the Middle East.
- FGL – finished ground level, the designed surface after the works.
- Design surface or DGL – design ground level, the modelled surface a machine control system will build to.
If you are working through a set full of level abbreviations, my A-Z list of engineering drawing abbreviations covers the related ones alongside this term.
AU and UK versus US
In Australia and New Zealand, NGL and natural surface are the usual terms, with levels in metres to AHD or an assumed site datum. In the UK the more common phrase is existing ground level, with heights to Ordnance Datum; “original ground level” turns up on earthworks drawings. US drawings talk about existing grade or natural grade, abbreviated EG on civil sheets, as elevations in feet and inches – and US height limits are typically measured from existing or finished grade as defined by the local zoning ordinance, so the reference surface can differ from one municipality to the next. Australian readers should not assume the planning meaning of “natural” in their own state carries across – it does not always carry across state lines, let alone overseas.
Where to check on your own set
- The feature or detail survey – the only authoritative source for NGL. Check the surveyor, the date, and whether it states AHD or an assumed datum.
- Title block datum note – assumed levels cannot be mixed with AHD levels.
- Site plan contours and spot levels – and whether they are labelled natural, existing, or finished.
- Cut and fill sections – the clearest place to see how far the design departs from natural ground.
- Geotechnical report and borehole logs – recorded depths of existing fill are the giveaway that today’s surface is not natural ground.
- The planning scheme or LEP that applies – and specifically which surface its height definition names, because that decides which level you draw the measurement from.
- Town planning drawings and the planning report – if building height is being measured, this is where the adopted NGL is argued and recorded.
References
- Colin Biggers & Paisley – Natural ground level and building height disputes
- Altus Planning – When is natural ground level, natural ground level? (Western Australian perspective)
- Best Hooper – Ground level: what is natural?
- Land Victoria – Australian Height Datum
Other meanings of NGL
In oil and gas, NGL means natural gas liquids – a real collision, not a trivia note. If you are reading a process, piping or plant drawing rather than a building or civil one, NGL almost certainly refers to the product stream, not a ground level. In messaging slang it means “not gonna lie”, which is what most general search results will show.