Requesting sewer flooding records from the undertaker

A sewer flooding records request goes to the sewerage undertaker, not to the council or the Environment Agency, and on a site where the drainage strategy depends on a connection it should be the first enquiry you make. The undertaker holds the public sewer records it is required to keep under section 199 of the Water Industry Act 1991, the register of properties at risk of sewer flooding, and its own capacity assessments. All three come back through a pre-development enquiry, and the response takes weeks rather than days, so the request belongs at the start of the programme rather than the week before submission.

What the undertaker actually holds

Four sets of information matter to a flood risk assessment or a drainage strategy.

The public sewer map shows foul, surface water and combined sewers, rising mains, pumping stations and treatment works, with diameters and in most cases invert levels and manhole references. The at-risk register records properties that have suffered or are considered at risk of internal and external sewer flooding, long known as the DG5 register even though the reporting measure Ofwat uses has changed. Incident records cover reported flooding, blockages and collapses. Capacity information covers hydraulic modelling of the network downstream of your site.

What the undertaker does not hold is anything about private drainage. Lateral drains and private sewers that transferred into public ownership in October 2011 are often recorded poorly or not at all, and drainage entirely within a private curtilage was never its responsibility.

The register of properties at risk of sewer flooding

Expect the response to be aggregated rather than specific. Undertakers do not identify individual properties, because flooding history is personal data, so the answer usually confirms whether incidents have been recorded within the postcode or within a stated distance of the site, and how many.

That is still usable evidence. A confirmed history of internal sewer flooding in the immediate area changes the design conversation completely: it makes a surface water connection harder to justify, it strengthens the case for infiltration or attenuation with a restricted discharge, and it puts finished floor levels and non-return protection on the agenda.

Where the answer is nil, say so explicitly in the flood risk assessment and give the date and reference of the enquiry response. An officer reading "no recorded sewer flooding" with no source will assume nobody asked.

Making the pre-development enquiry

Every undertaker runs its own developer services process, and the names differ, but the content of a good enquiry is the same.

1. Site boundary as a plan with a scale and grid reference, not a postcode alone.
2. The proposed development described in terms the undertaker can model: number of dwellings or floorspace by use, estimated foul flow, impermeable area.
3. The specific questions asked separately, so each gets an answer: asset records, flooding history, foul capacity, surface water capacity, point of connection, any protected assets crossing the site.
4. The programme, including when you need the response and when development would start.
5. Whether you are seeking a formal capacity assessment or a preliminary view.

Ask for the point of connection to be identified rather than confirmed, because a request phrased as confirmation gets a yes or no while a request to identify gets an engineer's view on where the network can take the flow.

Asset plans and their accuracy

Treat positions on a sewer record as indicative. The records were compiled over a long period from varied sources, and the caveat on the plan means what it says: do not excavate on the basis of it.

Where a public sewer crosses the site, the easement and build-over position matters to the layout as much as to the drainage. Undertakers apply standing distances for building near their apparatus and a build-over or build-near agreement will be needed. Find that out before the plots are drawn rather than after, because a 3 m easement through the middle of a terrace is a layout problem.

Using the response in a flood risk assessment

Sewer flooding is one of the sources a flood risk assessment has to consider alongside fluvial, tidal, surface water, groundwater and artificial sources. The Planning Practice Guidance on flood risk and coastal change expects each source to be addressed, so the undertaker's response is the evidence for one of them.

Write it up as source, pathway, receptor. The source is surcharge of the combined or surface water network in a storm exceeding its capacity. The pathway is usually manhole or gully surcharge and overland flow along the road. The receptor is the proposed development, and the design response is levels, thresholds, non-return valves and a discharge rate the network can accept.

Groundwater interacts with this more than people expect, because infiltration into a rising water table fails at exactly the time the sewers are surcharged. That interaction is covered in groundwater flooding evidence and how to assess it, and basement schemes face a compounded version of the same problem, set out in basement development and the flood risk objection.

The undertaker's response belongs in the Flood Risk Assessment as an appendix and in the Drainage Strategy as the basis for the discharge hierarchy.

Timescales and fees to allow for

Pre-development enquiries are charged services and turnaround is measured in weeks. Build the request into the programme at the point you fix the site boundary, because three things downstream depend on it: the discharge hierarchy in the drainage strategy, the layout where an asset crosses the site, and the capacity argument if a connection has to be requisitioned.

Where a formal capacity assessment is needed, allow longer. Network modelling is queued work, and the undertaker's response may itself recommend improvement works with their own programme.

One point on the right to connect. Section 106 of the Water Industry Act 1991 gives a right to communicate with a public sewer for foul drainage, subject to the undertaker's conditions, and it is not the same for surface water, where a connection can be resisted on capacity grounds and where national policy pushes the discharge hierarchy towards infiltration and watercourses first. Plan the strategy on that basis rather than assuming a surface water connection is available.

Where the response contradicts the council's view

It happens. The lead local flood authority holds surface water flooding records and the council's strategic flood risk assessment may map a critical drainage area over a site the undertaker says has capacity, or the reverse.

Do not pick the favourable answer. Report both, cite both with dates and references, and design for the worse case. Then say in the assessment why the two differ, because usually they are measuring different things: the undertaker is describing its pipe network while the lead local flood authority is describing overland flow that never enters a pipe. An officer who sees both records acknowledged will accept a design that addresses both, and one who spots a missing record will ask for the assessment again.

PlanScope drafts a Flood Risk Assessment and a Drainage Strategy structured to the sections the consultees look for, and flags every site-specific figure it cannot verify rather than filling it in, which is exactly right for data that has to come from an undertaker's written response. For specialist technical reports, qualified professional input may be required.

Enter the site postcode on PlanScope to see which flood and drainage documents the scheme is likely to need.

This is general guidance, not legal advice, so check the position with your local planning authority, the sewerage undertaker or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.