Discharging surface water to a public sewer: the consent route

Surface water discharge to a public sewer is the last option in the drainage hierarchy, and getting it agreed is a separate process from getting planning permission. The right to connect under section 106 of the Water Industry Act 1991 is real but qualified, the sewerage undertaker can refuse on defined grounds, and the discharge rate it will accept is a matter for negotiation rather than entitlement. Start that conversation before the layout is fixed, because the answer determines how much attenuation you have to build.

Why the sewer sits last in the hierarchy

National policy and Building Regulations Part H both set the same order of preference: infiltration to ground first, then discharge to a watercourse, then a surface water sewer, and a combined sewer last of all.

The reasoning is capacity. Combined systems carry foul flow and surface water in the same pipe, so rainfall entering them consumes capacity that treatment works and storm overflows have to absorb. Undertakers have been reducing the volume of surface water entering combined systems for years, and a proposal that adds to it starts from a defensive position.

That order has consequences for the evidence you assemble. To reach the sewer legitimately you have to show that the tiers above it were tested and failed, which means infiltration testing results rather than an assertion about clay, and a documented assessment of any watercourse within reach. Where a high water table rules out soakaways, groundwater flooding evidence is the material that makes the case. Where a watercourse exists but is culverted or in third party ownership, set out the reason it is unavailable, and note that a presumption against burying a watercourse cuts the other way, as the presumption against burying a stream explains.

The statutory right to connect, and its limits

Section 106 of the Water Industry Act 1991 gives the owner or occupier of premises the right to have their drains communicate with the public sewer. It is exercised by serving notice on the sewerage undertaker.

The undertaker may refuse, and the grounds are specific: that the mode of construction or condition of the drain or sewer does not satisfy its standards, or that the connection would be prejudicial to the sewerage system. Refusal has to be given within the statutory period after the notice, and there is a right of appeal to a magistrates' court. In practice disputes rarely reach that stage, because the negotiation is about rate and point of connection rather than about the principle.

The right is narrower for surface water than for foul flow. Legislation passed in 2010 anticipated a separate approval regime for sustainable drainage, and Schedule 3 to the Flood and Water Management Act 2010 has still not been brought into force in England, so lead local flood authorities exercise influence as consultees rather than as approving bodies. Check whether it is in force when you write a strategy, because implementation has been announced and deferred more than once.

What that means practically: the undertaker sets conditions through its own connection process and through the planning condition it asks the authority to impose, rather than through a statutory approval of the drainage design.

What the undertaker asks for before it agrees

Make a pre-development enquiry early, and give the undertaker enough to answer properly. A thin enquiry produces a holding response that helps nobody.

1. A site location plan with the red line boundary and the proposed connection point identified by manhole reference from the sewer records.
2. Existing and proposed impermeable area schedules in m², separating roofs, roads, parking and paving.
3. The existing discharge arrangement, including any historic connections, with evidence for the pre-development runoff rate on a previously developed site.
4. Infiltration testing results and a statement of why infiltration is not viable, plus the watercourse assessment.
5. The proposed discharge rate and the storage volume that supports it, with the design events and climate change allowance stated.
6. Foul flow figures, since most enquiries are answered for foul and surface water together and a Foul Sewage Assessment covers the other half of the question.
7. Phasing, where the connection is needed before the whole site is built.

Ask for three things in the response: capacity confirmation at the proposed point, the maximum discharge rate the undertaker will accept, and whether any off-site works, diversion or build-over consent will be required. Get them in writing, because the planning submission will be read against them.

Agreeing a discharge rate and getting it conditioned

The rate is the commercial heart of the matter, because every litre per second you lose has to be replaced with storage volume, and storage costs land.

On greenfield sites the starting point is the greenfield runoff rate calculated by an accepted method. On previously developed sites, national standards ask for a rate as close to greenfield as reasonably practicable, and undertakers commonly require a stated reduction against the existing rate rather than full greenfield equivalence. The size of that reduction varies between companies and between catchments, so obtain the figure rather than assuming a percentage from another scheme.

Three practical points shape the negotiation. A minimum practicable orifice size limits how low the rate can go before the control becomes a blockage risk, and a vortex control or pumped solution may be needed at very low rates. Phasing matters, because an interim rate during construction is often available where the final rate is not. And betterment has value: offering to remove an existing unattenuated connection elsewhere on the site is sometimes worth more to the undertaker than a marginal reduction in the new rate.

The planning condition then has to match the agreement. Conditions that require the rate to be agreed with the undertaker before work starts are common and workable, but a condition fixing a numerical rate that the undertaker has not accepted creates a conflict you will have to resolve by a section 73 application later. Check the wording before the decision is issued.

Adoption, diversion and building over a sewer

Three other parts of the Act come into play on most sites of any size.

Section 104 covers adoption of new sewers built by a developer, under the current code for adoption. Design to the adoption standards from the outset, because retrofitting them to a private layout is expensive, and unadopted drainage becomes a management company liability that affects sales.

Section 185 covers diversion of an existing public sewer. If a sewer crosses the developable area, diversion is a long lead item with its own design approval, easements and cost, and it should be priced and programmed at acquisition rather than discovered at technical design.

Building over or close to a public sewer needs the undertaker's agreement, usually through a build-over process with standard requirements on foundation type, clearances and access to manholes. A layout with a building sitting over an unmapped sewer is the classic reason a scheme has to be redrawn after permission, so get the asset records and, where the records are old, a survey.

Where several utilities interact on a constrained site, a Utilities Assessment that sets out capacity, connection points and diversion requirements together prevents the sequencing problems that show up on site.

Timescales and what they do to a programme

The statutory notice period for a connection is short. Everything around it is not.

A pre-development enquiry response takes weeks. A capacity assessment requiring modelling takes longer. Section 104 adoption agreements, section 185 diversions and any off-site reinforcement work run to months, and they involve legal agreements, easements from third parties and, occasionally, their own planning permissions.

So the order of work matters more than the length of any single step. Enquiry at feasibility stage, before the layout is fixed. Rate agreed in principle before the drainage strategy is finalised. Diversion or build-over identified before the site plan is drawn. Formal applications submitted once permission is granted, with lead times built into the programme rather than assumed away.

What the planning submission should say

The Drainage Strategy has to tell the whole story in a form the lead local flood authority and the undertaker can both check.

Set out the hierarchy assessment with its evidence, the proposed connection point with its manhole reference, the undertaker's pre-development enquiry response as an appendix, the agreed or proposed discharge rate, the storage volume and design events that support it, the position on adoption, and any diversion or build-over requirement with its status. Say plainly where a matter remains to be agreed, and propose the condition wording that would allow the authority to grant permission while the detail is settled.

That last move is underused. Offering a condition you can actually discharge, drafted precisely, is more useful to a case officer than a strategy that implies everything is already resolved.

PlanScope drafts these documents structured to UK planning policy, with the site specific figures flagged as fields to complete rather than invented, so the drainage engineer supplies the calculations and the undertaker's correspondence. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice. Check the current position with your sewerage undertaker, your lead local flood authority and a qualified drainage engineer before relying on it.