Separating surface water from a combined sewer
Taking roof and yard drainage out of a combined sewer is often the only betterment available on a tight urban site. It starts with dye testing, not drawings.
Sewerage undertakers and lead local flood authorities both want surface water taken out of combined sewers, and on a redevelopment that gives you something to offer. Surface water separation from a combined sewer means disconnecting roof and yard drainage from the pipe that also carries foul flow, and discharging it to a soakaway, a watercourse, a separate surface water sewer or an attenuated connection instead. Where the existing site drains everything to one combined pipe, removing that flow is a measurable betterment, and it is often the argument that gets a constrained site through drainage consultation.
It is also one of the few drainage improvements you can deliver on a tight urban plot without space for a basin.
Why undertakers press for it
A combined sewer carries foul and surface water in the same pipe. In heavy rain the combined flow exceeds capacity, and the system relieves itself through overflows or through flooding at the lowest point, which is usually somebody's basement. Every square metre of roof disconnected is flow that never enters the pipe.
Undertakers therefore treat surface water removal as capacity creation. On a redevelopment where the proposed foul flow is higher than the existing, offering surface water separation is the trade that makes the capacity enquiry work. The published surface water risk mapping is a useful cross-check on where the pressure is, although it has to be read with the care set out in reading the surface water flood risk map without overstating it.
The national drainage hierarchy also points this way. Discharge to ground first, then to a surface water body, then to a surface water sewer, with a combined sewer last. A scheme that keeps its surface water in the combined sewer has to explain why the three options above it were rejected.
Finding the existing connections
You cannot offer to separate what you have not found. This is survey work, and on an older site it takes longer than anyone allows for.
- Obtain the undertaker's sewer records for the site and the surrounding highway, and note which pipes are recorded as combined.
- Commission a drainage survey with closed circuit television and manhole schedules, covering every chamber within the site and the first chamber in the highway.
- Dye test or trace each downpipe and gully to establish what it actually connects to, because records and reality diverge on sites that have been altered.
- Record invert levels and pipe sizes, so the receiving option can be checked for gravity fall.
- Mark private drains, shared drains and public sewers separately on the survey drawing.
- Identify any connection crossing third party land, and check whether an easement exists.
The dye testing is the step people skip and the step that changes schemes. It is common to find that half a roof already drains to a surface water sewer nobody knew about, or that a yard gully believed to be surface water discharges to the foul line.
The options when there is no obvious outfall
Order the options the way the hierarchy does, and record why each was accepted or rejected.
| Option | What it needs | Common obstacle |
|---|---|---|
| Infiltration to ground | Permeable strata, depth to groundwater, infiltration testing | Clay subsoil, contamination, shallow water table |
| Discharge to a watercourse | A hydraulic connection and consent for the outfall | Distance, third party land, consent timescales |
| Separate surface water sewer | An existing separate sewer with capacity | Rare in older areas |
| Attenuated discharge to the combined sewer | Agreed discharge rate and storage volume | Still uses combined capacity |
| Rainwater harvesting and reuse | Demand that matches supply, tank space, maintenance | Only reduces volume, does not remove the peak |
Infiltration needs testing rather than assumption, and the testing has to be done at the depth and location of the proposed feature. Where ground conditions or contamination rule it out, say which test showed that and give the figures.
Where the receiving route is an ordinary watercourse, the outfall works need land drainage consent from the lead local flood authority or the internal drainage board, and that consent runs on its own timetable alongside the planning application.
Rainwater harvesting deserves one honest caveat. It reduces the volume discharged across a year and it can be counted towards water efficiency, but a tank that is already full when the storm arrives does nothing for the peak. Present it as a water resource measure with a secondary drainage benefit, not as attenuation.
Quantifying the betterment
The betterment claim has to be numerical, and it has to compare like with like.
State the existing impermeable area draining to the combined sewer, in square metres, taken from the survey rather than from the title plan. State the proposed impermeable area and where each part discharges. Then state the existing and proposed discharge rates to the combined sewer, with the method and the return periods used.
Where the existing site is wholly combined and the proposal removes all surface water, the reduction is simply the whole of the existing surface water flow. Where some remains, give the percentage reduction in area and the reduction in peak rate separately, because they are different numbers and officers conflate them.
An illustrative example. A former workshop with 640 square metres of roof and yard, all draining to a combined sewer, is redeveloped as eight flats. Infiltration testing shows the made ground is unsuitable, but a separate surface water sewer runs in the side street. Roof water from 410 square metres is connected to it through a small attenuation tank, 120 square metres of yard becomes permeable paving over a lined sub-base discharging to the same tank, and only the remaining 110 square metres of covered bin and cycle store area stays on the combined line. The strategy presents the area reduction and the peak rate reduction as two separate figures, each with its calculation appended.
Securing it, and keeping it
Separation is worth nothing if it is not built and maintained, so expect conditions.
The conditions that work require the drainage to be constructed in accordance with approved details before occupation, require a verification report confirming what was actually built with photographs and a post-construction survey, and require the maintenance arrangements to be submitted and approved. Ask for the verification condition rather than resisting it. It is the document that protects you on sale.
Where any part of the separated system crosses or sits near a public sewer, the undertaker's build over or build near agreement is a separate consent, and it governs foundation design as well as drainage. On a redevelopment it is sensible to resolve that at the same time as the capacity enquiry.
The Drainage Strategy should carry the survey, the hierarchy assessment, the calculations and the maintenance plan in one document, and it should cross-reference the finished floor levels and exceedance routes used in the Flood Risk Assessment so the two do not contradict each other.
And get the survey done before the layout is fixed. A scheme designed around where the water can actually go costs less than a scheme redesigned after the drainage consultee responds.
This is general guidance, not legal advice; check the position with your local planning authority, the sewerage undertaker or a qualified professional before relying on it. PlanScope produces AI-assisted draft reports for planning applications, and reports should be reviewed before submission. For specialist technical reports, qualified professional input may be required. PlanScope does not guarantee planning approval.
Frequently asked questions
Why do water companies want surface water removed from combined sewers?
Because a combined sewer carries foul and surface water in one pipe, so rainfall adds to the flow and the system relieves itself through overflows or by flooding at the lowest point. Every square metre of roof disconnected is capacity created. On redevelopments with higher foul flows, separation is often what makes the capacity enquiry work.
What survey work is needed before offering separation?
Obtain the undertaker's sewer records, then commission a closed circuit television drainage survey with manhole schedules covering every chamber on site and the first in the highway. Dye test or trace each downpipe and gully, because records and reality diverge on altered sites. Record invert levels and pipe sizes so gravity fall can be checked.
Does rainwater harvesting count as attenuation?
Not reliably. A harvesting tank that is already full when a storm arrives provides no attenuation, so it reduces annual volume rather than the peak rate. Present it as a water efficiency measure with a secondary drainage benefit, and provide separate storage for attenuation if the discharge rate has to be controlled.
How is the betterment quantified?
State the existing impermeable area draining to the combined sewer in square metres, taken from the survey, then the proposed area and where each part discharges. Give the existing and proposed peak discharge rates to the combined sewer with the method and return periods. Report the area reduction and the rate reduction separately.
What conditions should I expect?
Construction in accordance with approved drainage details before occupation, a verification report confirming what was built with photographs and a post-construction survey, and approval of the maintenance arrangements. The verification condition is worth asking for, because it is the document that evidences compliance when the site is later sold.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.