Drainage and flood risk in London
CCTV drainage surveys, drainage design, SuDS and flood risk assessments, and Thames Water build-over applications.
About these services
Drainage is easy to overlook until it holds up a project. This group covers the full range, from CCTV surveys of existing drains to drainage strategies for planning, build-over applications, soakaway testing, flood risk assessments and sewer adoption submissions. It also includes repair specifications when a survey finds cracked or root-damaged pipes, which are a common hidden cause of localised subsidence on clay soils.
Homeowners planning an extension often discover that a public sewer runs under the footprint and needs Thames Water's agreement. Developers need foul and surface water strategies, and often a sustainable drainage strategy, to satisfy planning requirements. Buyers and landlords use CCTV surveys to check drains before purchase or after a blockage. Architects need flood risk assessments when a site sits within a mapped flood zone, even for small schemes.
London's mix of combined and separate sewers, its slow-draining clay soils and its tight urban sites shape every drainage design. Soakaways rarely work on London Clay, so surface water often needs attenuation and a controlled discharge instead. Near the Thames and its tributaries, in boroughs such as Richmond upon Thames, Hounslow and Newham, flood risk becomes a planning question too. We design drainage that works on the ground you actually have.
Services in this group
- CCTV drainage survey and reportGCE-DR-001from £250
- Drainage survey technical reviewGCE-DR-002from £300
- Build over or near sewer technical packageGCE-DR-012from £850
- Foul drainage strategyGCE-DR-004from £600
- Surface water drainage strategyGCE-DR-005from £700
- Sustainable drainage (SuDS) strategyGCE-DR-006from £850
- Soakaway and infiltration testing reportGCE-DR-019from £600
- Drainage maintenance and management planGCE-DR-016from £450
- Minor development flood risk assessmentGCE-DR-013from £450
- Flood risk assessment (major development)GCE-DR-017from £1,500
- Sequential and exception test reportGCE-DR-018from £500
- Sewer adoption technical submissionGCE-DR-020from £2,500
- Drain repair specification and remedial worksGCE-DR-022from £350
Which drainage or flood report do you need?
If you are building an extension and a sewer runs close to or under it, start with a CCTV drainage survey to confirm the layout and condition, then the build over or near sewer technical package for the water company application. Work over or within about three metres of a public sewer generally needs approval before you build, and Thames Water will confirm what applies to your site.
For planning applications, a surface water drainage strategy or a fuller SuDS strategy is often a validation or condition requirement. Where the site lies in a flood zone, a minor development flood risk assessment covers extensions and small conversions, while larger schemes need the full version. Our guide on building over a public sewer explains the build-over process step by step.
Projects using these services
Frequently asked questions
How do I know if there is a public sewer under my extension?
Sewer records show the approximate line of public sewers, but they are not always accurate, particularly for older shared drains behind terraces. A CCTV survey traces the actual route and depth. Since 2011 many private shared drains have become public sewers, so a pipe you assumed was yours may now need water company approval to build over.
Will a soakaway work in my garden?
On London Clay, often not, because the clay drains too slowly to accept water from a roof. On sand, gravel or chalk it may work well. The only reliable way to know is an infiltration test in a trial pit on site. If the test fails, rainwater is usually stored and released slowly into the sewer instead.
Does a small extension need a flood risk assessment?
It may, if the site lies within a mapped flood zone or an area the council has identified for surface water flooding. Councils generally require a proportionate assessment with the planning application. For a minor development this covers the flood source, floor levels, flood-resistant construction and safe access. The council's validation list will confirm whether one is needed.
What is a SuDS strategy and when is it required?
Sustainable drainage manages rainfall close to where it falls, using features such as permeable paving, green roofs, rain gardens and attenuation tanks, rather than sending it straight to the sewer. London planning policy expects it on most new developments, and many councils attach drainage conditions to smaller schemes. The strategy sets out the features, flow rates and maintenance needed.