Drainage Surveys Before and During London Extension Projects: CCTV Drain Survey, Build-Over Agreements, and Drainage Implications

**Why London extensions are particularly likely to have drainage complications**:

London's Victorian and Edwardian sewers are typically brick-built oval or circular pipes, originally designed to serve far lower population densities than exist today. Many London gardens have a public sewer running through them — particularly in mid-terrace and end-of-terrace properties where the back garden is the logical route for the Victorian combined sewer that serves the row of houses. In some areas, the sewer runs parallel to the rear boundary; in others, it crosses the garden diagonally; in some terrace rear gardens, it runs directly under the proposed extension footprint.

• Additionally, many London Victorian houses have:
• •Private drainage runs (soil pipes; gulley connections; yard drains) that run under the garden in unexpected directions
• •Combined drainage (foul water and surface water in the same pipe) — relevant for understanding any drainage diversions required
• •Pre-existing drainage defects (cracked pipes; root ingress; collapsed sections; tree root blockages) that should be addressed before the extension is built over them

**When to commission a CCTV drainage survey for a London extension**:

• A CCTV drainage survey should be commissioned:
• •Before or during RIBA Stage 2 (Concept Design) for the extension — so that the sewer route is known before the extension footprint is fixed in the design
• •At the latest, before planning permission is submitted for extensions that are likely to be near or over a public sewer — some London Boroughs require a drainage impact assessment as part of the planning application
• •Before any groundworks commence on site, if a survey has not already been done — to confirm the drainage position and condition before excavation near or over the drainage run

**What a CCTV drainage survey involves**:

*1. Initial desktop study*: before commissioning a survey, download or request the Thames Water sewer map for the area. Thames Water's 'Digital Plan of Record' (DPOR) sewer maps show the approximate route of public sewers — available via Thames Water's 'Build Over' online tool or their asset records team. The sewer map is an approximate guide — actual sewer positions can vary from the mapped line.

• *2. CCTV drain survey on site*: a CCTV drainage surveyor accesses the inspection chambers on the property (the manhole covers in the garden or rear of the property) and passes a remote-controlled CCTV camera through each drainage run. The camera records:
• •The internal condition of the pipe (cracks; root ingress; displaced joints; deposition; corrosion)
• •The dimensions of the pipe (diameter; shape — circular; oval; egg-section; rectangular)
• •The direction and depth of the run
• •Connections from other drainage runs
• •The pipe material (vitrified clay; UPVC; concrete; brick; pitch fibre — the latter is common in 1950s–1970s extensions and is a maintenance concern)

*3. CCTV survey report*: the surveyor produces a written report with the CCTV footage and a drainage layout plan showing the mapped runs, invert levels (the level at the inside bottom of the pipe), pipe diameter, condition grading (using WRc or MSCC coding), and any defects identified.

*4. Drainage layout drawing*: from the CCTV survey, the engineer or drainage surveyor produces a drainage layout plan showing the existing drainage runs superimposed on the proposed extension plan — confirming whether the extension footprint is over or near any drainage runs.

**What a CCTV drain survey may reveal**:

• •*Public sewer within the extension footprint*: if a public sewer runs under the proposed extension footprint, Thames Water's build-over agreement regime applies (see below). This may require the extension design to be modified to avoid the sewer, or a build-over agreement to be obtained before works proceed
• •*Existing drainage defects*: cracked, displaced, or root-invaded drains under the proposed extension footprint should be repaired before the extension is built — otherwise the extension foundation will be at risk of undermining as the defective drain continues to deteriorate (and drain water may escape into the formation, saturating the ground under the foundation)
• •*Pitch fibre pipes*: pitch fibre drainage (installed extensively in 1950s–1970s extensions) collapses over time — if pitch fibre is identified, it should be replaced before being built over
• •*Unexpected drainage runs*: many London gardens have drainage connections that are not fully documented — previous drainage connections to a rear outbuilding; gulleys from a former conservatory; yard drainage runs. The CCTV survey maps these so they can be accounted for in the extension foundation and floor design
• •*Invert levels*: the invert levels from the CCTV survey are critical for understanding whether the proposed floor slab or foundation can be constructed without impacting the drainage run headroom

**What is a build-over agreement?**

A build-over agreement is a formal agreement with Thames Water (the sewerage undertaker for London) that is required before any structure is built over or near a public sewer. The statutory basis is the Water Industry Act 1991 and the Sewerage Undertakers' Technical Standards for Build Over and Near to Public Sewers.

• The purpose of the build-over agreement is to protect the public sewer — to ensure that:
• •The sewer can still be accessed for maintenance and repair by Thames Water operatives
• •The structure built over or near the sewer does not impose additional loading on the sewer pipe that could cause it to collapse
• •If the sewer needs to be relaid in the future, this can be done without the extension needing to be demolished

**When a Thames Water build-over agreement is required**:

• *The Thames Water build-over requirements apply where*:
• •A new structure (an extension; outbuilding; conservatory; access road; car park; raised bed) is proposed within **3 metres** of the centreline of a public sewer of 160mm diameter or less; or
• •Within **6 metres** of a public sewer of diameter greater than 160mm (including brick-built Victorian combined sewers, which are often much larger than 160mm)
• •Within **12 metres** of a public pumping main or rising main

*Note*: the distances above are from the CENTRELINE of the sewer — not from the edge of the sewer. A 450mm diameter sewer has its centre 225mm from its outer wall. A structure built 3.0m from the edge of the sewer centreline is 2.775m from the outer edge of the sewer.

**How Thames Water's build-over process works**:

*Step 1 — Check if a sewer is present*: use Thames Water's 'Build Over' online tool (developers.thameswater.co.uk/Build-Over-Or-Near-A-Sewer) to check whether a public sewer is shown on the Thames Water record near the proposed development. Input the site address; the tool returns the DPOR sewer map and an indication of whether a build-over agreement may be required.

*Step 2 — Assess the proposed works*: is the proposed structure within the 3m/6m/12m zones?

*Step 3 — Submit a self-certifying build-over application (for straightforward cases)*: For structures over or near sewers of up to 160mm diameter where the proposed structure meets Thames Water's technical standards for self-certification, the applicant (typically the architect or structural engineer on behalf of the client) can submit a 'Self-Certifying Build Over Application' to Thames Water. Thames Water reviews the submission and issues an approval (or a request for a full build-over agreement instead).

• The Thames Water technical standards for self-certification are broadly:
• •The proposed structure does not exceed 3 storeys (8m height at eaves)
• •The foundation is at or below the depth required to be outside the influence zone of the sewer (typically determined by a 45-degree line drawn from the inner edge of the sewer at the sewer invert level)
• •The structure does not place additional load on the sewer
• •A CCTV pre-commencement survey of the sewer is undertaken and submitted to Thames Water
• •A CCTV post-completion survey of the sewer is undertaken and submitted to Thames Water (to confirm the sewer has not been damaged during construction)

*Step 4 — Full build-over agreement (for non-standard cases)*: Where the self-certification route is not available (e.g., structure is over a larger sewer; the proposed foundation is within the 45-degree influence zone; the structure is more than 3 storeys), Thames Water requires a full build-over agreement.

• A full build-over agreement involves:
• •A structural engineer's assessment of the proposed loads on the sewer
• •A proposal to modify the foundation or structure to protect the sewer (bridging foundations; reinforced concrete raft bridging the sewer; diversion of the sewer)
• •Thames Water's technical review and approval
• •A legal agreement registered against the property title
• •Thames Water fee: typically £ varies based on complexity — £300–£800 for straightforward cases; higher for complex schemes

*Step 5 — Sewer diversion (where build-over agreement cannot be obtained)*: Where Thames Water does not agree to a build-over — because the sewer is too large; the proposed structure would prevent future sewer maintenance; or the sewer condition is too poor to be built over safely — sewer diversion may be required. Sewer diversion involves re-routing the public sewer around the proposed extension footprint in a new pipe. Thames Water must approve the diversion design and the works must be carried out by a contractor approved by Thames Water. Costs: sewer diversion in London typically £5,000–£30,000+ depending on the length and complexity of the diversion, access constraints, and ground conditions. Sewer diversion must be complete and accepted by Thames Water before the extension foundation can be constructed over the existing sewer route.

**The CCTV pre-commencement and post-completion surveys for Thames Water**:

• Where a build-over agreement (self-certifying or full) is obtained, Thames Water requires:
• •*CCTV pre-commencement survey*: a CCTV survey of the relevant sewer section BEFORE any groundworks commence — to establish the baseline condition of the sewer
• •*CCTV post-completion survey*: a CCTV survey of the same sewer section AFTER the extension is completed — to confirm that the sewer has not been damaged during construction

Both surveys must be submitted to Thames Water as part of the build-over process. If the post-completion survey shows damage to the sewer caused during construction, the developer is responsible for the remediation costs.

**Cost overview for drainage investigation and compliance on a London extension project (2025)**:

| Item | Typical cost range | |---|---| | Thames Water DPOR sewer map search | Free (via Thames Water online tool) | | CCTV drain survey (single-storey extension, up to 3 chambers) | £300–£600 | | Extended CCTV survey (larger site; 4–6 chambers) | £600–£1,200 | | Drainage layout plan from CCTV survey | £200–£500 (from drainage engineer) | | Thames Water self-certifying build-over application fee | £ varies (typically £300–£500 to Thames Water) | | Thames Water full build-over agreement fee | £300–£1,000+ to Thames Water depending on complexity | | CCTV pre-commencement survey for Thames Water (single sewer section) | £250–£500 | | CCTV post-completion survey for Thames Water (single sewer section) | £250–£500 | | Drainage defect repair (within garden, before extension) | £500–£5,000 depending on defect and depth | | Sewer diversion (short diversion, London) | £5,000–£15,000 | | Sewer diversion (complex, long distance, deep) | £15,000–£30,000+ |

**Planning implications of drainage for London extensions**:

Several London Boroughs require a drainage impact assessment or drainage strategy as part of the planning application for an extension or new dwelling:

*Surface water drainage strategy*: the NPPF and the lead local flood authority (typically the London Borough's drainage team) require developments to manage surface water runoff sustainably. For extensions over a certain floor area threshold (typically 1,000m² for major development; but some Boroughs apply it at lower thresholds for residential development), a surface water drainage strategy demonstrating sustainable drainage systems (SuDS) is required. For smaller domestic extensions, the planning requirement for a drainage strategy is less common — but it is increasingly being requested by inner London LPAs.

*Foul drainage capacity*: where the extension adds new bathrooms or kitchens, the planning application may need to confirm that the existing foul drainage connection to the public sewer has adequate capacity for the increased flow. A Thames Water drainage capacity assessment may be needed for larger schemes.

**How to manage drainage effectively on a London extension project — a sequence**:

1. Commission Thames Water sewer map search as soon as the site is identified (free; 10 minutes) 2. If sewer is shown within 3m/6m of proposed extension: commission CCTV drain survey before design is finalised 3. Share CCTV survey results and drainage layout with the architect and structural engineer so the extension footprint and foundation depth are designed with the drainage position in mind 4. Submit Thames Water build-over application (self-certifying or full) if required — before planning permission is submitted where possible, or as a condition of planning permission 5. Commission CCTV pre-commencement survey immediately before groundworks begin 6. Repair any drainage defects identified in the survey before the foundation is constructed over or near them 7. Commission CCTV post-completion survey after the extension is complete and submit to Thames Water to close the build-over agreement 8. Retain all CCTV survey reports and build-over agreement documentation in the property's permanent planning and compliance file