Where is the pipe?
A trace or survey confirms route, depth, diameter, direction, connections and chambers rather than inferring them from a plan.
Plan drains before extension foundations
Build-Over Agreements & Extension Drains: Surrey Guide
A practical route through drain tracing, sewer ownership, build-over approval, diversion options and foundation coordination before an extension reaches site.
The direct answer
Do you need a build-over agreement?
You may need written agreement from the relevant water company if an extension will be built over or close to a public sewer. The exact route, pipe size, depth, condition, ownership and proposed foundations all matter. Start by confirming the drain—not by relying on an old drawing or the position of one inspection cover.
- Establish whether the pipe is a private drain, public lateral drain or public sewer.
- Trace its real line and depth in relation to the proposed walls and foundations.
- Compare build-over, diversion and redesign options before construction information is fixed.
- Start with
- Drain trace, ownership check, condition evidence and proposed plans
- Possible routes
- Build over, divert, redesign or keep foundations clear
- Coordinate with
- Designer, building control, water company and groundwork contractor
Make the right decision
Four questions to resolve before digging
A drain can affect foundation depth, wall position, inspection access, floor build-up and programme. Resolve the interfaces on paper while changes are still manageable.
Who owns it?
Ownership affects whose permission is needed and who must retain future access. Shared or lateral drains may be public even inside the boundary.
What condition is it in?
A camera survey may be required. Defects should be understood before the pipe becomes less accessible beneath a new structure.
How will foundations work?
The design must protect the pipe, avoid transferring load onto it and meet the relevant separation and access requirements.
Compare clearly
Build over, divert or redesign?
Swipe across to compare every column
| Option | Potential advantage | Key constraint |
|---|---|---|
| Build over subject to approval | May preserve the preferred extension footprint | Technical rules, pipe condition, access and future risk |
| Divert the drain or sewer | Keeps pipework outside the new footprint | New route, falls, connections, approvals and added groundwork |
| Alter the extension layout | Can simplify approval and reduce drainage risk | May compromise space or require a design revision |
| Bridge or locally adapt foundations | Can protect a crossing in an approved detail | Needs competent structural and drainage coordination |
Devil in the detail
The best time to find the drain is before excavation
Late discovery can stop work, change concrete quantities and force rushed design decisions. Early tracing lets the foundation, drainage and inspection sequence be planned as one package.
Talk through your siteFrom question to clear scope
A practical pre-construction sequence
No unexplained leap from problem to price. Each step reduces uncertainty and makes the next decision easier.
- 1
Review records
Check sewer maps, property information, planning drawings and visible chambers.
- 2
Trace & survey
Confirm the physical route, depth, condition and active connections.
- 3
Confirm ownership
Identify the correct undertaker and whether a formal agreement is required.
- 4
Choose the strategy
Coordinate build-over, diversion or redesign with the structural scheme.
- 5
Secure approvals
Submit the requested plans, survey evidence and construction details.
- 6
Build & record
Follow inspection requirements and retain clear as-built information.
Prepare once, quote better
Information to assemble early
The application and build team need a shared set of facts. Missing depths, ownership or foundation details are common reasons a drainage issue resurfaces on site.
- Existing and proposed plans
- Sewer map and undertaker details
- Drain trace and CCTV report where needed
- Pipe diameter, depth and direction
- Proposed foundation design and levels
- Diversion detail if applicable
- Building control and approval correspondence
Useful questions
Before you commit to the work
Clear answers now prevent expensive assumptions being buried later.
What is the difference between a drain and a public sewer?
A private drain generally serves one property before it joins wider shared pipework. Public lateral drains and sewers are normally the undertaker’s responsibility. Ownership can be complex, so confirm it from records and site evidence.
Can foundations cross a drain?
Sometimes, with an appropriate approved detail that avoids loading or damaging the pipe. Public sewers have specific undertaker rules; private drainage still needs sound structural and building-control coordination.
What happens if we discover a drain during excavation?
Pause work around it, protect it and establish route, condition and ownership. Do not encase, bridge, divert or connect to an unknown pipe without the appropriate design and approvals.
Is a CCTV survey always required?
Not in every case, but it is often valuable and may be required by the undertaker, particularly when a pipe will become less accessible or its condition is uncertain.
How long does a build-over agreement take?
Times vary with the undertaker, application route and completeness of the information. Treat approval as a pre-construction task and check current timescales directly rather than programming around a generic estimate.
A practical next step
Turn the site details into a clear scope
Send photos, the property location and what happens during rain or construction. We will help identify the most useful next survey.