A gas diversion is the process of moving an existing gas main or service to accommodate a new building, structure, or highway works. They are a common requirement on commercial and infrastructure projects where new development clashes with buried gas assets.
Gas diversions are one of the most under-estimated line items on UK construction projects. Programme slippage on a single diversion can delay practical completion by months. This guide explains how diversions work, what they cost, and how to run them without blowing your programme.
When You Need a Gas Diversion
A diversion is required whenever a proposed new structure, excavation, or ground-level feature conflicts with an existing gas main or service. Common triggers include:
A new building footprint crossing an existing gas main.
Foundation works or piling in the vicinity of a main that would compromise its integrity.
Highway realignment or new road crossings.
Ground-level changes (major cut or fill) that alter the cover depth above the main.
New utilities being installed that cannot clear existing gas assets.
The first warning sign is usually a GPR survey or a statutory utility plan showing a main in or near the proposed works.
Who Owns the Asset
Your diversion is managed by whoever owns the gas main. In most cases, this is the Gas Distribution Network (GDN) for the region: Cadent, Northern Gas Networks, SGN, or Wales and West Utilities. Some mains are owned by Independent Gas Transporters (IGTs), and a small number of industrial private networks have third-party ownership.
The asset owner has the final say over the diversion design and the works. They will not allow a third party to touch the main directly.
Cost Drivers
Gas diversion costs vary from a few thousand pounds for a short, low-pressure diversion to several hundred thousand pounds for a long, medium-pressure diversion on a busy road. The main cost drivers are:
Length of diversion. Longer routes mean more excavation, more pipe, more reinstatement.
Pressure tier. Low-pressure diversions are simpler than medium-pressure or intermediate-pressure diversions, which require more sophisticated welding, testing, and isolation.
Ground conditions and surface type. Hard ground, tarmac carriageway, and obstructed routes all push costs up.
Traffic management requirements. A road closure on a principal route adds significant cost for signage, diversion routing, and notice periods.
Service disruptions. If the diversion requires a temporary shut-off of gas supply to any properties, alternative supply (such as mobile LPG) or advance notice procedures apply.
Interface with other services. Clashes with water, electricity, or telecoms assets require coordination and sometimes additional diversions.
Typical Timeline
Diversion projects have a long lead time. The typical sequence is:
Initial enquiry and site visit: two to four weeks.
Scoping and feasibility design: four to eight weeks.
Detailed design and approvals: six to twelve weeks.
Programming and notice periods: four to eight weeks.
On-site works: two to twelve weeks depending on scope.
Reinstatement and snagging: two to four weeks.
Total elapsed time from first enquiry to completion is often six to twelve months. Complex medium-pressure diversions on busy highways can exceed eighteen months.
The “C3” Problem
Gas diversions fall under the Ofgem C3 charging regime, which determines how the cost is split between the asset owner and the requesting party. For most commercial developers, the reality is that the developer bears the full cost of the diversion.
There are limited circumstances where a cost share applies. These include highway authority-driven diversions and certain statutory utility schemes. For most commercial development, assume the diversion is 100 per cent your cost.
How to Reduce the Pain
Find out about gas assets at the feasibility stage. A gas main discovered during a C4 survey is an affordable problem. A gas main discovered after piling has started is a very expensive problem.
Design to avoid, not divert. If the building layout can be adjusted to keep the main clear with a reasonable setback, that is usually cheaper than any diversion.
Engage the GDN as early as possible. Their programme is the binding constraint. Getting on their diary six months ahead of site start is normal.
Coordinate with other utility diversions on construction sites. If electricity, water, or telecoms also need moving, running them in one combined programme reduces total disruption and total traffic management cost.
Consider temporary supply strategies. On long construction programmes, a temporary gas diversion followed by a permanent reinstatement after building completion can be the only viable route.
What Good Looks Like
A well-managed gas diversion starts with utility awareness at RIBA stage 2 or 3, feasibility cost input by stage 4, detailed design and pricing locked by stage 5, and execution slotted into the build programme with at least four weeks of float.
The common failure mode is diversions being added to the programme at stage 5 or 6, at which point the GDN’s lead time exceeds the available construction window and the whole build slips.
The Bottom Line
Gas diversions are slow, expensive, and largely non-negotiable. The only way to manage them is to identify them early and build their lead time into the construction programme from the start. Everything else is firefighting. It is also worth reading our guide to reducing utility connection costs on developments for ways to keep diversion spend in check.