Build Over Agreements for Extensions: A Complete Guide
In the UK, extending your property often requires more than just planning permission and building control approval. If your extension is near or over a public sewer or drain which is especially typical for terraced properties, you’ll likely need a Build Over Agreement (BOA) with your local water authority. These agreements ensure the integrity and accessibility of vital underground infrastructure.
This guide covers every major aspect of BOAs, including:
• How to locate sewers on your property
• How to submit an application
• Structural requirements like bridging foundations
• Manhole relocation options
• Costs involved in the process
1. What Is a Build Over Agreement (BOA)?
A Build Over Agreement is a legal document between the property owner and the local sewerage undertaker (e.g. Thames Water, Severn Trent, Yorkshire Water), allowing you to build within 3 metres of a public sewer or directly over it.
Without this agreement, you risk:
• Enforcement action or being required to remove or modify the structure
• Legal issues during property sale
• Insurance or mortgage problems
2. How to Locate Public Sewers
Before planning your extension, the first step is locating any underground sewers or drains.
Methods to Locate Sewers:
A. Water Authority Sewer Maps
• Request via website or directly from the water company.
• Often available online or via request for a small fee.
• Shows only public sewers, not private drains.
B. Drainage Surveys (CCTV)
• Hire a drainage company to perform a CCTV inspection.
• Confirms the exact location, depth, and condition of the pipes.
• Highly recommended before submitting a BOA application.
C. Visual Inspection
• Look for inspection chambers (manholes) within your property or nearby.
• Track direction and flow by lifting covers (with care and appropriate equipment).
Tip: Just because a pipe is within your boundary doesn’t mean it’s private—since 2011, shared sewers beyond the property edge are usually classed as public.
3. How to Submit a Build Over Application
Step-by-Step Process:
Step 1: Gather Documents
• Site location plan
• Existing and proposed drainage layout
• Foundation drawings showing proximity to sewer
• CCTV survey (often required for sewers over 225mm or poor condition)
• Structural engineer’s report (for complex builds or heavy loads)
Step 2: Submit to Water Authority
Each company has its own portal. For example:
• Thames Water: developers.thameswater.co.uk
• Severn Trent: stwater.co.uk
Step 3: Pay the Fee
Fees range between £300–£800, depending on sewer size and project complexity.
Step 4: Wait for Approval
Decisions usually take 2–4 weeks, unless further investigation is required.
Step 5: Post-Completion
Water company may inspect works. A certificate of consent is issued upon compliance.
4. Bridging the Foundations Over a Sewer
You cannot place loads directly onto a public sewer. Instead, foundations must bridge over or divert around the pipe.
Techniques to Bridge Foundations:
A. Concrete Lintels/Bridging Slabs
• Used when sewer is small and shallow.
• Concrete lintels span the sewer trench to distribute load away from it.
B. Pile and Beam Foundations
• For deeper sewers or poor ground.
• Vertical piles on either side of the sewer, supporting a reinforced concrete beam spanning across.
• Minimises load and vibration.
C. Raft Foundation
• Suitable for cohesive soils.
• Spreads the building load across a wide slab—may be used if not directly over sewer.
Key Requirements:
• No foundation within 150mm vertically or 500mm horizontally of the pipe.
• Provide access for future maintenance (access panels, removable flooring if within building).
Important: The structural solution must be signed off by a qualified engineer and approved by the sewerage authority.
5. How to Relocate Manholes
If a manhole is located within the proposed footprint of the extension, it generally must be moved, as building over it is discouraged or prohibited.
Manhole Relocation Options:
A. Move Outside of Extension Footprint
• Divert pipework and rebuild the chamber in a new location.
• Requires hydraulic assessment (ensure correct gradient and flow).
• May need a Section 106 sewer connection approval.
B. Replace with Rodding Access
• For smaller, straight-line sewers.
• Replace manhole with inspection pipe and rodding access outside the building.
• Only possible if no change of direction or junction occurs at the chamber.
C. Enclose in Extension (Rarely Permitted)
• Only in special cases (e.g., garages).
• Requires sealed, double-sealed, vented access cover.
• Risk of smell, damp, or blockages makes this a last resort.
CCTV survey and detailed plans are usually required before permission is granted for any relocation.
6. Costs Involved in BOAs and Related Works
Here’s a breakdown of typical costs (approximate):
Build Over Agreement Fee: £300 - £800
CCTV Drain Survey: £200 - £400
Structural Engineer's Report: £400 - £1,000+
Foundation Bridging (Lintels): £500 - £2,000
Pile and Beam Foundations: £5,000 - £15,000+
Sewer Diversion/Manhole Move: £2,000 - £8,000+
Note: These costs vary significantly based on property size, ground conditions, sewer diameter, and access.
7. Conclusion and Recommendations
A Build Over Agreement is more than a formality—it ensures the long-term integrity of both your extension and public sewer infrastructure. Neglecting it can lead to major headaches down the line.
Key Takeaways:
• Always check for sewers before finalising extension plans.
• Get a CCTV survey and consult a structural engineer early.
• Be ready for extra costs and time due to foundation adjustments or manhole diversions.
• Engage with the water authority as soon as sewers are identified.
A well-managed BOA process leads to a safer, compliant, and more valuable extension project.
Disclaimer: This article is for general informational purposes only and does not constitute legal or professional advice. Always consult relevant professionals and local authorities before undertaking any development or change of use.