Build Over Agreements with Thames Water in London: When You Need One, How to Get It, and What It Involves

**The legal basis for Build Over Agreement requirements**:

Section 174A of the Water Industry Act 1991 (as amended by the Water Industry Act 2003 and subsequent regulations) gives sewage undertakers (Thames Water for London) the right to require a Build Over Agreement where a development proposes to build over or in close proximity to a public sewer. Thames Water's requirements are set out in their 'Build Over or Near to a Public Sewer' guidance (currently the Build Over/Near guidance document aligned with the Water Industry (Build Over Sewers) Regulations).

**The 3m rule — when a BOA is required**:

• *Thames Water currently requires either a Build Over Agreement or self-certification (depending on the sewer size and depth) where:*
• •A proposed building (extension; outbuilding; conservatory) is to be constructed within **3 metres** of the centreline of a **public foul, combined, or surface water sewer** that is up to **225mm internal diameter (9")**, OR
• •A proposed building is within **6 metres** of a public sewer that is **greater than 225mm diameter** (a 'strategic sewer' — larger main sewers running through London residential areas)

*The 3m measurement*: the 3 metres is measured from the **centreline of the sewer pipe** to the **nearest part of the proposed building structure**, including its foundations. If your extension is, for example, 2m from the sewer centreline, it falls within the 3m zone. If the foundation trench for the extension falls within the 3m zone even if the above-ground wall is slightly further away, the BOA is still required.

**Identifying public sewers on or near your London property**:

*How to check for public sewers*: 1. **CON29DW search (drainage and water enquiry)**: included in the standard conveyancing search pack obtained by a solicitor when purchasing a property. Section 4 of the CON29DW indicates whether there is a public sewer within the property boundary or passing through the rear garden. However, the CON29DW search is a snapshot and may not always reflect the exact sewer location precisely 2. **Thames Water Sewer Records portal**: Thames Water provides an online portal (accessible via their website: thameswater.co.uk) where you can search for sewer records by address or postcode and download the sewer map for the area showing the location, type, and size of public sewers on or near the property. This is the most reliable source for the purpose of BOA assessment 3. **CCTV survey**: a CCTV survey of the existing drainage in the rear garden will locate and verify the position of any drain pipes — but it does not necessarily distinguish between a public sewer and a private drain without reference to Thames Water records

• *Public sewer vs. private drain — the key distinction*:
• •**Public sewer**: owned, operated, and maintained by Thames Water. Subject to BOA requirements. Thames Water has rights to access the sewer from your land for maintenance and emergency repair — with 7 days' notice in non-emergency situations; no notice in an emergency
• •**Private drain**: serving only your property and maintained by you as the property owner. Not subject to Thames Water BOA requirements — but subject to your own maintenance obligations
• •**Shared private sewer (adopted)**: since the Water Industry Act 2011 transfer (October 2011), shared private drains serving two or more properties were transferred to Thames Water ownership and became public sewers. If the drain in your rear garden was shared with an adjacent property before 2011, it is now almost certainly a public sewer — check the CON29DW and Thames Water sewer records.

**Build Near Agreement vs. Build Over Agreement**:

• Thames Water uses two related terms:
• •**Build Over Agreement (BOA)**: required where the extension is to be built directly over the public sewer — that is, the extension roof or floor structure extends over (above) the sewer centreline
• •**Build Near Agreement**: required where the extension foundation or structure comes within the 3m zone but the extension does NOT extend over the sewer centreline — that is, the extension is adjacent to (beside) the sewer but not above it. In practice, Thames Water's guidance treats Build Near and Build Over under the same regulatory framework, and the same application process applies to both

**Is a BOA required for the extension or only for the foundations?**

• The BOA requirement applies to any part of the development that falls within the 3m or 6m zone — including:
• •The building footprint (walls; columns; any loadbearing structure)
• •The foundations of the extension (trench fill or strip foundations; piled foundations; raft slab foundation)
• •Any new drainage connections or pipes that pass through the 3m zone
• •External hard standing (patio; driveway) directly over the sewer — for hard standing only (not a building), the requirement is less formal but Thames Water still discourages paving directly over a sewer without a manhole access point

**The two routes for managing Build Over requirements with Thames Water London**:

Thames Water's Build Over process has two routes depending on the depth and diameter of the affected public sewer:

• *Route 1 — Self-certification (Build Over Sewer Notification)*:
• For construction over or near public sewers that are:
• •**Shallow** (depth to soffit of sewer ≤3.0m below proposed foundation level) AND
• •**Small** (internal diameter ≤225mm) AND
• •**Suitable for self-certification** based on the proposed structural arrangement

In the self-certification route, the homeowner/contractor/architect submits a notification to Thames Water (via Thames Water's online Build Over Agreement portal at buildoverapplication.thameswater.co.uk) confirming that the design complies with Thames Water's standard structural requirements. Thames Water issues a self-certification consent in response. No formal structural design review by Thames Water is required — but the project must be designed and certified by a structural engineer who confirms it meets the standard requirements.

• *Route 2 — Formal Build Over Agreement application*:
• Required where the sewer is:
• •**Deeper** than 3.0m to soffit, OR
• •**Larger** than 225mm diameter (strategic sewers), OR
• •**Affected by proposed piling** (piled foundations within the 3m or 6m zone require a formal BOA regardless of depth), OR
• •**The design cannot meet the standard self-certification requirements** (for example, where the standard structural requirements cannot be achieved)

The formal BOA application involves: 1. Submission to Thames Water's Build Over team (via the online portal) 2. Thames Water's structural team reviewing the proposed design 3. Thames Water issuing a formal consent letter (the Build Over Agreement) with conditions 4. The conditions typically include: a pre-construction CCTV survey; compliance with specified structural requirements; a post-construction CCTV survey; and notification of Thames Water before construction within the zone begins

**Pre-construction CCTV survey — required for all formal BOA applications and recommended for self-certification**:

A CCTV survey of the public sewer in the vicinity of the proposed extension is required as a condition of all formal Build Over Agreement applications, and is strongly recommended (and often required by Building Control and mortgage lenders) for self-certification routes.

• *The purpose of the pre-construction CCTV survey*:
• •To record the existing condition of the sewer before any construction activity nearby — establishing a baseline record of any pre-existing damage or defects
• •To identify the exact location, depth, and condition of the sewer in relation to the proposed extension
• •If the sewer is in poor condition at the outset, Thames Water may require repair or relining before the BOA is issued (or may refuse the BOA and require diversion of the sewer instead)

*CCTV survey process*: 1. A CCTV drainage contractor inserts a push-rod or mainline CCTV camera into the sewer manhole upstream or downstream of the affected length 2. The camera is driven through the affected sewer length, recording video and capturing the internal condition of the pipe 3. A drainage survey report is produced, typically coded to WRc (Water Research Centre) CCTV coding standards, recording defects by type and severity 4. The report and DVD/video file are submitted to Thames Water as part of the BOA application 5. A matching post-construction CCTV survey (after the extension is built) is required to confirm no damage occurred to the sewer during construction

*CCTV survey costs for London BOA applications*: approximately £350–£800 for a standard residential CCTV pre-construction survey (push-rod, up to 30m sewer run). Post-construction survey approximately the same cost.

**Structural requirements for building over a public sewer in London**:

Thames Water's standard structural requirements for Build Over/Near applications (which the design must meet for self-certification, or which Thames Water will check for formal applications) include:

• *1. Protecting the sewer from loading*:
• The proposed foundation design must ensure that no additional structural load from the extension is transferred directly to the public sewer pipe. This is typically achieved by:
• •Designing the foundation so that its load is spread to ground at a sufficient angle that the pressure bulb of stress does not reach the sewer pipe
• •Using reinforced concrete strip foundations that span over the sewer and bear on undisturbed ground on either side — with no founding directly above the sewer
• •Bridging the sewer: where the extension foundation crosses over the sewer, the foundation must bridge over the sewer on both sides — with a minimum clear span between the bearing points of foundation on either side of the sewer

• *2. Maintaining access to the sewer for Thames Water*:
• Thames Water must retain access to the sewer for maintenance and emergency repair. Requirements include:
• •An inspection chamber (manhole) with removable cover must be provided at the point where the sewer passes under the extension, so that Thames Water can insert CCTV camera equipment or drainage rods into the sewer without requiring demolition of the extension floor or roof
• •The inspection chamber cover must be at finished floor level (not buried under screed) and must be accessible — a locked inspection chamber cover with a key kept by the homeowner is acceptable
• •Thames Water must be able to access the sewer via the inspection chamber without entering the dwelling (where possible) — an external access route via the garden is preferred

• *3. Foundation specification for extensions over public sewers*:
• •Strip foundations in unreinforced concrete are NOT acceptable directly over or bridging a public sewer — reinforced concrete foundation is required to distribute loads and bridge the sewer
• •Minimum concrete specification: C30/37 reinforced concrete (minimum)
• •Minimum reinforcement: typically 16mm bar top and bottom, with additional link bars — SE to specify based on span
• •Foundation depth: not less than the crown (top) of the sewer pipe plus at least 150mm clearance above the sewer crown — so the foundation base does not bear on or damage the sewer

*4. Pile-free zone above the sewer*: Piled foundations (drilled or driven piles) must not be located within the 3m zone of the sewer — even outside the 3m zone, piled foundations require a formal BOA, and within the 3m zone, piling above the sewer is almost always refused by Thames Water. On sites where piling is proposed (deep made ground; weak soils), alternative foundation types (reinforced raft; deepened strip foundation) should be used in the 3m zone.

**Typical costs and timeline for a Thames Water BOA in London**:

| Cost element | Typical cost | |---|---| | Thames Water self-certification fee | Approximately £300–£500 (Thames Water application fee, reviewed periodically) | | Thames Water formal BOA application fee | Approximately £500–£1,500 (Thames Water application fee, depending on sewer size and complexity) | | Structural engineer design for BOA-compliant foundations | £500–£1,500 (included in the overall extension structural design; the BOA requirement may add £200–£500 to the SE fee) | | Pre-construction CCTV survey | £350–£800 | | Post-construction CCTV survey | £350–£800 | | Inspection chamber provision (where required) | £500–£1,500 (supply and install of inspection chamber and cover at finished floor level within the extension) | | **Total BOA budget (typical London residential extension, formal route)** | **£1,500–£4,000** |

*Timeline*: Thames Water's target response time for BOA applications is approximately 4–6 weeks for a standard formal application. Self-certification is faster — typically 2–3 weeks. Factor the BOA timeline into the extension pre-construction programme — the BOA must be issued before work in the sewer protection zone begins.

**Indemnity insurance as an alternative to a BOA for historical extensions**:

Where a previous owner of a London property has already built an extension over or near a public sewer without obtaining a Build Over Agreement, the options for the current owner or a prospective buyer are:

*Option 1 — Retrospective BOA application*: Thames Water accepts retrospective BOA applications for works that have already been completed. A retrospective application requires: a CCTV survey of the sewer in its current condition; the same structural information that would have been required before construction; and Thames Water's review and agreement. If the sewer is in good condition and the extension meets the structural requirements, a retrospective BOA can be issued. Typical cost: same as a forward BOA application plus the retrospective application process (£1,500–£4,000 total).

*Option 2 — Indemnity insurance (legal indemnity policy)*: An insurance-backed legal indemnity policy provides financial protection to the property owner and their mortgage lender against the risk that Thames Water exercises its rights to access or demolish the extension to carry out sewer maintenance — without the need to resolve the BOA with Thames Water. The indemnity policy pays: the cost of Thames Water's access works and making good any damage to the extension; and the diminution in value of the property if access was required and the extension damaged.

• *Indemnity insurance considerations*:
• •Cost: a residential BOA indemnity policy typically costs £150–£600 depending on the property value and the risk assessment
• •No notification to Thames Water: a condition of the indemnity policy is that Thames Water is NOT notified about the missing BOA — if Thames Water are notified, the policy becomes void (as Thames Water can then formalise their position)
• •The policy runs with the land and protects future buyers — it can be assigned to successive property owners
• •Mortgage lenders: most high-street mortgage lenders accept a BOA indemnity policy as sufficient protection for a historical missing BOA — but check with the specific lender before relying on this
• •Indemnity insurance does NOT resolve the underlying position — it provides financial protection against the risk, not a formal resolution. Some buyers prefer to seek a retrospective BOA for certainty

*Which option to choose (retrospective BOA vs. indemnity insurance)*: Where the extension appears to be structurally adequate and the sewer is in good condition, a retrospective BOA provides the cleanest solution and eliminates the risk entirely. Where the cost or complication of a retrospective BOA is disproportionate to the level of risk (particularly for older extensions that have co-existed with the sewer without incident for many years), indemnity insurance is a pragmatic and widely accepted alternative. Your solicitor and surveyor should advise on the appropriate approach for the specific property.

**What happens if an extension is built without a BOA when one was required?**

The consequences of building over or near a public sewer without a Thames Water Build Over Agreement:

1. **Thames Water's right of access**: Thames Water has a statutory right under the Water Industry Act 1991 to access public sewers for inspection, maintenance, and emergency repair — including access through private land and through buildings that have been built over or near the sewer. If Thames Water needs to access the sewer under your extension (e.g., to repair a collapsed section or clear a major blockage), they can excavate through your extension floor or roof to access the sewer. They are required to make good the excavation — but 'making good' means reinstating the structure to a serviceable condition, not restoring a premium kitchen extension to its original specification and finish.

2. **Difficulty in selling the property**: a missing BOA will be flagged on the CON29DW drainage search during the conveyancing process for any future sale. The buyer's solicitor will require either a retrospective BOA or indemnity insurance before exchange of contracts. Failure to resolve the BOA issue will delay or prevent the sale.

3. **Building Control completion certificate**: some Building Control bodies (BCBs) in London require evidence of a BOA (or a statement that the extension is outside the 3m sewer zone) before issuing a completion certificate for a new extension. An extension built without a BOA where one was required may not be able to obtain a Building Control completion certificate.

4. **Mortgage implications**: some mortgage lenders will not lend on a property with an unresolved BOA issue — requiring either a retrospective BOA or indemnity insurance before they release mortgage funds. This can delay a purchase and in some cases may cause a lender to withdraw an offer.