Separating foul and surface water on a conversion

Separating foul and surface water on a conversion is expected by almost every local planning authority and by every sewerage undertaker, and on a Victorian building served by a single combined drain it is often only partly achievable. The sequence that works is to survey what is there, establish what the receiving sewer actually is, then separate as much as the site allows and explain the rest. Designing a theoretical separated system before anyone has lifted a manhole cover wastes money.

The expectation comes from two directions. Approved Document H of the Building Regulations sets a hierarchy for surface water disposal, to a soakaway first, then a watercourse, then a sewer. Undertakers resist new surface water connections to combined sewers because that water ends up at the treatment works or in a storm overflow.

Why separation is asked for on conversions

A conversion usually increases foul load and changes the roof and hard surface arrangement at the same time. Turning a house into four flats multiplies the sanitary appliances. Adding a rear extension and a bin store adds impermeable area.

If both go into the same combined drain, the conversion makes the downstream capacity problem worse on two counts. Separating the surface water removes the part that is easiest to deal with on site and the part that causes the biggest peak.

There is a water quality reason too. Combined sewers overflow in heavy rain, and every litre of clean roof water kept out of them is a litre that does not contribute. Councils increasingly reflect that in their validation requirements, and the lead local flood authority will comment on the drainage approach even on minor applications in a critical drainage area.

Survey before you design

The existing private drainage on an older building is rarely as recorded. Branches have been added, gullies have been connected to whatever was nearest, and previous extensions have built over runs that nobody can now find.

Commission a CCTV survey with a drainage plan as the output, not just a video file. The survey should establish the layout and gradients, the material and condition of each run, the position of the boundary with the public sewer or lateral drain, and whether any misconnections exist. Dye or tracing tests resolve the ambiguous connections.

Two findings change the scheme. A collapsed or heavily root-damaged run means repair or replacement is happening anyway, which is the least costly moment to separate. And a run that crosses beneath a neighbouring property, which is common in terraces, may mean the practical separation point is inside your building rather than in the yard.

Record the invert levels. Separation is a gravity problem, and a new surface water run that cannot reach its outfall without a pump is a maintenance liability for the life of the building.

Where a combined public sewer limits the benefit

Here is the part that surprises people. If the only receiving sewer is combined, separating foul and surface water within the site and then recombining them at the connection achieves very little hydraulically.

It is still worth doing, for three reasons. It future-proofs the building for the day the network is separated. It lets you deal with some of the surface water on site, through a soakaway, permeable paving or a water butt arrangement, before anything reaches the sewer. And it satisfies the undertaker and the council, who will ask for it regardless.

What it does not do is justify claiming a betterment that the hydraulics do not deliver. Establish the sewer type from the undertaker's records, which you obtain through a drainage and water enquiry, and describe the position honestly in the drainage statement. An overstated claim is the kind of thing that unravels at the discharge of conditions stage.

Partial separation and what it achieves

On most conversions the answer is partial separation, and the useful version is to take the new and the easy areas out of the combined system first.

• New roof areas from an extension or dormer to a soakaway or to a separate surface water run.
• Rear yard and parking areas to permeable paving with a sub-base reservoir.
• Existing rear roof slopes, where the downpipes are accessible and a route to a soakaway exists.
• Front roof slopes and the front yard last, because the route usually crosses under the building.

Soakaway feasibility rests on infiltration testing and on groundwater levels, and the recognised design method is set out in the BRE soakaway digest. Test before you commit, because clay subsoils and high water tables defeat infiltration more often than developers expect. Where infiltration fails on part of the site, our note on managing runoff on a steeply sloping site covers the alternatives in more detail, and the rainfall data you use to size any storage should follow the approach in our note on which rainfall dataset to cite.

Foul capacity, and the question the undertaker asks

Increasing the number of dwellings raises the foul flow, and the undertaker may need to confirm that the receiving network has capacity. A pre-development enquiry is the route, and on larger conversions the answer can involve off-site reinforcement with its own programme.

A Foul Sewage Assessment is the document that sets out the proposed foul arrangement, the estimated flow, the point of connection and the undertaker's response. On a small conversion it can be short. On a conversion of a large commercial building into flats it is the document that stops the application stalling at consultation.

Pumped foul arrangements for basement flats deserve a mention in the statement, with the pump station, its emergency storage and its maintenance responsibility identified. Councils have become more careful about these since basement conversions became common.

Connections, build over and the consents nobody budgets for

Three separate approvals sit outside the planning permission and get forgotten.

1. A connection to a public sewer, applied for under section 106 of the Water Industry Act 1991. The right to connect is not unconditional, and the undertaker can refuse on grounds including the mode of construction and the condition of the drain.
2. A build over or build near agreement where the works are over or close to a public sewer or lateral drain. Since the 2011 transfer of private sewers, many runs that look private are public, and an extension across one needs the undertaker's agreement.
3. Any discharge to a watercourse, which needs the consent of the riparian owner and may need land drainage consent from the lead local flood authority or the Environment Agency.

Each has a lead time. Build over agreements in particular can take longer than the planning application, and starting them after consent is a scheduling error rather than a prudent sequence.

What the drainage statement should conclude

Keep it short and make it answer the question. A Drainage Strategy for a conversion should state the existing arrangement as surveyed, the sewer type and point of connection confirmed by the undertaker, the proposed foul arrangement with estimated flows, the proposed surface water arrangement with the extent of separation achieved and the reason for any part not separated, infiltration test results where a soakaway is proposed, and the consents still to be obtained.

The honest conclusion on many conversions is partial separation with a stated reason. Officers accept that. What they do not accept is a statement claiming full separation on a building where the front elevation drains under the party wall to a combined sewer in the street, because the first contractor on site will find it and the discharge of conditions will fail.

This is general guidance rather than legal advice. Check the requirements with your local planning authority, your sewerage undertaker or a qualified professional before relying on it.

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