Drainage Strategies & SuDS Design in London

We prepare drainage strategies, SuDS designs and flood risk assessments for planning applications across London, and help discharge drainage conditions once permission is granted. We work for developers, self-builders, architects and homeowners, from single new homes and extensions to small developments.

Get a quote or call 020 3835 2633

What London boroughs expect
Each London borough is its own Lead Local Flood Authority, and drainage is assessed against London Plan Policy SI 13. It asks development to aim for greenfield runoff rates, manage water as close to its source as possible, and follow this drainage hierarchy:
1. Use rainwater as a resource, for example rainwater harvesting or blue roofs
2. Infiltrate rainwater into the ground at or near its source
3. Attenuate rainwater in green infrastructure, such as green roofs and rain gardens
4. Discharge directly to a watercourse
5. Controlled discharge to a surface water sewer
6. Controlled discharge to a combined sewer
The policy also resists new impermeable surfacing, including front gardens and driveways, unless it is unavoidable. Many boroughs ask for a sustainable drainage proforma with the application.

National standards
Defra’s National Standards for Sustainable Drainage, published in June 2025, now apply to new development, and drainage strategies are expected to show how each standard has been met. The revised NPPF also expects SuDS on development that could affect drainage. Read our NPPF article →

Ground conditions
Much of London sits on London Clay, which drains poorly, so attenuation is often needed, using blue and green roofs, permeable paving or storage crates with a controlled discharge. River terrace gravels, especially near the Thames, can allow infiltration, but groundwater levels need checking.

Local area pages
See structural engineers in London for our London services and borough pages.

What we provide
• Drainage strategies and SuDS design for planning applications, from single new homes to small developments
• Infiltration testing to BRE 365 and soakaway design
• Flood risk assessments where the site needs one
• Discharging pre-commencement drainage conditions, and responses to the council and the Lead Local Flood Authority
• Water company applications: sewer connections (Section 106), adoption (Section 104), diversions (Section 185) and build-over agreements
• Structural design for the same project, so the drainage and foundations work together

Our drainage work in London
• Wellesley Road, Twickenham: structural and below-ground drainage design for a rear extension, two-storey side extension and loft conversion.
• Whitton Road, Twickenham: rear extension and remodelling with new below-ground drainage.
• The Ridge, Twickenham: structural and below-ground drainage design for a two-storey rear extension.
• Vicarage Road, Twickenham: structural engineering and below-ground drainage design for a large rear extension.
See our projects →

Common questions

Do I need a drainage strategy for a small project in London?
Often, yes. Many boroughs ask for a drainage strategy or proforma for new homes, and some for larger extensions, especially in areas at risk of surface water flooding.

Can I pave my front garden?
London Plan policy resists new impermeable surfacing, including front gardens and driveways. Permeable paving, or draining the surface to a planted area, usually solves this.

Who handles sewer connections in London?
Thames Water across almost all of London. We prepare the connection and build-over applications for you. Build-over agreements explained →

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

SuDS and drainage design →
Civil engineering services →
New build structural and civil engineering →
Flood risk assessments →
Other areas: Sussex · Kent · Surrey
Get in touch or call 020 3835 2633