What is the datum level?
The datum level is the agreed zero point from which all height dimensions of a building are measured. On a construction drawing it is almost always the top of the finished ground floor, marked as 0.000 or ±0.00. In Dutch practice this point is called the peil.
Everything above it gets a plus dimension (+2.600 for a ceiling, +6.400 for a ridge), everything below a minus dimension (−0.900 for a foundation trench, −2.400 for a basement floor). Nobody on site has to work with absolute heights, and a single setting-out error does not corrupt the whole drawing.
Site datum, national datum and road level
The site datum is a local zero point, but it is nearly always tied to a national height system — Ordnance Datum in the UK, NAP in the Netherlands, NHN in Germany. The site plan therefore carries a note such as “0.000 = OD +12.40 m”. The surveyor transfers that height from a benchmark nearby.
There is also the road level: the height of the road crown in front of the plot. Authorities often require the ground floor to sit a set number of centimetres above it, so rainwater cannot run into the building and the driveway has enough fall.
Watch out for one trap: in planning law, the point from which permitted building height is measured is defined separately, usually as the adjoining ground level or the road crown. That is not automatically the same as the finished floor level on the drawing. On an extension or a roof addition the difference can easily amount to several tens of centimetres of permitted height. Read the definition in the local planning policy before you start calculating.
In practice
- Fix the datum securely. It is set on a point that survives the whole build: a concrete peg, the wall of an existing building, or a benchmark outside the excavation.
- Finished floor levels per storey are always referenced back to the main datum.
- Finished floor, not structural slab. Zero sits on top of the screed and floor finish. Forget that build-up and your stair rise or door heights will not work out.
Related terms
- Ordnance datum
- Ground level
- Setting out
- Screed
- Benchmark
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