Site Survey and Setting Out: A Groundworker's Guide

Quick Answer: Setting out transfers the positions shown on drawings to physical pegs and profiles on site. It must be accurate to the building's design dimensions — errors in setting out propagate through the entire build and are expensive to correct later. Use a total station or laser level for accuracy; builder's squares and profile boards for control. Check your datum point, confirm the OS grid reference with the client's drawings, and verify the building's relationship to the site boundary before driving the first peg.

Summary

Setting out is one of the most critical early tasks on any construction site and one of the most commonly rushed. A 50mm error in the position of a foundation, an out-of-square corner, or a slab 100mm too high can mean thousands of pounds in remedial work once the build has progressed. Building inspectors will check finished foundation sizes and depths; a building that is significantly out of position relative to the planning permission boundary can trigger enforcement action.

The principles of setting out are straightforward — the challenge is the rigour required to apply them correctly. Every dimension must be checked twice, from a known datum, with tolerances understood. The basic tools are a tape measure, builder's square, and boning rods, but a laser level and total station make the job significantly faster and more accurate.

For groundworkers, competent setting out is a genuine skill that justifies a premium. The ability to set out from a drawing independently, without relying on the main contractor or structural engineer to hold your hand, marks the experienced operator from the beginner.

Key Facts

• Datum point — a fixed reference point of known height; the OS Ordnance Datum (OD) in metres Above Ordnance Datum (AOD) is the national reference; site datum is usually a TBM (temporary benchmark) transferred from an OS benchmark or adjacent structure
• OS National Grid — all planning drawings are referenced to the OS National Grid (Eastings and Northings); modern total stations can import coordinates directly; match your grid north to the drawing's north point
• Profile boards — timber boards and pegs set back from the building lines (typically 1–1.5m clear of excavation), used to re-establish corner positions after excavation has removed the foundation pegs
• Building lines — the face of the building as set out; offset marks on profile boards allow recovery of the exact line after excavation
• Tolerances — BS 8000 Part 1 specifies setting out tolerances; typically ±10mm positional accuracy for residential foundations, ±5mm level on structure
• Squareness check — diagonals of a rectangle must be equal; 3-4-5 rule for right angles; for large buildings, calculate expected diagonal from drawing dimensions and measure
• Reduced levels — all heights on a construction drawing are given as reduced levels (RL) — height above site datum. The first task is establishing the TBM and its RL
• Boundary checks — confirm the position of the building relative to site boundaries matches planning approval before beginning excavation; retrospective enforcement is costly
• Setting out errors — the most common: wrong starting datum, accumulating tape measure errors over long distances, ignoring magnetic deviation on compass bearings, not checking the as-built position matches planning drawings
• Party Wall relevance — setting out must ensure excavations stay clear of party wall notice distances (3m from neighbour's building per Section 6 of the Party Wall Act)

Quick Reference Table — Common Setting Out Equipment

Detailed Guidance

Setting Up the Site Datum (TBM)

The temporary benchmark (TBM) is the height reference for the entire site. It must be:

• Fixed and immovable — a concrete pad, the top of a kerb, or a nail in a mature tree stump; never a peg in soft ground
• Referenced to a known level — typically transferred from an OS benchmark (cut marks on lamp posts, bridges, buildings) or taken from the LiDAR survey data used to produce the topographic drawing; confirm with the structural engineer
• Recorded and marked clearly — write the RL on the timber or mark with paint; photograph it as a site record

Transfer the OS benchmark level to your TBM using a level and staff. Book the readings in a level book (BSB method: backsight, intermediate sight, foresight) and calculate the height of instrument (HI) at each instrument position. Check your arithmetic: sum of backsights minus sum of foresights must equal first RL minus last RL.

Reading a Topographic Drawing

Before setting out, extract from the drawings:

• Site boundary lines — confirmed by the Land Registry title plan or a measured boundary survey
• Building position relative to boundaries — expressed as dimensions to faces of walls or outer corners; check this matches the planning permission
• Finished floor level (FFL) — the reduced level of the ground floor; this is the primary vertical control
• Foundation depths — reduced levels of foundation formation levels for each strip or pad
• North point — orientation; confirm this matches your OS grid north (buildings in planning drawings are typically oriented to true or grid north, not magnetic)

Establishing Building Lines

For a rectangular building, you need four corners accurately positioned. Method using profile boards:

1. Mark one corner — from the site boundary or a fixed reference, measure the dimension shown on the drawing to your first building corner. Drive a peg.
2. Set the first building line — drive profile boards 1.2–1.5m beyond each end of the first wall, aligned with the first corner peg. Stretch a string line between them.
3. Set the first perpendicular — use the 3-4-5 method or a builder's square to set a second string line at exactly 90°. Check: measure 3m along wall 1, 4m along wall 2; the diagonal between these two points must be exactly 5m. For large buildings, use multiples (6m-8m-10m or 9m-12m-15m) for better accuracy.
4. Set the remaining corners — measure the building dimensions along the two control lines to establish corners 3 and 4.
5. Check the diagonals — measure both diagonals of the building rectangle. They must be equal. Calculate what they should be from the drawing: diagonal = √(width² + length²). An out-of-square building within 10mm of the calculated diagonal is acceptable.
6. Mark profile boards — for each wall, mark the face of the wall on the profile board using a saw cut; also mark offsets for the centre of the foundation trench. These reference marks allow the string lines to be re-set after excavation.

Levelling for Foundation Depths

With the TBM established and the FFL known, calculate the required foundation formation level:

• Foundation bottom RL = FFL − floor construction depth − minimum foundation depth
• For a typical domestic build: FFL − (150mm screed + 100mm insulation + 100mm slab) − 1000mm foundation = FFL − 1.35m
• Confirm the formation level with the structural engineer and Building Control

Set up a level over the TBM area. Take a backsight reading on the TBM staff to establish height of instrument (HI). Calculate the required staff reading at formation level: staff reading = HI − required RL. Use a boning rod set in the trench to indicate when the correct depth has been reached. Alternatively, use a laser level set to the HI and scan the trench.

Check foundation depth at each corner and midspan of each strip before proceeding — Building Control will inspect at this stage.

Using a Laser Level for Setting Out

A rotating laser level makes levelling operations significantly faster and removes the need for a second person to hold a staff. Set up:

1. Level the instrument (self-levelling types do this automatically within ±5°)
2. Set the instrument height over the TBM: hold a staff on the TBM, observe the laser line on the staff, record the reading (e.g. 1.530m above TBM)
3. Calculate target staff readings at each point: required reading = TBM RL + instrument reading − required RL
4. Move the staff to each survey point and adjust until the laser line reads the target value

Laser levels are affected by bright sunlight outdoors; use a laser detector (receiver) clipped to the staff for outdoor work in daylight.

Common Setting Out Errors and How to Avoid Them

Frequently Asked Questions

Do I need a surveyor to set out my building?

For simple domestic extensions and new dwellings, a competent groundworker with the right equipment can set out the foundations. For anything that requires OS coordinate accuracy (e.g. the precise position of a building on a large plot), a new-build where the planning permission specifies exact setback distances, or where the boundary is disputed, a specialist land surveyor with a total station and GPS kit should carry out the initial control survey. The cost is £200–600 for a typical domestic setting out survey — cheap insurance against position errors.

How accurate does setting out need to be?

BS 8000 Part 1 specifies ±10mm positional accuracy for residential buildings. In practice, the structural engineer and Building Control will accept ±20–25mm for foundation positions. For the building position relative to the boundary, check the planning condition — some permissions specify setbacks to ±100mm; others are less prescriptive. The closer to boundaries or planning limits, the more precise you need to be.

What if the ground isn't level and I'm reading gradients?

All height references must be reduced levels — height above the site datum, not height above the sloping ground surface. On sloped sites, use a level and staff (or laser level) to transfer reduced levels to all areas of the site. Do not measure depths from the existing sloping surface unless explicitly shown that way on the drawings.

Regulations & Standards

• BS 8000-1 — Workmanship on building sites: code of practice for excavation and filling; includes setting out tolerances

• BS 7334 — Measuring instruments for building construction: calibration and accuracy classification

• Planning conditions — check all planning permission conditions for position constraints before setting out

• Party Wall etc. Act 1996 — Section 6: setting out must confirm excavations are clear of 3m/6m notice distances from neighbouring buildings

• CDM Regulations 2015 — setting out is part of the pre-construction phase; hazards from existing services must be identified and marked before excavation begins

• Ordnance Survey National Grid — OS benchmark data, National Grid explanation

• ICE Manual of Construction Materials — setting out methodology and equipment guidance

• CIRIA — Site Investigation and Setting Out — practical guidance for construction professionals

• trench excavation safety — safety requirements once setting out is complete and excavation begins

• soil types and bearing capacity — ground investigation before setting out foundation levels

• concrete mix ratios and foundation concrete — specification to match the levels set out

• structural calculations and what tradespeople need to know — working with engineer's drawings during setting out