Drainage surveyed first — not discovered halfway through your project.
Drainage connections, soakaways, foundation work, underpinning and basement tanking across East London and the Home Counties. BS EN 752 compliant, Building Regs Part H, Thames Water approved.
Most builders discover drainage problems after they've started. We survey first.
Drainage and groundworks are the most common source of project overruns in residential construction. A builder prices an extension, starts digging, and finds a drain in the wrong place, a build-over agreement that was never obtained, or ground conditions that do not match expectations. The client absorbs the cost. This is not bad luck — it is the result of not investigating before pricing.
RCB carries out a drainage survey before any groundworks project is priced. We identify drain positions, depths, and conditions. We check for existing build-over agreements with Thames Water or the relevant utility. We assess ground conditions and flag any risk of tree root interference or contaminated ground. The price you receive reflects reality — not optimism.
All drainage work complies with BS EN 752 (Drain and sewer systems outside buildings) and Building Regulations Part H. Connections to public sewers are carried out under the correct Thames Water or utility approval. Foundation and underpinning work is designed by a structural engineer and signed off by building control.
What's included
Properly designed drainage eliminates subsidence risk, satisfies building control, prevents damp penetration, and protects your foundations for the life of the building
How It Works
The RCB process for this service.
Drainage Survey
Before any price is produced, we carry out a drainage survey — identifying drain positions, depths, flow directions, and conditions. Where required, CCTV inspection is used. Build-over requirements are identified and Thames Water records checked.
Fixed Quote & Approvals
A fully itemised quote covering excavation, drainage works, foundation construction, and reinstatement. Any required Thames Water build-over agreement or utility notifications are identified upfront and managed by RCB.
Groundworks & Drainage
Excavation and drainage works carried out in sequence — drainage first, then foundation construction. Building control inspections called at the correct stages. No guesswork, no rework.
Inspection & Sign-Off
Drainage tested under water or air pressure before backfilling. Foundation inspections passed before structure proceeds. Full building control sign-off on completion. As-built drainage drawings provided.
Planning & Regulations
Drainage work that connects to the public sewer requires approval under the Water Industry Act 1991. Building over or near a public sewer requires a build-over agreement with Thames Water (or the relevant utility) before work begins — failure to obtain this can result in a requirement to expose and potentially remove the work at the owner's cost. All drainage design must comply with BS EN 752 (Drain and sewer systems outside buildings) and Building Regulations Part H (Drainage and Waste Disposal). Soakaway design must follow BRE Digest 365 and is subject to percolation testing. Underpinning and foundation works require a structural engineer's design and building control approval at every stage. Basement waterproofing design should follow BS 8102 and be carried out by a contractor with BS 8102 knowledge.
FAQs
Common questions.
What is a sewer build-over agreement and do I need one?
A build-over agreement is required from Thames Water (or your local water company) before constructing over or within 3 metres of a public sewer. Without one, you cannot legally build — and any structure built without approval may need to be removed at your expense. RCB checks for public sewer positions before pricing any extension or groundworks project and manages the application process.
How do I know if I have drainage problems before starting a project?
The most reliable method is a CCTV drain survey — a camera is passed through your drainage system to identify cracks, root intrusion, blockages, incorrect falls, or connections to unexpected drain runs. RCB recommends a survey before pricing any extension, loft conversion, or basement project. The cost is typically £200–£500 and almost always prevents much larger problems.
What is the difference between a soakaway and a French drain?
A soakaway is a pit filled with rubble or a plastic crate system that allows surface water to percolate into the ground. It is used for roof water and surface water where ground conditions permit. A French drain is a perforated pipe in a gravel-filled trench that redirects water away from a structure — typically used for waterlogged ground, retaining wall drainage, or preventing damp at foundations. RCB will carry out a percolation test to determine which system is appropriate for your ground conditions.
How deep do foundations need to be for an extension in London?
London clay is notoriously prone to shrinkage and swelling — foundations in clay soil typically need to be 900mm to 1.2m deep, and in areas with mature trees can need to be 1.5m to 3m deep (via traditional strip or pile and beam methods). Building control will specify the required depth based on a soil investigation. RCB coordinates this with a structural engineer and building control from the outset.
What is basement tanking and when is it needed?
Basement tanking is the application of a waterproofing system to prevent water ingress below ground level. It is required for any habitable basement room or any below-ground space used for storage where water penetration would be damaging. There are three approaches: Type A (barrier protection — tanking), Type B (structurally integral concrete), and Type C (drained cavity system). RCB will specify the correct approach based on your ground conditions and use.
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