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. Final Thoughts 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.