Drainage Engineers in the UK

Browse 185 verified drainage engineers across the UK. Get quotes for Drainage Strategy, SuDS Design, Drainage Layout and more.

185 verified drainage engineers across the UK on The Planning Review.

What does a drainage engineer do?

A drainage engineer designs the surface water and foul water systems that a development needs in order to satisfy planning policy, the lead local flood authority (LLFA), and the water company. In the UK planning system, applicants must demonstrate that their development will not increase flood risk on or off site and that adequate drainage capacity exists. The drainage engineer provides this technical evidence, preparing the drainage strategy and sustainable drainage system (SuDS) design that LPAs require before they will grant permission for most developments.

For surface water, the engineer designs SuDS features (such as permeable paving, swales, rain gardens, attenuation tanks, and detention basins) that manage runoff at source rather than piping it directly into the sewer network. Since the Flood and Water Management Act 2010, LLFAs are statutory consultees on surface water drainage for major applications, and they expect discharge rates to be restricted to greenfield runoff rates for all storm events up to and including the 1 in 100 year event plus a climate change allowance. Infiltration testing is usually required to determine whether the ground can absorb surface water or whether attenuation and controlled discharge are needed instead.

On the foul side, the engineer assesses whether the existing sewer network has capacity for the new development or whether upgrades, pumping stations, or private treatment plants are required. Where new drainage infrastructure will be adopted by the water company, the engineer prepares the technical design to the required standard and manages the Section 104 adoption application. Early instruction is important because ground investigation results and LLFA requirements frequently influence the site layout and housing mix.

When do you need a drainage engineer?

  • The development is a major application (10 or more dwellings or over 1,000 sqm) and the LLFA is a statutory consultee on surface water drainage
  • The LPA's validation checklist requires a drainage strategy or surface water management plan
  • The site is in Flood Zone 2 or 3 and a flood risk assessment with drainage design is required
  • You are developing a greenfield site and need to demonstrate that post-development runoff rates do not exceed greenfield rates
  • The development involves significant new impermeable areas (roofs, roads, car parks, driveways)
  • There is no available public sewer connection and an alternative foul drainage solution is needed
  • The water company or LLFA has objected to the drainage proposals on a current application
  • A planning condition requires a detailed drainage design to be approved before construction commences
  • The development is in Wales and SAB (SuDS Approving Body) approval is required under Schedule 3 of the Flood and Water Management Act 2010

Services you can get local quotes for

  • Drainage Strategy & SuDS Design
  • Flood Risk Assessment (FRA)

Frequently asked questions

How much does a drainage strategy cost?

For a small residential development of up to 10 units, a drainage strategy with SuDS design typically costs between £2,000 and £5,000. For medium-sized schemes (10 to 50 units), fees of £4,000 to £10,000 are common, particularly where infiltration testing, hydraulic modelling, or complex SuDS design is required. Large-scale residential or commercial developments may require drainage engineering fees of £10,000 to £25,000 or more, especially where Section 104 adoption applications and detailed engineering design are included. Ground investigation costs (trial pits and infiltration testing) are usually an additional £1,000 to £3,000.

How long does the process take?

A drainage strategy for a standard residential development can be prepared within 3 to 6 weeks, provided ground investigation data (infiltration test results and groundwater levels) is available. Ground investigation itself may take 2 to 4 weeks to commission and complete. The LLFA typically responds to consultation within 21 days of the planning application being validated, but may request amendments that require further design work and re-consultation. Section 104 adoption applications to water companies can take 3 to 6 months to process. Overall, from initial instruction to an agreed drainage strategy, the process may take 2 to 4 months for a standard scheme.

What are SuDS and why are they required?

Sustainable Drainage Systems (SuDS) are drainage solutions designed to manage surface water runoff in a way that mimics natural drainage processes. Instead of piping rainwater directly into the sewer network, SuDS use features such as permeable paving, swales, rain gardens, ponds, and underground attenuation tanks to slow the flow of water, reduce peak discharge rates, improve water quality, and provide biodiversity and amenity benefits. SuDS are required because conventional piped drainage systems contribute to downstream flooding, combined sewer overflows, and pollution of watercourses. National and local planning policy requires SuDS to be incorporated into all major developments, and many LPAs expect SuDS on smaller schemes as well.

Do I need infiltration testing?

In most cases, yes. Infiltration testing (carried out in accordance with BRE Digest 365) is needed to determine whether the ground conditions on your site allow surface water to soak away. If infiltration rates are sufficient, the drainage strategy can be based on soakaways or infiltration basins, which are the preferred option in the SuDS hierarchy. If the ground is impermeable (heavy clay, high water table, or contaminated land), the drainage design will need to use attenuation and controlled discharge to a watercourse or surface water sewer instead. LLFAs will typically require evidence of infiltration testing before they will approve a drainage strategy, even if the geological mapping suggests infiltration is unlikely.

What is a Section 104 agreement?

A Section 104 agreement is a legal agreement between a developer and the sewerage undertaker (water company) under Section 104 of the Water Industry Act 1991. It provides for the adoption of new sewers and drainage infrastructure built by the developer, so that they become part of the public sewer network maintained by the water company. This protects future homeowners from being responsible for the maintenance of shared drainage systems. The water company will require the drainage design to meet their technical standards (typically Sewers for Adoption or the Design and Construction Guidance), and they will inspect the works during construction before formally adopting the infrastructure.

Legal and regulatory framework

  • Flood and Water Management Act 2010
  • NPPF
  • Planning Practice Guidance (Flood Risk and Coastal Change)
  • Building Regulations Approved Document Part H
  • CIRIA C753 (The SuDS Manual)
  • Water Industry Act 1991 (Sections 104 and 106)
  • Non-Statutory Technical Standards for Sustainable Drainage (Defra, 2015)
  • Environment Agency guidance on climate change allowances

Professional accreditations

  • Chartered Institution of Water and Environmental Management
  • Engineering Council
  • Institution of Civil Engineers
  • Society for the Environment

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