PAS 128 explained — the standard for utility surveys
PAS 128 is the UK specification for utility detection and mapping. Here is what it is, what each quality level means, and how to specify a survey to it.
PAS 128 is the UK standard for utility detection, mapping, and verification surveys. It was developed to bring consistency and accountability to a corner of the construction industry that previously varied wildly in quality and deliverable. If you are commissioning utility surveys in the UK, PAS 128 is the framework you should be specifying against. Here is what it is, what each quality level means, and how to use it on a project.
Why it exists
Before PAS 128, utility surveys in the UK varied from “we asked the statutory undertaker for records” to “we used GPR and an electromagnetic locator with verification trial pits”. The deliverables looked similar — a plan with services drawn on it — but they were not equivalent. A contractor reading a “utility survey” had no easy way to know how thoroughly the site had been investigated.
PAS 128 fixed that. It defines a quality framework with explicit levels, each with documented requirements. A survey delivered to PAS 128 carries a quality label that tells the reader exactly what was done.
The quality levels
PAS 128 defines four quality levels, lettered A to D, with B further subdivided.
Quality Level D (QL-D) — Records search only. The surveyor obtains statutory undertaker records and any other available information. No site work is done. This level is useful for early desktop assessment but is not a survey in any meaningful sense. Records are often incomplete, out of date, or simply wrong on detail.
Quality Level C (QL-C) — Records reconciliation with site walkover. The surveyor walks the site and reconciles the records with what is visible above ground (manholes, valve covers, hydrants, lamp standards). This confirms which records are likely to be relevant but does not measure or verify what is buried.
Quality Level B (QL-B) — Geophysical detection and mapping. This is where the survey becomes a survey. The site is searched with electromagnetic locators, ground penetrating radar, sonde tracing, and other geophysical techniques. Detected services are mapped with measured depth and position. Quality Level B is the standard level for most construction utility surveys.
QL-B is divided into sub-levels by accuracy:
- QL-B-1: highest geophysical accuracy
- QL-B-2: standard geophysical accuracy
- QL-B-3: lower-accuracy geophysical detection
- QL-B-4: detection only, position uncertain
For most live construction work, QL-B-1 or QL-B-2 is the appropriate target.
Quality Level A (QL-A) — Verification. Detected services are physically verified, typically by careful excavation (trial pit or vacuum excavation). Position and depth are measured directly. QL-A is the highest confidence level and is appropriate where a strike would be critical or where high-precision setting-out is required.
How to specify a PAS 128 survey
A defensible PAS 128 specification includes:
- The site boundary, clearly marked.
- The target quality level for the bulk of the survey (typically QL-B-1 or QL-B-2).
- Any specific services or areas requiring QL-A verification.
- The asset records the surveyor will have access to.
- The format and coordinate system for the deliverable.
- Any specific technologies required (multi-channel GPR for large or complex sites).
- Access constraints, working hours, and any specific induction requirements.
A reputable utility surveyor can talk through these in a brief planning call before quotation.
What a PAS 128 deliverable looks like
A PAS 128 survey delivers:
- A drawing showing every detected service, with position and depth annotated.
- A clear quality level indicator on each service — sometimes the whole survey is one level, sometimes individual services are at different levels (a buried fibre that was confirmed with sonde may be QL-B-1 while an inferred service in a difficult area may be QL-B-3).
- A separate layer for services taken from records but not verified on site.
- A list of services searched for but not detected.
- A method statement and equipment record.
- A surveyor sign-off.
The deliverable is provided in DXF/DWG and PDF formats, suitable for direct use in design and construction documentation.
Where the gaps are
PAS 128 is a quality framework, not a guarantee. Even a well-executed QL-B-1 survey has limits:
- Services running parallel to each other can be hard to separate without verification.
- Non-metallic, non-pressurised services (e.g. abandoned drains) are sometimes invisible to all geophysical methods.
- Made-ground variability and conductive soils degrade GPR penetration.
- Services below the practical penetration of the equipment used are missed.
A defensible PAS 128 deliverable acknowledges these limits. A surveyor offering “100% detection” should be regarded with suspicion — no honest survey delivers that.
When to scope QL-A verification
QL-A trial excavation is appropriate where:
- A service strike would be catastrophic (HV cables, gas, fibre, district heating).
- Excavation will pass within the geophysical uncertainty of a detected service.
- High-precision setting-out for new services is required.
- A previous QL-B survey identified an anomaly that needs to be resolved.
The cost of QL-A verification on a small number of critical services is small relative to the cost of an avoided strike or unplanned excavation halt.
Practical advice
For a contractor or designer commissioning utility surveys:
- Specify PAS 128 by quality level. Vague briefs invite vague deliverables.
- Read the deliverable and check the quality level on each service.
- Plan QL-A verification on the services where strikes would be critical.
- Keep the deliverable on file. A PAS 128 survey is part of the project’s safety case.
PAS 128 has done more to professionalise UK utility surveying than any other single framework. Use it.