Setting out is the process of transferring the dimensions and positions shown on construction drawings onto the actual ground or structure, so that the building is constructed in the right place, at the right level, and to the right shape. It is the practical reverse of surveying: instead of measuring what exists, a surveyor or engineer marks out where new work must go using pegs, profiles, lines and level marks.
What setting out means on site
On site, setting out establishes the physical reference points that trades follow when they dig, pour, build or fix. These can be timber pegs for drainage runs, profile boards offset from foundation trenches, marked lines on a slab for partition walls, or column positions on a steel-frame grid. Each mark carries a known position and, usually, a known level.
The work is split into two related tasks: fixing horizontal position (where something sits on the plan) and fixing vertical position (how high or deep it is relative to a datum). A datum is an agreed reference level from which all other levels are measured. Setting out continues throughout a project, not just at the start, because each new element — substructure, frame, services, finishes — needs its own marks derived from the same overall framework.
Why accurate control matters
It is the practical reverse of surveying: instead of measuring what exists, a surveyor or engineer marks out where new work must go using pegs, profiles, lines and level marks.
Control is the underlying network of fixed points that everything else is measured from. If that framework is sound, individual errors stay small and local. If it drifts, errors accumulate and show up later as walls that do not line up, services that clash, or cladding that will not fit the frame.
Accurate dimensional control — the discipline of keeping every measured position within set limits — matters because construction is unforgiving of compounded mistakes. A misplaced column near the start can force expensive rework once the structure is up. Poor levels can leave inadequate fall on drainage, or floors that do not meet at thresholds. Getting control right early is far cheaper than correcting it once concrete has cured or steel has been erected.
Good control also gives a clear record of who set out what, and from which reference. When several trades and sub-contractors work from the same grid and benchmarks, disputes about position are easier to resolve because everyone shares one source of truth.
How position is transferred from drawing to ground
The first step is usually establishing a site grid and one or more benchmarks. A grid is a set of reference lines, often labelled with letters and numbers, that the design is dimensioned against. A benchmark is a fixed point of known height; a temporary benchmark (TBM) is a stable, protected mark on site tied to the national height system or to a project datum.
From this framework, positions are set out using one or more of the following methods:
- Total station — an instrument that measures angles and distances electronically, letting an operator place a prism at a calculated coordinate. Most modern setting out uses coordinates rather than tapes and offsets.
- GNSS (satellite positioning) — used for larger or open sites where centimetre-level accuracy from satellites is acceptable, often for earthworks and initial grids.
- Optical and laser levels — used to transfer heights from a benchmark to working marks, such as datum lines on walls.
- Tape, line and offset methods — still used for short, simple work derived from established control.
The drawing coordinates are loaded into the instrument, the operator sets up over or near known control points, and the instrument guides the marking of each design position. Key points are then physically marked and, where practical, checked against a second independent measurement before any construction relies on them.
As-built and verification checks
An as-built survey records where elements were actually constructed, as opposed to where they were designed. It is taken once work is complete, or at staged points such as after foundations or after the frame is erected. The survey produces measured positions and levels that can be compared against the design.
Verification checks serve two purposes. During construction they confirm that work in progress is within tolerance before the next stage commits to it — for example, checking that a foundation slab is level and correctly positioned before steel is set on it. After completion, as-built records form part of the project handover, feeding into operation and maintenance information and informing future alterations.
As-built data also matters where services are concerned. Recording the actual line and depth of buried drainage and utilities, before they are covered, gives an accurate record that reduces risk during later excavation. Where deviations are found, they are reported so that designers can assess whether the variation is acceptable or needs correcting.
What tolerances apply
A tolerance is the permitted amount by which a measured position may differ from its design value. No element is built perfectly to its theoretical point, so tolerances define what counts as acceptable. They vary by the type of work and the standard being followed.
In UK practice, setting out and structural tolerances are commonly drawn from British and European standards and from the project specification. Relevant references include the National Structural Concrete Specification, the relevant parts of BS EN 13670 for concrete execution, BS EN 1090 for steelwork, and BS 5606 on accuracy in building. The contract documents normally state which apply and may tighten them for particular interfaces.
As a general indication, position tolerances on primary grid lines and columns are typically tighter than those on secondary or finishing elements, because errors at the structural level propagate further. Cladding, glazing and other components that fit together demand close control at their fixing points. A key principle is that tolerances should not be stacked thoughtlessly: if each stage uses its full allowance in the same direction, the combined deviation can exceed what the next element can accommodate. Sensible setting out works back from the tightest interface and controls the chain of measurements that feeds into it.
Reviewed: June 2026