Ground level is the height of the ground surface around a building: the boundary between earth and air. It is one of the key reference planes in construction, because almost every vertical dimension — building height, foundation depth, floor level, drainage invert — is ultimately related to ground level or to a datum derived from it. The Dutch term is maaiveld.
Ground level, datum and finished floor level
These three get mixed up constantly:
- Existing ground level — the actual, finished ground surface. It may slope and can differ on each side of the building
- Datum (± 0.00) — the zero point fixed for the project, usually the top of the finished ground floor. Every dimension on the drawings reads as + or − from this point
- National height system — Ordnance Datum in Britain, NAP in the Netherlands, NHN in Germany. Expressing levels in the national system makes heights comparable across plots and projects
So a drawing typically carries a note such as: datum = finished floor level = 200 mm above adjoining ground level = a stated height above Ordnance Datum.
Why ground level matters
- Planning heights. Maximum building height, eaves height and ridge height are measured from existing or finished ground level. Raising your ground does not automatically raise the height you may build to — what counts is how the local rules define the measuring point
- Keeping water out. Keep the finished floor a few hundred millimetres above adjoining ground level so rain runs away from the wall, not into it. Where a damp proof course is used, it must sit clear above the finished external level
- Falls around the building. Lay paving to fall away from the building, in the order of 1 to 2 per cent, and keep the paving below thresholds and below the damp proof course
- Foundation depth. Foundation depths are measured from ground level. In temperate climates a strip foundation typically needs to reach several hundred millimetres below ground; on shrinkable clay or near trees it needs to go considerably deeper. Follow the ground investigation and local guidance
- Made up ground. Raising the site adds load to the subsoil and can cause settlement in the surrounding ground
Related terms
- Datum
- Damp proof course
- Fall
- Foundation
- Settlement
- Frost depth
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