A utility survey finds and marks the cables and pipes buried under a site before anyone digs. Surveyors use cable locators (CAT and Genny) and ground penetrating radar (GPR), often to the PAS 128 standard, and give you a drawing and paint marks on the ground. HSE guidance is clear that plans alone aren't enough to dig safely.
Why you need to locate services first
Hitting an electricity cable typically causes severe, potentially fatal burns, and a damaged gas pipe can lead to fire or explosion. Under regulation 25(4) of the CDM Regulations 2015, construction work that could create a risk from an underground service must not go ahead unless suitable and sufficient steps have been taken to prevent the risk, so far as is reasonably practicable.
"Plans alone are not sufficient to identify and locate services before starting work." Assume all services are live, use locators frequently during the work, and use trial holes to confirm a service's position and depth.
How to dig safely: the three steps
- Plan and get the recordsSearch the free LinesearchbeforeUdig service and contact any asset owners who aren't on it. The National Underground Asset Register (NUAR) covers England, Wales and Northern Ireland, but is currently open to asset owners and their contractors. No records doesn't mean no services.
- Locate and markA trained operator or surveyor sweeps the area with a CAT and Genny, and GPR where needed, and marks each line with paint, crayon or pegs.
- Dig safelyExpose services with trial holes by hand or by vacuum excavation. HSE says keep power tools 500 mm or more from the marked line of a service, and don't use a mechanical excavator within 500 mm of a gas pipe.
CAT and Genny or GPR?
CAT and Genny
The CAT is a receiver that picks up signals on metal cables and pipes, from the power network, radio signals or a Genny (signal generator) clipped on or placed nearby. Using a Genny significantly increases accuracy.
Won't find plastic pipes unless they have a tracer wire or a sonde is pushed through them. Some power cables give off no signal.
Ground penetrating radar
Sends radar pulses into the ground and records the echoes, so it can pick up non-metallic services that a CAT can't.
Wet ground and clay cut how deep it can see, sometimes to centimetres, and boulders or disturbed ground give false readings.
PAS 128 survey quality levels
PAS 128:2022 is the British Standards Institution specification for detecting and mapping buried utilities. Ask which quality level a quote is for.
| Level | What it means |
|---|---|
| QL-D | A desktop search of utility records only |
| QL-C | Records checked against what's visible on site, such as covers and street furniture |
| QL-B | Services detected with equipment such as locators and GPR, graded B4 (suspected) to B1 (detected with high confidence) |
| QL-A | Services seen and measured where they're exposed, for example in a trial hole or chamber |
What a utility survey costs
- Site size and accessBigger areas and live roads take longer, and roads may need traffic management.
- Quality levelA records search costs far less than a full QL-B detection survey with GPR.
- CongestionTown centres with many services take more time to untangle.
- DeliverablesCAD or 3D drawings and trial hole verification add cost.
What you should get
- Services marked on the ground in paint, crayon or pegs
- A drawing (usually CAD or PDF) showing each service and its confidence level
- A note of what couldn't be detected, and why
- Recommended trial hole positions to confirm depths
Street Works UK guidance sets duct and pipe colours, for example yellow for plastic gas pipes and black or red for electricity cables, but it warns that not all services will match, and soil can discolour them.
Who can do it
HSE says anyone using a locator should be thoroughly trained in its use and limitations. The common one-day course is EUSR Category 1, locate utility services and HSG47, and certificates last three years. For surveys to PAS 128, ask the firm how its surveyors are trained and whether it holds a PAS 128 accreditation.
Questions to ask a utility survey company
- Which PAS 128 quality level is your quote for?
- Will you use GPR as well as CAT and Genny?
- Will you mark services on the ground as well as on a drawing?
- What format will the drawing be in, and how soon?
- How are your surveyors trained and accredited?
- Do you need traffic management or permits to survey the road?
Prices in this guide are typical ranges from suppliers' published price lists and cost guides. They change and vary by area, so always confirm the price with the supplier before you book. Rules, fees and tax rates are checked against official sources.