Reading a drainage asset record from a water company

A sewer asset record request is the least costly piece of due diligence on a development site and the one most often left until the drainage engineer asks for it. The record tells you where the public pipes run, how big they are, which way the water goes and roughly how deep, and that information decides whether your scheme drains by gravity or needs a pump, whether a building line has to move, and whether you are about to design over somebody else's asset.

It also tells you less than people assume. Read it as a starting point rather than a survey.

What the undertaker is required to hold

Under the Water Industry Act 1991 each sewerage undertaker has to keep records of the public sewers, disposal mains and lateral drains it is responsible for, and make them available for inspection. That duty covers public assets. It does not extend to every pipe in the ground.

The practical consequence is that a clean-looking asset plan showing one public foul sewer in the road can sit above a site criss-crossed by private drainage, surface water culverts in private ownership, land drains from a former agricultural use and a Victorian combined sewer nobody has looked at in fifty years.

How to request the records and what they cost

Every undertaker runs a developer services or asset information route, and most now issue plans digitally within a few working days. Ask for the full set rather than the single plan.

1. The sewer or asset location plan, covering the site and a sensible buffer around it rather than just the red line.
2. Manhole schedules or a levels schedule where the undertaker holds one, because the plan alone rarely carries invert levels.
3. A water main plan from the water undertaker, which is sometimes a different company from the sewerage undertaker in the same area.
4. A pre-development enquiry or capacity assessment, which is a separate application and takes longer than the plan.

Charges vary by undertaker and are published on their websites. Inspection of the public sewer records is free, and most undertakers make a modest charge for supplying copies, with the capacity enquiry priced separately. Order the capacity enquiry first, because it is the one with a lead time measured in weeks.

Reading the symbols, levels and flow direction

Four things matter on the plan itself. Pipe diameter in millimetres, pipe type, flow direction, and node levels.

Colour conventions differ between undertakers, so read the legend each time rather than relying on the colour you remember. Where levels are absent, the plan gives you position and nothing else, and a manhole survey is the only way to get the invert. On a flat site, a 300 mm error in assumed invert level is the difference between a gravity outfall and a pumping station with an adoption agreement attached.

Public, private and the transfer that changed everything

Private sewers and lateral drains that were connected to the public network transferred into the ownership of the sewerage undertakers on 1 October 2011, under regulations made using powers in the Water Industry Act 1991. Many of those pipes have never been mapped.

So the asset plan shows what the undertaker knew about before the transfer plus whatever has been recorded since. A shared drain running along the rear of a terrace, now public, frequently appears nowhere. The distinction still matters because a public sewer brings build over requirements, easement expectations and a right of access for the undertaker, while a private drain that never transferred is a matter between landowners.

A lateral drain is the length of drain outside the property boundary carrying flow to the public sewer. Those are commonly public and commonly unmapped, which is exactly the combination that causes trouble when a rear extension lands on one.

Why unrecorded pipes are the norm

Three sources of error recur. The transfer described above. Historic development, where combined drainage was laid without record and later partially separated. And former agricultural or industrial use, where land drains, culverted ditches and process drainage were installed outside any statutory regime. Historic mapping is the fastest way to spot the third category, and the approach set out for historic maps as evidence of former land use works equally well for drainage.

Verify before you design. A manhole survey with lids lifted and levels taken, a CCTV survey where the condition or route is doubtful, and a walkover looking for ditches, headwalls and outfalls. On a brownfield site, assume there is drainage you have not found yet and leave yourself a route.

Building over or near a sewer

Approved Document H to the Building Regulations deals with building over or close to an existing sewer, and the undertakers operate their own build over and build near agreement processes on top of it. The common trigger is construction within three metres of a public sewer, with lighter touch arrangements for small extensions over pipes below a stated diameter.

Three practical points. The agreement is a separate consent from planning permission and from building control, so a planning permission does not authorise building over a public sewer. The undertaker may require diversion at the developer's cost, which on a deep sewer is a serious figure. And where a pipe has to be diverted, the diversion needs designing before the layout is fixed, not after.

Plan the layout to keep buildings off the easement where there is any choice. It is the lowest cost decision available at the sketch stage and the most expensive one to revisit at technical design.

Using the records in a drainage strategy

The asset record feeds four parts of the submission. The existing drainage description, the proposed point of connection for foul flows, the surface water discharge hierarchy argument, and the levels strategy. A Drainage Strategy that states a connection point without a level and a capacity position is incomplete, and most lead local flood authorities will say so.

For foul drainage, section 106 of the Water Industry Act 1991 gives an owner or occupier a right to connect to a public sewer, subject to the undertaker's control over the manner of connection. Capacity in the receiving network is addressed through the pre-development enquiry, and where an upgrade is needed a Grampian condition tying occupation to the works is the usual planning answer. That material belongs in a Foul Sewage Assessment.

For surface water, the hierarchy runs infiltration first, then watercourse, then sewer, and a connection to a combined sewer is close to the bottom. If the asset plan shows only a combined system, expect the authority to ask for infiltration testing before accepting a sewer discharge. Flood risk work draws on the same material, and reading a flood map for planning extract correctly matters just as much as reading the asset plan, with the combined picture going into the Flood Risk Assessment.

Sequencing against the planning application

Order the asset plan and the capacity enquiry before the layout is drawn. Commission the manhole survey as soon as access allows. Then design, then submit. Applications for adoption of new sewers under section 104 and for connection under section 106 come after planning permission, so leave programme for them.

Illustrative example: a 25 unit scheme on a former depot where the asset plan showed a 225 mm foul sewer in the adjoining road at 2.1 m depth, and the manhole survey found the invert 600 mm shallower than assumed. The layout changed to drain the rear of the site to a second connection rather than accept a pumping station, which saved an adoption agreement and an ongoing maintenance liability. The change cost a week at sketch stage. Finding it at technical design would have cost a redesign.

Enter your postcode on PlanScope to see which drainage and flood documents your site is likely to need, then draft them and send the technical work for review by a drainage engineer.

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