A drainage design engineer designs the foul water and surface water drainage systems for new buildings and development sites - from individual house extensions to large residential estates. They work to Building Regulations Approved Document H, the sewerage undertaker's sewer adoption guide (Sewers for Adoption) and, for surface water, sustainable drainage standards. On housing developments, the drainage engineer typically prepares a drainage layout for planning, develops the detailed design for Building Regulations and manages the Section 104 sewer adoption agreement with the water company.
What they do
- Design foul water drainage systems to Building Regulations Part H
- Design surface water drainage to adoptable sewer standards
- Prepare drainage layouts and hydraulic calculations
- Size pipework, manholes and inspection chambers
- Manage Section 104 sewer adoption applications with the sewerage undertaker
- Design septic tanks and package treatment plants for off-mains drainage
- Carry out drainage impact assessments on existing sewer networks
- Prepare build-over agreement applications for work near public sewers
When you need one
A drainage design engineer is needed on any new development where drainage system design is required - from a single house with a septic tank to a large housing estate with adoptable sewers.
Typical costs (UK, 2025)
| Job type | Typical cost |
|---|---|
| Drainage layout for planning (small site) | £600 - £1,500 |
| Full drainage design for Building Regs | £1,200 - £4,000 |
| Section 104 adoption agreement management | £1,500 - £5,000 |
| Septic tank or treatment plant design | £400 - £1,200 |
| Build-over agreement application | £400 - £900 |
| Drainage impact assessment | £800 - £2,500 |
| Hourly rate | £75 - £140 / hr |
Sewerage undertaker Section 104 agreement fees and requisition costs are charged separately by the water company. These can be significant - typically £5,000-£50,000+ depending on the extent of new public sewers being adopted.
Qualifications to look for
MCIWEM is the primary qualification for drainage engineers. CIWEM members with drainage expertise have demonstrated competence in water management, drainage design and sewer adoption. The CIWEM member directory lists qualified drainage engineers by location.
Many drainage engineers hold CEng MICE rather than or alongside MCIWEM, particularly those working on infrastructure projects. ICE members can be found through the ice.org.uk member directory.
How to find a drainage design engineer
Frequently asked questions
What is a Section 104 agreement?
A Section 104 agreement under the Water Industry Act 1991 is the legal mechanism by which a sewerage undertaker (water company) agrees to adopt the foul and surface water sewers on a development as public sewers after construction. The developer's drainage engineer prepares the technical design to the sewerage undertaker's standard and submits it for approval. Following construction and satisfactory inspection, the sewers are adopted and the developer is discharged from their maintenance obligation.
What does Building Regulations Part H cover?
Approved Document H to the Building Regulations covers drainage and waste disposal for buildings. It includes requirements for foul water drainage (H1), wastewater treatment systems (H2), rainwater drainage (H3), building over sewers (H4), separate systems of drainage (H5) and solid waste storage (H6). A drainage design engineer prepares drainage designs that demonstrate compliance with Part H as part of the Building Regulations application.
What is a drainage impact assessment?
A drainage impact assessment (also called a sewerage impact assessment or sewer capacity assessment) evaluates whether the existing public sewer network has sufficient capacity to accept foul or surface water flows from a proposed development. Required by the sewerage undertaker where existing sewers are known to be at or near capacity. If capacity is insufficient, the developer may need to contribute to reinforcement works or provide on-site attenuation to limit flows to the sewer.