You Ask, We Answer

We’re technical experts in the complex and ever-changing world of civil and structural engineering design and technical approval.

Where can I drain my site to?

Our answer is based on the Link team’s collective experience, which includes involvement in hundreds of projects of varying scales from inception through to construction on site, across the UK over the past 15 years.

Our simple answer is: To the most suitable outfall location.

As part of this answer we shall consider foul and surface water separately.

Surface Water

To begin to explore the answer to this question we must first consider the surface water drainage outfall hierarchy, as set out in Section 3 of Part H of Building Regulations and Paragraph 7-080, Flood Risk and Coastal Change, Planning Practice Guidance. This sets out the order of preference of the various outfall options that should be considered when selecting a suitable outfall for the surface water from a proposed development.

1. Infiltration to ground;

2. Discharge to a watercourse;

3. Discharge to a surface water sewer/highway drain;

4. Discharge to a combined sewer.

In order to infiltrate the surface water to ground the ground conditions need to be suitable, typically granular gravels and sands. A preliminary indication of the underlying strata can be obtained by visiting the British Geological Survey (BGS) website. The information on this website shall give an indication as to whether the ground conditions shall be cohesive or granular. Should the underlying strata be found to be granular then infiltration testing should be undertaken in line with BRE365 to determine a design infiltration rate for the site. It is common for the Lead Local Flood Authority (LLFA) to request infiltration testing to confirm whether a suitable infiltration rate can be achieved before this outfall option can be discounted. Where a development site is wholly dependent on the ability to drain surface water to the ground, the infiltration rate available at the site should be obtained to inform an outline planning application. If there are other available options including infiltration to ground, then the test to prove the suitability can be provided as a condition on the development.

Once it has been determined that a surface water drainage network that utilises infiltration would not be feasible, the next outfall option is to discharge the surface water to a watercourse or land ditch. Typically most greenfield sites are drained informally by a network of ditches and as such a site walkover or topographical survey can identify these features. A surface water drainage network can then be designed to discharge to a watercourse at a restricted rate with attenuation provided.

Should no watercourses be present in the vicinity of the site, the next suitable outfall is to a surface water sewer. To establish the location of any surface water sewers in the vicinity of the site a developer enquiry application should be made to the local water company. The response to this shall typically provide the sewer records for the area around your site, as well as confirmation of the capacity of these sewers to accept the flows from the proposed development. Should a surface water sewer not be available either then an outfall to a combined sewer should be explored with the local water company, in this case early engagement with them is advised as it is not the preferred solution when it comes to discharging surface water and should be considered as a last resort.

Foul Water

The foul flows from a development should be discharged to an adoptable combined or foul sewer wherever possible. It should be noted that under the Water Industry Act (1991) the local water company is obligated to provide an outfall for a proposed development subject to reasonable costs and timescales. As such should there not be a suitable sewer in the vicinity of the site a sewer requisition can be utilised to bring a sewer to the site. A water company would typically prefer a sewer requisition over a pumped solution in order to mitigate the long term maintenance liability, subject to the upfront cost of the requisition. It should be noted that the sewer requisition shall be constructed by the water company’s appointed contractor though the cost for the works shall need to be covered by the developer.

Should the site be a minor development, in the absence of any adoptable sewers, a suitable foul treatment works could be considered as an alternative, with the treated effluent either discharging to a drainage field or watercourse. This mode of foul water disposal is covered by The General Binding Rules, above which a permit shall need to be sought from the Environment Agency. In order to comply with the general binding rules the proposals must comply with the following criteria:

  • Sewage should be domestic in nature;
  • Not cause pollution;
  • Not be more than 2m3 when discharging to ground or 5m3 when discharging to a watercourse.

It should be noted that other criteria may impact the feasibility of a package treatment works such as, if the development proposals are to discharge to an aquifer, well or borehole and whether the site is located in a groundwater source protection zone.

If you are looking at a new site and have a concern about how you shall drain the proposed development, please contact us at and we would be more than happy to provide you with some site specific advice.

Apr 19, 2022