Planning a home extension in London? Drainage is often the hidden complexity that catches homeowners off guard. With Victorian-era combined sewers running beneath most properties and Thames Water's strict Build Over requirements, getting drainage right from the start is essential for a smooth project.
We've evaluated London's drainage consultants based on residential extension experience, Thames Water approval success rates, response times, and client feedback. This guide focuses specifically on consultants who understand the unique challenges of domestic extension projects rather than large commercial developments.
When Do You Need a Drainage Consultant?
Mandatory Situations
Building within 3m of public sewer
Build Over agreement required
Extension in Flood Zone 2 or 3
Flood Risk Assessment needed
Basement or lower ground floor
Pumped drainage likely required
New bathroom or kitchen
Foul drainage connection needed
Extension over 50m²
Surface water management required
Planning condition requirement
Drainage strategy pre-commencement
Even when not strictly mandatory, a drainage survey is highly recommended for any extension project. Discovering sewer issues mid-construction can add £5,000-£15,000 to project costs and cause significant delays.
Typical Drainage Consultant Costs
| Service | Price Range | Turnaround |
|---|---|---|
| CCTV Drain Survey | £250 - £450 | Same day - 3 days |
| Drainage Strategy Report | £400 - £800 | 5 - 10 days |
| Build Over Agreement Application | £500 - £900 | 4 - 8 weeks (inc. TW) |
| Soakaway Design & Testing | £600 - £1,200 | 7 - 14 days |
| Flood Risk Assessment | £500 - £1,500 | 7 - 14 days |
| Full Drainage Package* | £1,200 - £2,500 | 2 - 4 weeks |
*Full package typically includes CCTV survey, drainage strategy, and Build Over application
Top 10 Drainage Consultants for London Extensions
Drainage Design London
Specialists in residential drainage solutions across Greater London
Starting Price
£295 (CCTV survey)
Thames Water Success Rate
98%
Response Time
24-48 hours
Best for: Homeowners wanting a complete drainage package. Known for handling complex Build Over applications and providing clear, jargon-free reports. Strong track record with Victorian properties in South and East London.
Metro Rod London
National brand with dedicated residential extension team
Starting Price
£275 (CCTV survey)
Coverage
All London boroughs
Response Time
Same day available
Best for: Fast turnaround needs and emergency surveys. Extensive equipment including push-rod and crawler CCTV systems for complex drainage networks. Well-established relationships with Thames Water.
JMC Drainage Consultants
Chartered engineers specialising in residential projects
Starting Price
£350 (CCTV survey)
Qualification
ICE Chartered
Response Time
2-3 days
Best for: Complex projects requiring engineering sign-off. Excellent for basement extensions needing pumped drainage systems and projects with challenging ground conditions. Reports accepted by all London building control bodies.
Lanes for Drains
Established drainage specialists with 50+ years experience
Starting Price
£285 (CCTV survey)
Experience
Since 1970
Response Time
Same day - 48 hours
Best for: Properties with suspected drainage problems that need diagnosis before extension planning. Combined survey and repair capabilities mean issues can be resolved quickly. Strong presence in North and West London.
Capital Drainage Solutions
Boutique consultancy focused on domestic extensions
Starting Price
£320 (CCTV survey)
Specialisation
Residential only
Response Time
3-5 days
Best for: Homeowners wanting personalised service and direct access to the surveyor. Particularly good at explaining drainage requirements to non-technical clients. Works closely with architects and structural engineers.
Aspect Drainage Surveys
Survey specialists with advanced diagnostic equipment
Starting Price
£260 (CCTV survey)
Technology
HD & sonar mapping
Response Time
24-72 hours
Best for: Properties needing detailed drain mapping and condition assessment. HD footage and sonar technology excellent for unclear situations. Competitive pricing for CCTV surveys. Covers all London postcodes.
SuDS Design Ltd
Sustainable drainage specialists for eco-conscious projects
Starting Price
£450 (SuDS design)
Specialisation
Sustainable drainage
Response Time
5-7 days
Best for: Extensions requiring SuDS compliance or homeowners wanting green drainage solutions. Expert in permeable paving, rain gardens, and attenuation systems. Ideal for larger extensions in flood-sensitive areas.
Thames Drainage Experts
Build Over specialists with direct Thames Water liaison
Starting Price
£550 (Build Over pkg)
Build Over Success
99%
Response Time
3-5 days
Best for: Projects where Build Over agreement is the primary concern. Streamlined process with Thames Water and excellent at navigating complex sewer situations. Can expedite urgent applications.
HomeDrain Surveys
Budget-friendly option for straightforward projects
Starting Price
£199 (basic survey)
Coverage
Inner London focus
Response Time
Same day available
Best for: Simple rear extensions where you just need to verify drain locations. Good entry-level option but may need to engage specialists for complex Build Over applications. Best for budget-conscious projects.
London Basement Drainage
Basement extension drainage specialists
Starting Price
£650 (basement pkg)
Specialisation
Below-ground only
Response Time
5-7 days
Best for: Basement conversions and lower ground floor extensions. Expert in pumped drainage systems, anti-flood valves, and waterproofing coordination. Works closely with basement construction specialists across prime London boroughs.
Understanding Thames Water Build Over Agreements
If your extension is within 3 metres of a public sewer, you legally require a Build Over Agreement from Thames Water before construction can begin. This applies even if the sewer crosses your neighbour's property but runs within 3m of your proposed extension.
Build Over Process
CCTV Survey
Professional survey to assess sewer condition and exact location
Application Submission
Submit plans, survey results, and structural details to Thames Water
Thames Water Assessment
4-6 week review period (can be longer for complex cases)
Agreement Issued
Approval with conditions for construction methodology
"Starting the Build Over process early is crucial. I've seen projects delayed by months because homeowners didn't realise a sewer ran through their garden. Get the CCTV survey done before finalising extension designs."
— Drainage consultant, 15 years experience
Common Drainage Issues Found in London Extensions
Combined Sewers
Most of London uses Victorian combined sewers where surface water and foul water share the same pipes. This means more complex drainage calculations and potential Thames Water restrictions on additional surface water connections.
Shared Drains
Terraced and semi-detached properties often share drainage with neighbours. Your extension may require agreement from adjacent properties if the shared drain needs modification or if your works could affect their drainage.
Insufficient Fall
Extensions can struggle to achieve adequate drainage fall, especially on flat sites or when connecting to existing drains. Minimum 1:80 gradient required for foul drains (1:40 for surface water). May require pumped systems.
High Groundwater
Parts of London, particularly near the Thames and its tributaries, have high water tables. This affects soakaway viability and may require alternative surface water drainage solutions.
Frequently Asked Questions
Do I need a drainage survey for a small rear extension?
Yes, in most cases. Even small extensions can be built over or near existing drains. Building control will typically require evidence that drainage has been considered. A basic CCTV survey costs around £250-£350 and could save thousands if issues are discovered later.
How long does a Thames Water Build Over agreement take?
Standard applications take 4-6 weeks from submission. However, if the sewer is in poor condition or the proposed works are complex, Thames Water may require additional information or impose conditions that extend this timeline. Allow 8-12 weeks in your project planning.
What happens if I build without a Build Over agreement?
Building without required permissions is illegal and can result in Thames Water requiring the structure to be removed at your expense. It also creates problems when selling the property, as solicitors will identify the missing documentation during conveyancing.
Can my architect handle drainage design?
While architects understand drainage principles, most recommend using specialist drainage consultants for detailed design and Build Over applications. Drainage consultants have the equipment for surveys and established relationships with Thames Water that streamline approvals.
What is a soakaway and do I need one?
A soakaway is an underground chamber that allows rainwater to gradually disperse into the ground. You may need one if your extension increases roof area and the existing drainage cannot handle additional surface water. Soakaway viability depends on soil permeability and groundwater levels—percolation testing determines suitability.
Planning an Extension in London?
Check your property's planning constraints including flood zones and drainage requirements with our instant property report.
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