Civil Engineering: Drainage, SuDS and Flood Risk

Kelp Studio are civil and structural engineers working across London, Surrey, Sussex, Kent and Essex. Our civil engineering work covers everything below ground and around your building: drainage, sustainable drainage (SuDS), flood risk and the agreements you need with the water company and the council. Because we also design the structure, foundations, retaining walls and drainage are coordinated from the start.

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Our civil engineering services

Drainage strategies and SuDS design
Surface water drainage strategies for planning, runoff and attenuation calculations, and SuDS such as soakaways, permeable paving, attenuation tanks and green roofs.

Flood risk assessments
Site-specific flood risk assessments for planning, prepared alongside the drainage strategy so both tell the same story.

Below-ground drainage design
Foul and surface water drainage layouts, levels and details for extensions, new homes and developments.

Soakaway design
Soakaways sized from BRE 365 infiltration test results, with calculations, drawings and a specification for Building Control. We arrange the testing through a specialist we regularly use. Soakaway design →

Water company agreements
Sewer connections (Section 106), sewer adoption (Section 104), sewer diversions (Section 185) and build-over agreements.

Flood and drainage scheme feasibility studies
Early studies that look at the flood risk, drainage constraints and options for a site or area, so you can choose the right scheme before committing to detailed design.

Retrofitting SuDS in towns and cities
Adding sustainable drainage to existing streets, estates, schools and public spaces, such as rain gardens, tree pits, permeable paving, green roofs and disconnecting downpipes from the sewer, to reduce local flooding and pressure on the sewers.

Off-mains drainage
Septic tanks, package treatment plants and drainage fields for sites with no mains sewer, checked against the Environment Agency’s rules for small sewage discharges. We specify the system and coordinate with the manufacturer, who designs and installs it.

Pumping stations and petrol interceptors
Specified where drains can’t reach the sewer by gravity, and for parking areas and yards where oil could get into the drainage.

Utility searches
Locating existing sewers, water mains and services before design and construction starts.

Discharging planning conditions
Applications to discharge pre-commencement drainage and flood risk conditions, and responses to the council and Lead Local Flood Authority.

Drainage requirements by area
Each area has its own flood authority and drainage rules. See what applies in London, Sussex · Brighton & Hove, Kent and Surrey.

Building new homes? Structural and civil engineering for new builds →

Who we work with
Developers, architects, planning consultants, builders, self-builders and homeowners, from single extensions to new housing developments.

Recent civil and structural work
• Meads Court, Bexhill-on-Sea: structural and below-ground drainage design to convert a disused commercial unit into shops and flats, with a new timber-frame storey on top.
• Downey Close, Hastings: a new townhouse development on a brownfield site in Hollington.
• Combermere Road, Hastings: structural design and below-ground drainage for a new-build house, plus help discharging the planning conditions.
• Rotherfield Close, Brighton: new-build house on an infill site, with below-ground drainage and a Section 106 sewer connection application to Southern Water.
• Woodlands Park Drive, Great Dunmow: planning application, structural engineering and below-ground drainage design for a rear extension.
• Twining Avenue, Twickenham: structural and below-ground drainage design for a rear extension, for a local developer.
See more of our projects →

Flood Re: help with home flood insurance
Flood Re is a joint initiative between the Government and insurers that makes flood cover more affordable for homes at higher risk of flooding. It applies to most homes built before 2009. Its Build Back Better scheme can provide up to £10,000 towards flood resilience measures as part of an eligible insurance claim, and Flood Re is developing Flood Performance Certificates, planned to give lower premiums to homes with resilience measures in place.
We can assess your property’s flood risk and design the drainage and SuDS that help reduce it. Eligibility and cover are decided by Flood Re and your insurer. Visit Flood Re →

Common questions

What’s the difference between a civil and a structural engineer?
A structural engineer designs the parts of a building that carry load, such as beams, walls and foundations. A civil engineer deals with the infrastructure around it, such as drainage, flood risk and connections to sewers. We do both, so you have one point of contact.

When do I need a civil engineer?
Usually when your planning application needs a drainage strategy or flood risk assessment, when a drainage condition must be discharged before work starts, or when you need to connect to, divert or build over a public sewer.

Do you work on small projects?
Yes. Alongside developments, we regularly help homeowners with soakaways, drainage for extensions and build-over agreements. SuDS for homeowners →

12+ years’ experience · £1,000,000 PI insurance

Our structural design services →
Is my property at risk of flooding? →
SuDS explained: a plain-English glossary →