According to the GOV.UK NUAR Economic Case Summary, there are roughly 60,000 strikes on buried pipes and cables in the UK every year, costing the industry an estimated £2.4 billion annually. Around 70 people suffer serious injuries from contact with underground electricity cables alone.
If you hit a high-voltage cable or gas pipe, the consequences can be devastating.
That is exactly why PAS 128 exists. It is the industry-recognised framework for detecting, verifying, and locating underground utilities before you break ground. And it defines four survey types, each suited to a different stage of your project.
Here is what each one involves, when you need it, and how to choose the right one.
What Is PAS 128?
PAS 128 is a specification published by the British Standards Institution (BSI). It sets out how underground utilities, whether active, abandoned, redundant, or unknown, should be detected, verified, and located on any construction or development project.
The current version is PAS 128:2022, updated in April 2022 with improved guidance on method limitations, practitioner qualifications, and what clients should expect (BSI Group).
Here is something many people miss. PAS 128 is not legally mandatory. But the Construction (Design and Management) Regulations 2015 (CDM 2015) require that “suitable and sufficient steps” are taken to identify underground services before excavation. PAS 128 is the recognised way to meet that duty (HSE, CDM 2015).
One more thing. PAS 128 compliance requires a topographical survey as a baseline. The topo provides the base map for positioning and overlaying all other survey data. Without it, your utility survey will not be PAS 128 compliant.
Around 50% of our topographical surveys now include a utility detection element, and that number has steadily increased as health and safety has become more ingrained across the industry.
The Four Types of PAS 128 Survey
PAS 128 defines four survey types: D, C, B, and A. Each builds on the last, increasing in detail, confidence, and cost.
Type D: Desktop Utility Records Search
This is a desk-based exercise. No one visits your site.
A Type D survey involves gathering existing utility records from statutory undertakers: water, gas, electricity, telecoms, and drainage providers.
When it is appropriate: Very early feasibility stage, initial risk assessment, or as a baseline before commissioning more detailed surveys.
The limitation: Records are often inaccurate, incomplete, or out of date. A Type D gives you a starting point, but you should never treat it as the full picture.
It provides QL-D data, the lowest confidence level.
Type C: Site Reconnaissance
Type C includes everything from Type D, plus a physical site walkover.
A surveyor visits the site and identifies visible surface features: manholes, valve covers, marker posts, cable routes, and any other evidence of buried services. They then correlate what they see on the ground with the desktop records.
When it is appropriate: Early design stage. It helps confirm whether the desktop records match reality.
The limitation: It still cannot detect what is below the surface. You are working with what you can see, not what is hidden.
It provides QL-C data.
Type B: Detection Survey (The One Most Clients Need)
This is where the real detection work happens, and it is the survey type most clients actually need.
A Type B survey uses two complementary technologies:
- EML (Electromagnetic Location), also known in the industry as “cat and genny”, detects metallic utilities such as metal pipes and cables. It is quick but limited to conductive materials.
- GPR (Ground Penetrating Radar) detects both metallic and non-metallic targets, including plastic pipes and fibre optic cables.
But here is what most people miss. Many clients call us asking for “a GPR survey” or “a radar survey”. GPR is the equipment, not the survey type. And GPR alone will not effectively locate underground services without EML.
GPR on its own is suited for locating underground tanks, voids, and culverts. For utility detection, you need both technologies working together.
Within Type B, the survey intensity is defined by a Methodology level (M-level). This determines the spacing of the survey grid used on site. A tighter grid means more coverage and higher confidence:
- M1 (5 m grid): Best suited for low-risk, undeveloped or open sites. A broad overview.
- M2 (2 m grid): A good balance of coverage and detail. This is the methodology we recommend for most projects.
- M4P (0.5 m grid with post-processing): Maximum detail. GPR data is collected across the entire site area and post-processed back in the office. Better for congested sites or locating specific features like culverts.
Once we explain the different levels to clients, they usually require a PAS 128 M2 survey. It offers the best balance of coverage, confidence, and cost.
A Real Example of Why the Level Matters
We had a project where the client was looking for a culvert running through the site. This was not mentioned to us when commissioning the survey. On paper, it looked like a straightforward PAS 128 M2 job, and we had all the information required to quote.
When the surveyors arrived on site, the client explained about the culvert. The team advised that locating it without an open access point would be difficult with a standard M2 survey.
Had the client commissioned a PAS 128 M4P, we would have had a better chance: GPR over the entire site area with post-processing back in the office.
The lesson? Choosing the correct PAS 128 level depends entirely on what you are trying to achieve. If you have a specific target, mention it when commissioning. It may change the recommended survey level entirely.
Type B provides QL-B data. This quality level is subdivided into four categories (B1 to B4) based on how many detection methods confirmed the utility and the resulting positional confidence.
Type A: Verification Survey
Type A is the highest certainty level. It involves physically exposing the buried utilities.
Methods include trial pits, vacuum excavation, and hand digging. You get the exact position, depth, size, material, and condition of each utility.
When it is appropriate: Immediately before excavation in high-risk areas, or to confirm critical utility positions for detailed design.
The limitation: It is intrusive, more expensive, and may require permits and traffic management. You do not usually need Type A across an entire site. It is typically targeted at critical crossing points or high-risk zones.
It provides QL-A data, the highest confidence level.
How to Choose the Right Survey Type
The right survey type depends on your project stage and risk level:
- Feasibility stage: Type D (minimum)
- Early design: Type C
- Detailed design: Type B (M2 recommended)
- Pre-construction or high-risk zones: Type A
Remember, surveys build on each other. You cannot skip straight to Type A without Types D and C informing the process.
The most common mistake? Most clients do not fully understand what they are asking for when they request a utility survey. A phone call before commissioning is the best first step. It allows us to understand what you are trying to achieve and advise on the right type of survey for your project.
Choosing a Surveyor You Can Trust
Not all utility survey providers are equal. Here is what to look for.
Verify TSA membership. Check The Survey Association website for affiliated and verified survey companies. The survey is the foundation of your project. If you cut corners here, you run the risk of expensive mistakes further down the line. (The Survey Association).
Choose on experience, not price. Focus on the firm’s sector experience and the quality of deliverables they can show you. Equipment is a tool. What matters is knowing what to measure and how to present it usefully. That comes from experience.
Confirm they can deliver the topographical survey too. As mentioned above, PAS 128 compliance requires a topo survey. Your surveyor should deliver both, so you are not managing multiple providers for what should be an integrated process.
Terrain Surveys is a TSA member, ISO 9001 registered, and accredited through Constructionline Gold, CICES, CSCS, and Acclaim. We have delivered utility surveys across England and Wales for over 22 years.
Next Steps
The right PAS 128 survey type prevents costly strikes, project delays, and serious safety incidents.
If you are unsure which type you need, pick up the phone. A conversation before commissioning is the single best way to get the right survey for your project.
Not Sure Which PAS 128 Survey Type You Need?
We will talk through your project and recommend the right survey level. No obligation.
Get a free, no-obligation quote from Terrain Surveys. Call us on +44(0)143 884 1300 or request a quote.
