Build Over Agreement London: Thames Water Guide for Extensions
If there is a public sewer running under or near your property, you may need a build over agreement from Thames Water before you can start your extension. It is one of the most commonly overlooked requirements in London, and getting it wrong can cause serious problems.
Quick Answer
A build over agreement is Thames Water's approval to build over or within 3 metres of a public sewer. Application costs £300-1,000 in total (including surveys) and takes 6-12 weeks for approval. Building without one risks enforcement action, insurance problems, and difficulty selling your property.
£300-£1k
Application Cost
6-12 weeks
Approval Time
Within 3m
Trigger Distance
Check your specific property constraints
Free Property CheckWhat Is a Build Over Agreement?
A build over agreement is formal approval from Thames Water (or your local water company) that permits you to build a structure over or close to a public sewer. Public sewers are the pipes that carry wastewater from multiple properties to treatment works. They run beneath many London properties, often through rear gardens where extensions are most commonly built.
Thames Water has a legal right to access and maintain public sewers. If you build over one without agreement, you could obstruct their ability to repair or replace the sewer in the future. The build over agreement ensures that your extension is designed and built in a way that protects the sewer and preserves Thames Water's access rights.
This requirement applies to all types of construction over or near public sewers, including house extensions, conservatories, garages, garden walls, and even some landscaping works. It is separate from planning permission and building regulations, and you need all three to be in order before starting work.
When Do You Need a Build Over Agreement?
You need a build over agreement from Thames Water if you are proposing any of the following:
- Building directly over a public sewer: Any structure with foundations above the line of a public sewer
- Building within 3 metres of a public sewer: Even if you are not directly over it, foundations and construction activity within 3m can damage or displace the sewer
- Building within 1 metre of a manhole or inspection chamber: Thames Water needs to maintain access to these points
How common is this? More common than most homeowners realise. Public sewers run through a significant proportion of London residential properties, particularly in Victorian and Edwardian areas. Your architect or builder should check the sewer maps early in the design process.
How to Check If You Need One
Checking whether a public sewer runs under or near your proposed extension is straightforward and should be done at the earliest stage of your project.
Three ways to check:
- Thames Water sewer maps: Available online through Thames Water's developer services portal. Shows the approximate position of public sewers on your property. Cost: £30-50 for a sewer map search
- CCTV drainage survey: A camera inspection of your drains to physically confirm the sewer position, condition, and connections. More accurate than maps, which can be up to 1m off. Cost: £300-600
- Your architect should check: Any competent architect or architectural designer will check the sewer maps as part of their initial site appraisal. If they do not mention it, ask
Note: Thames Water sewer maps show approximate positions only. The actual sewer may be up to 1 metre from the position shown on the map. A CCTV survey is the only way to confirm the exact location and condition. Most build over agreement applications require a CCTV survey anyway.
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The Application Process Step by Step
The build over agreement application process involves several stages. Starting early is essential because the 6-12 week approval timeline can delay your construction start date if not factored into your project plan.
- 1Commission a sewer map search
Order from Thames Water's developer services. Shows the position of public sewers on and around your property. Takes 5-10 working days. Cost: £30-50.
- 2Arrange a CCTV drainage survey
Hire a drainage specialist to survey the sewer with a camera. This confirms the exact position, depth, condition, and diameter. Required by Thames Water in most cases. Cost: £300-600.
- 3Submit the build over agreement application
Submit your application to Thames Water with your proposed extension plans, structural details, foundation design, and CCTV survey results. Your architect or builder can do this. Application fee: £0-400 (Thames Water may waive the fee for simple applications).
- 4Thames Water assessment
Thames Water reviews your application and may request additional information or modifications to your foundation design. They assess the risk to the sewer and the adequacy of your protective measures. Timeline: 4-8 weeks.
- 5Approval with conditions
Thames Water issues approval, usually with specific conditions about foundation design, protective measures, and access requirements. You must comply with all conditions during construction.
Cost Breakdown
| Item | Cost | Notes |
|---|---|---|
| Sewer map search | £30-£50 | Online via Thames Water developer services |
| CCTV drainage survey | £300-£600 | Required for most applications |
| Application fee | £0-£400 | Thames Water may waive for simple cases |
| Protective measures | £1,000-£5,000 | Concrete slab, modified foundations, etc. |
| Total typical cost | £1,330-£6,050 | Including all surveys and protection works |
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The build over agreement process typically takes 6-12 weeks from start to finish. This can run in parallel with your planning application, so it does not necessarily add to the overall project timeline if you start early enough.
- Week 1-2: Order sewer map search, arrange CCTV survey
- Week 2-3: Receive sewer map, conduct CCTV survey
- Week 3-4: Submit application with all documentation
- Week 4-10: Thames Water assessment (may request additional info)
- Week 10-12: Approval issued with conditions
Pro tip: Start the build over agreement process at the same time as your planning application. Both run for approximately the same duration (8-12 weeks), so submitting them in parallel means you avoid adding months to your project timeline. Your architect should coordinate both processes.
What Happens If You Build Without Agreement?
Building over a public sewer without a build over agreement is a serious matter with potential consequences at every stage of property ownership.
- Enforcement action: Thames Water can require you to expose the sewer, demonstrate it is undamaged, and install protective measures retroactively. This is far more expensive than doing it properly from the start
- Sewer damage liability: If your extension damages the sewer (e.g., through foundation settlement or construction loads), you are liable for all repair costs. A sewer collapse under a building can cost £10,000-50,000+ to repair
- Insurance complications: Your buildings insurance may not cover damage related to an extension built over a sewer without agreement. Some insurers add specific exclusions
- Problems when selling: Conveyancing searches reveal the absence of a build over agreement. Buyers' solicitors will flag this, potentially delaying or collapsing the sale. You may need to obtain retrospective approval or indemnity insurance to proceed
Many homeowners discover this after starting work
One of the most common scenarios: the builder starts digging foundations and discovers a sewer pipe that was not on the original plans. Work stops, a retrospective application must be made, and the project is delayed by weeks or months. The CCTV survey before construction begins is a small investment that prevents this expensive surprise.
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Common Conditions Thames Water Impose
When Thames Water approves a build over agreement, it almost always comes with conditions. These conditions are designed to protect the sewer and ensure Thames Water can access it for maintenance and repair in the future.
- Concrete protection slab: A reinforced concrete slab placed over the sewer before your foundations are built. This distributes the load from the building and protects the sewer from damage. Adds £1,000-3,000 to construction costs
- Foundation design restrictions: Thames Water may specify minimum clearances between foundations and the sewer, or require piled foundations instead of traditional strip foundations to avoid loading the sewer
- Access maintenance strip: You may need to maintain an accessible area (typically 1.5-3m wide) above the sewer line. This could mean leaving a removable floor section or not building directly over the sewer at all
- Pre and post-construction CCTV surveys: Thames Water may require a CCTV survey before construction starts and another after completion to confirm the sewer has not been damaged. Additional cost: £300-600 for the post-construction survey
- Sewer diversion: In some cases, Thames Water may require the sewer to be diverted away from your extension footprint entirely. This is the most expensive outcome, costing £5,000-15,000+, and is more common for larger developments
Frequently Asked Questions
How do I know if there is a public sewer on my property?
Order a sewer map search from Thames Water's developer services portal (cost: £30-50). This shows the approximate position of public sewers on and around your property. The maps are indicative, so the actual position may vary by up to 1 metre. For precise positioning, commission a CCTV drainage survey (£300-600). Your architect or builder should check this as part of their standard site appraisal.
What is the difference between a public sewer and a private drain?
A private drain serves only your property. A public sewer serves two or more properties and is maintained by Thames Water. Many drains that homeowners believe are private are actually public sewers, especially in older London properties. The Transfer of Private Sewers Regulations 2011 transferred most shared drains to public sewer status. If in doubt, a CCTV survey can confirm whether the drainage serving your property is public or private.
Can Thames Water refuse a build over agreement?
Yes, but outright refusal is uncommon for residential extensions. Thames Water is more likely to impose conditions (modified foundations, protective slabs, access strips) rather than refuse entirely. Refusal is more common for large sewers (over 300mm diameter), very deep sewers, or sewers in poor condition that Thames Water plans to replace. If refused, your options include diverting the sewer, redesigning the extension to avoid the sewer, or appealing the decision.
Do I need a build over agreement for a conservatory?
Yes, if the conservatory is built over or within 3 metres of a public sewer. Many homeowners assume conservatories are exempt because they are lighter structures, but Thames Water's requirement applies to all permanent structures with foundations. The good news is that conservatories typically have shallower foundations, so the conditions imposed are usually less onerous than for full extensions.
Can I get retrospective approval if I have already built over a sewer?
Yes, you can apply for a retrospective build over agreement. Thames Water will typically require a CCTV survey to confirm the sewer's condition, and may impose additional conditions or require remedial works if the sewer has been damaged. Retrospective approval is commonly sought when selling a property and the buyer's solicitor identifies the missing agreement. Budget £500-2,000 for the retrospective application process, plus any remedial works required.
Does a build over agreement transfer with the property?
Yes. Once granted, a build over agreement is linked to the property and transfers to new owners when the property is sold. The conditions continue to apply, meaning the new owner cannot modify the structure in a way that contravenes the agreement without applying for a new one. The agreement should be included in your property deeds and disclosed during conveyancing.
Summary: Build Over Agreements in London
What it is: Formal approval from Thames Water to build over or within 3 metres of a public sewer. Required for extensions, conservatories, and other structures with foundations near public drainage.
Cost: £300-1,000 for surveys and application, plus £1,000-5,000 for protective measures during construction. A small cost relative to your overall extension budget.
Timeline: 6-12 weeks from start to approval. Can run in parallel with your planning application to avoid adding to the overall project timeline.
Key action: Check the sewer maps early in your project. A £30-50 sewer map search and £300-600 CCTV survey prevents nasty surprises during construction and ensures you can sell the property without complications in the future.
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