How Do You Survey and Set Out a Site for Construction?

Quick Answer: Site survey establishes existing levels, services and boundaries; setting out transfers the design onto the ground using survey methods to BS 5606:1990 and BS 5964 (or current ISO equivalents). For domestic work the standard tools are an optical level or rotating laser level with staff, a builders' square, profile boards, line and pins. Tolerances under BS 5606 typically require setting out to ±15mm for slab edges and ±10mm for column or wall lines, with diagonal checks confirming squareness on every rectangle.

Summary

Setting out is the bridge between drawings and the build. If it's wrong, every subsequent trade pays the cost — doors won't square, partitions arrive at corners not in plan, drains miss the chamber by a half metre. Most disputes about "the brickie built it wrong" actually start with a setting-out error from the groundworker.

The basic toolkit for a domestic site is straightforward: a rotating laser level for transferring levels around the plot, a builder's measuring tape (30m), profile boards at all building corners, masonry line and pins, a builder's square or 3-4-5 method, and a string line with plumb bob. On a larger site a total station or GPS survey is used, but the same geometric principles apply.

The survey before setting out captures: existing ground levels, the position of underground services, boundary references (party walls, fence lines, neighbour windows), trees subject to TPO, and any features that will constrain the build (manhole covers, BT lines, gas mains).

Key Facts

Quick Reference Table

Spending too long on quotes? squote turns a 2-minute voice recording into a professional quote.

Try squote free →
Tolerance Class (BS 5606) Permitted Deviation Typical Use
Class 1 (precise) ±5mm Steelwork bolts, lift shafts, balcony lines
Class 2 (normal) ±10mm Slab edges, wall centre lines, columns
Class 3 (coarse) ±15mm Foundation lines, drain runs, paving edges
Class 4 (very coarse) ±25mm Bulk excavation, hardcore levels
Instrument Range Accuracy Typical Cost
Builder's optical level 30m ±2mm £200–£400
Rotating laser level 100m radius ±1–3mm £400–£1,200
Cross-line laser 15–30m ±2mm at 10m £150–£400
Total station 1000m+ ±2mm + 2ppm £3,000–£8,000
GNSS / RTK GPS unlimited ±10–20mm £8,000–£20,000

Detailed Guidance

Pre-survey checks

Before any pegs go in:

  1. CAT and Genny scan — sweep the working area for buried services. HSG47 (HSE) and PAS 256 cover safe digging near services. Mark service positions in spray paint:
    • Yellow = gas
    • Blue = water
    • White = telecom/comms
    • Red = electricity
    • Green = drainage
  2. Service plans — request plans from utility companies via LSBUD or directly from each provider; plans are indicative only, not exact
  3. Boundary check — locate boundary by reference to title plan, fence lines or party wall award
  4. Tree survey — identify trees subject to TPO; check RPA (root protection area) per BS 5837:2012
  5. Existing levels — establish a Temporary Bench Mark (TBM) referenced to OS datum or to a stable feature

Establishing the TBM

A TBM is a fixed point on or near the site whose height (Above Ordnance Datum or arbitrary site datum) is known and used for all level transfers. Choose a feature that won't move during the build — top of a manhole cover, kerb, threshold of a neighbouring building. Paint a small mark and record its height. All site levels are then measured up or down from the TBM.

Setting out a rectangular building

For a typical extension or new dwelling:

Setting Out Sequence
1. Locate the front building line from boundary reference (title plan offset)
2. Drive a peg at the front-left corner (A)
3. Measure along to drive a peg at the front-right corner (B)
4. Use 3-4-5 method to square from A — mark a point 4m along front face
   - Stretch a tape 3m perpendicular to AB at A
   - Stretch a second tape 5m from the 4m mark
   - Adjust the perpendicular until the diagonal is exactly 5m
5. Extend the perpendicular line and drive peg C
6. Repeat from B for peg D
7. Check both diagonals AC and BD — they should be equal
8. If diagonals differ by more than ~5mm per 10m, re-set

Profile boards

Once the corners are established, set up profile boards 1m clear of the dig line. Each profile carries the line for the outside of the wall, the outside of the footing, and the inside of the wall, marked by nails in the top edge of the timber. Profiles allow lines to be re-strung after each stage (top of footing, top of brickwork) without re-surveying.

Transferring levels

A rotating laser level with a staff is the standard method. Set the laser on a tripod somewhere central, level it (auto-levelling on most modern units), and use the staff and detector to read heights at any point within 30–100m. Subtract or add the difference between TBM reading and target reading to set the new level.

For longer or higher transfers (basement work, multi-storey), a total station with prism gives metre-accurate distance plus millimetre vertical at long range. The principle is the same.

Setting out drains and falls

Drains are set out from the connection point back to the source, working uphill. The invert level of the connecting manhole or sewer chamber is the fixed datum.

Checking squareness during the build

Diagonal measurement is the trade's standard. On any rectangle:

For internal layouts (partition walls, kitchen unit sets), use a builder's square (BS EN 14730 framing square) against a known straight edge as a reference. Cumulative error builds across long runs — return to a baseline and re-check after every major dimension.

Common errors and how to avoid them

Error Cause Prevention
Building set out at angle to boundary Reference line taken from wrong fence Reference from title plan, not fence (fence may be wrong)
Footing trenches don't square 3-4-5 not used; corners pegged by eye Always 3-4-5 the front face
Level error 50mm across slab Laser self-levelled incorrectly Re-check the TBM reading; warm-up time after travel
Drain runs to wrong fall Datum invert level wrong Verify chamber invert against drainage drawing before digging
Site set out clear of building line Boundary peg disturbed by plant Mark boundary with paint AND offset peg; check at start of each day
Wrong height of slab Damp-proof course level not transferred DPC level should be on the profile, not in the foreman's head

Working from a setting-out drawing

A typical drawing shows the proposed building with:

Always cross-check critical dimensions on the drawing — a typo of a digit in a level will track right through the build if nobody verifies it.

Frequently Asked Questions

Do I need a chartered surveyor?

Not usually for domestic extensions and new builds — competent groundworkers can set out from a builder's optical or laser level. Chartered surveyors (RICS) are typically engaged for boundary disputes, party wall surveys, topographic surveys for design, or where structural setting out has very tight tolerances (steel frames, lift shafts).

How do I find an OS benchmark?

The Ordnance Survey provides datasheets for trig points and benchmarks via Datalink. Most older OS BMs (cut into kerb stones, telegraph poles) are still discoverable but no longer maintained — for new design work use the OS GeoData service or an RTK GPS reading to give a value in OSGB36 datum.

Can I use a phone laser distance meter for setting out?

A phone-paired laser distance meter is fine for short measurements (room dimensions, single-line checks) but not for setting out a building. Setting out needs a calibrated steel tape, optical/laser level, and known reference points — not a single distance reading from one corner.

What's the difference between a TBM and a permanent benchmark?

A Permanent Benchmark (PBM) is an OS-maintained, geodetically known height value. A Temporary Bench Mark (TBM) is a site-specific datum chosen by the contractor for convenience — its height is established by reference to a PBM (or by assumed level if no PBM is available). A site can run perfectly well on a TBM only — but if the design is referenced to OS datum, the TBM must be tied in.

How do I set out a curved wall or radius?

Establish the centre point of the radius, then use a swing tape (or a chalked string) from the centre to mark the arc. For larger or more complex curves, use co-ordinates from the architect's setting-out drawing and check between three or more points on the curve.

Regulations & Standards