Rules and Guidance
Each government body has its own rules and guidance. The simple rule to remember is the Exclusion Zone. If you are inside the Exclusion Zone the Infrastructure Act will apply.
- 3m either side
- 6m above & below
There are some other rules: Cranage must not oversail a certain zone, consideration for collapsed cranes and scaffolds that can fall onto an Infrastructure asset.
Process and paperwork
The paperwork is similar to that of most government bodies as follows:
- Pay their fee…. there is always a fee!
- Provide an Approval in Principle (AIP, or Form 1 for Network Rail). This document lays out the principles that are to be used. The codes designed to, the idea of the structural assumptions and applied loads. Importantly it defines the Category of Design check to be undertaken for the permanent and temporary works, known as CAT I, II & III checks. The design approach requires the approval of the government body.
- The detailed designs are undertaken and checked by the client’s consultant team.
- CAT I – Design is reviewed and signed off by a Chartered Engineer involved on the project
- CAT II – once the design is completed the drawings and reports are provided to a separate design engineer and an inline design is undertaken again. Internally Croft sets up an internal “Firewall” so that the second design team that was not involved on the project, can provide a fresh opinion on the design.
- CAT III – this is similar to CAT II but the design must be provided by an independent external company. It provides a different approach, on one project over a network rail tunnel, Mott Macdonald undertook a design led by a geotechnical engineer, that considered the effects differently to our structural engineers. Both resulted in the same outcome but approached the design from a different angle.
[Diagram] - During this stage LUL (London Underground), Highways and Network Rail take a back seat. They have approved the design in principle. It is for the design team and the CAT checkers to ensure it is completed in detail. They will ask for the completed design checks, but not the detailed design information. A final sign off is sent for approval and typically a one-page signed sheet is received.
- Construction Approval - With the design approved, the construction approvals will be needed. This is where RAMS (Risk Assessment & Method Statements) are provided and approval is gained. The areas of concern may be construction sequences and loading, cranage, scaffolds, surveys etc.
Highway Design & CG300 – Overview
CG300 is part of the UK’s Design Manual for Roadsand Bridges (DMRB) and deals with technical approval of highwaystructures.
- CG300: “Technical Approval of Highway Structures”
- Managed by: National Highways, but TfL, local authorities, and Network Rail apply similar rules where their infrastructure is involved.
This standard ensures that any structure affecting thehighway (e.g., bridges, culverts, retaining walls, sign gantries) is safe,appropriately designed, and reviewed independently.
Category (CAT) Checks
These are independent checks of the design, based oncomplexity and risk: A CAT check is not just a peer review — it's anindependent validation of the calculations, drawings, and assumptions.
CAT I Simple structures, low risk
CAT II Medium complexity or risk
CAT III High complexity, safety-critical, unusual
Technical Approval Process (TAP)
Steps:
- Propose Structure – early design with scope and complexity.
- Submit Form A – Approval in Principle (AIP), outlining the structure, design standards, loads, geotechnical context, and proposed CAT level.
- Approval of Form A by the Technical Approval Authority (TAA).
- Design & CAT Check – Detailed design carried out and checked per agreed CAT level.
- Submit Form B – Certificate of Design and Check.
- Construction & Form C – Certificate of Supervision once built.
TfL Implements CG300 in London
Transport for London (TfL) adopts the CG300 process forstructures affecting or near the TfL Road Network (TLRN).
TfL Process Highlights:
- TAA is typically TfL's Structures Team.
- All bridge works, basement extensions, underpasses, retaining walls near Red Routes or major TfL roads must go through this.
- Form A must be submitted early – even for third-party developments.
- TfL often require CAT II or CAT III checks for work near tunnels, stations, or heavily trafficked roads.
- Additional consideration for pedestrian safety, utilities, and bus infrastructure.
TfL also enforces LU Standard S1052 for works nearLondon Underground assets (including deeper CAT III scrutiny).
Network Rail Implements Design Checks
Network Rail uses a parallel but stricter process,called:
- Engineering Assurance under NR/L2/CIV/003 (Engineering Assurance of Building and Civil Engineering Works).
Network Rail Approach:
- Form 001 – Approval in Principle (AIP)
– Similar to CG300 Form A, but must consider railway loading, clearance, derailment containment, etc. - Design CRE & Checker CRE
– All design must be signed off by a Competent Responsible Engineer (CRE) and independently checked by a Checker CRE. - Form 002/003 – Design check certificates.
- Form 004/005 – Construction check and sign-off.
Risk-Based Categories:
- Complex or novel designs require CAT III checks.
- All works near live railway or track interfaces need intense scrutiny.
- Interfaces like overbridges, underbridges, footbridges, or OLE supports are subject to full assurance process.
For any third-party developer or council working near arailway bridge or embankment, Network Rail must approve both temporary andpermanent works, including geotechnical impact assessments.
When Should a the highways codes be consider on a project?
- Designing any structure adjacent to a highway, railway, or TfL route.
- Designing temporary works like hoarding, scaffolding near roads or railways.
- Planning a basement extension near a railway cutting or viaduct.
- Designing retaining walls, culverts, or service diversions under roads.
