PAS 128 Utility Survey Categories Explained — Which Level Do You Actually Need?

PAS 128 is the British Standard for detecting and mapping buried utilities, and it defines four survey categories — D, C, B and A — that describe how confident you can be in the result. Most delays and disputes over utility surveys come down to one thing: the wrong category was specified for what the project actually needed, either too little detection to be safe or more than the project could justify paying for.

For QSs, PMs and developers scoping survey work across Hertfordshire, Essex, East Anglia and Cambridgeshire, here’s what each category actually involves and how to pick the right one.

GPR utility detection on an active UK highways excavation site — coloured service markings on the road surface show located buried utilities prior to excavation. AKN Engineering carries out PAS 128 Type B utility surveys across Hertfordshire, Essex, East Anglia and Cambridgeshire

PAS 128 Category D — desktop records only

Category D is a desktop study: gathering utility records from statutory undertakers and other sources, with no site detection at all. It tells you what should be there according to records, which are frequently incomplete or wrong. On its own, Category D is not sufficient to excavate safely — it’s a starting point for planning further survey work, not a substitute for it.

PAS 128 Category C — a site walkover added to records

Category C adds a site walkover to the desktop study — visually identifying surface features like manhole covers, valve boxes and marker posts, and correlating them against the records. It improves confidence over records alone but still doesn’t detect what’s actually buried; it’s still a planning tool rather than a basis for safe excavation.

PAS 128 Category B — actual detection

Category B is where genuine detection begins, using ground penetrating radar (GPR) and electromagnetic location (EML) to physically locate buried utilities and assign each one a quality level (from B1 to B4) reflecting how confident the survey is in its position and depth. This is the category most construction and groundworks projects need before excavating — it tells you what’s actually in the ground, not just what the records claim.

PAS 128 Category A — verified by exposure

Category A confirms utility position and depth by physically exposing them, typically through trial holes or vacuum excavation. It’s the highest confidence level available and is usually reserved for high-risk excavation near critical services, or where Category B detection has flagged something that needs verifying before work proceeds.

Which category do you actually need?

For most UK construction and groundworks projects, a Category B survey — genuine GPR and EML detection — is the right level: it gives you a confident, defensible picture of what’s underground before you dig, without the cost of exposing every service. Category D or C alone should be treated as planning inputs, not a green light to excavate. Category A is worth adding only where a specific service needs physical verification — usually high-voltage cable, gas mains, or anywhere a strike would be genuinely dangerous, as covered in HSE guidance HSG47 on avoiding danger from underground services.

What affects the cost

Cost is driven mainly by site size, ground conditions, utility density and the category specified — a Category B survey across a dense urban site with multiple utility types costs more than the same category on a simple rural plot. Specifying the right category from the outset avoids the most expensive scenario: commissioning a Category C desktop-plus-walkover, discovering it isn’t sufficient once excavation starts, then paying for a second Category B visit under time pressure.

Working with AKN Engineering

AKN Engineering carries out PAS 128 utility surveys, including Category B GPR and EML detection, for main contractors, developers and QSs across Hertfordshire, Essex, East Anglia and Cambridgeshire — helping you specify the right category from the start and avoid a repeat visit once work is underway.

If you’re scoping a utility survey and aren’t sure which category your project needs, speak to AKN Engineering on 01279 927 033 or visit www.aknengineering.co.uk.