Build over agreement or land drainage consent

These are two different consents from two different bodies, and a self-builder often needs both. A build over agreement comes from the sewerage undertaker and deals with building over or close to a public sewer. Land drainage consent comes from the lead local flood authority and deals with works that affect the flow of an ordinary watercourse. Neither is planning permission, neither is building control, and a planning permission grants you no right to do either.

The practical difference that catches people out: land drainage consent has a statutory determination period with a deemed consent if the authority does not decide in time, while a build over agreement has no such backstop and depends entirely on the undertaker's process.

Two consents with different owners

The sewerage undertaker owns the public sewer network and controls what is built over and near it. Its powers and duties come from the Water Industry Act 1991, and its interest is the long-term maintainability of an asset it may have to dig up in forty years' time.

The lead local flood authority, which is the county council or unitary authority, consents works in or affecting ordinary watercourses under the Land Drainage Act 1991. Its interest is flood risk and the free passage of water. Where the watercourse is a designated main river, the Environment Agency takes that role instead, through a flood risk activity permit rather than a land drainage consent.

That gives three possible consenting bodies for what a homeowner thinks of as one drainage question, and the first task on any site with water on or near it is working out which asset you are dealing with and therefore who you have to ask.

Building near or over a public sewer

The trigger is proximity rather than position directly above. In practice an undertaker will want an agreement where any part of the structure, including foundations, comes within three metres of a public sewer, and the requirement bites on foundations more often than on walls because a strip footing spreads.

Since the 2011 transfer of private sewers in England and Wales, lateral drains and sewers that used to be private became the undertaker's responsibility, so a shared drain running across a back garden between terraced houses is very often now a public sewer. That transfer is the single most common surprise on an extension project. The drain looks domestic, the homeowner has always treated it as theirs, and it is on the undertaker's asset plan.

What the undertaker assesses is access for maintenance, loading on the pipe, the depth and condition of the asset, and whether a manhole would be enclosed. Typical outcomes are agreement subject to a lintelled or piled foundation detail bridging the pipe, agreement subject to a CCTV survey before and after, a requirement to divert the sewer at your cost under the statutory diversion procedure, or refusal where a manhole would be built over.

Several undertakers operate a simplified or self-certification route for small domestic extensions over small-diameter pipes at modest depth. Check your own undertaker's published criteria, because the thresholds differ between companies and the easy route disappears as soon as the pipe is larger or deeper than the stated limits.

Building control will ask for the agreement, because Approved Document H deals with building over or near drains and sewers, so the two processes interlock even though they are separate.

Works to an ordinary watercourse

An ordinary watercourse is any watercourse that is not a designated main river: a stream, a ditch, a field drain, a culverted brook under a garden. Consent under section 23 of the Land Drainage Act 1991 is needed for any works that would obstruct or alter the flow, which covers culverts, pipes, bridges, headwalls, weirs, dams, outfalls and in many cases infilling.

Two points matter for a small site. A ditch that is dry for most of the year is still a watercourse. And the consent regime applies to the works themselves, so installing a new surface water outfall into a roadside ditch needs consent even though nothing is being built over the ditch.

Where the watercourse is a main river, the Environment Agency's permitting regime applies instead, and the scope is wider: works within eight metres of the top of the bank of a main river, or within sixteen metres of a tidal defence, generally need a permit. Check the main river map before assuming your stream is an ordinary watercourse.

Flood risk and water quality issues can run alongside. Where a catchment is subject to nutrient or water neutrality requirements, those are separate matters again with their own evidence, as our note on nutrient neutrality and water neutrality explains.

Who issues each and how long it takes

The two month period for land drainage consent, with consent deemed to be given if the authority does not determine the application in that time, is a genuine protection. Keep the submission date and the acknowledgement, because the clock only runs from a valid application.

Build over agreements have no equivalent, and the undertaker's turnaround depends on the complexity of the detail and whether it asks for a survey. Allow longer than the published service standard suggests, and submit before you need it rather than when the groundworker is on site.

Culverting, and why it is resisted

National and Environment Agency policy is against culverting open watercourses, and lead local flood authorities apply that consistently. The reasons are practical: a culvert is a blockage risk, it is hard to maintain, it removes habitat, and it converts a visible flood route into an invisible one.

Expect a presumption against any new culvert and a requirement to justify it on grounds that cannot be met another way, such as a necessary access crossing with no alternative route. Where a culvert is accepted, the consent will usually require a minimum diameter or soffit level, headwalls with trash screens designed for maintenance access, a maintenance regime, and sometimes a commuted sum.

The corollary is that de-culverting scores well. Where a scheme can open up a piped watercourse, that is a betterment the authority will weigh in your favour, and it may also help the drainage strategy and biodiversity position for the planning application.

Where a planning condition depends on one

Planning officers routinely impose conditions requiring drainage details to be approved before work starts, and sometimes requiring evidence of a build over agreement or land drainage consent.

That creates a sequencing problem. Where the condition requires the third party consent before any development, and the third party will not engage until the design is fixed, you can end up unable to start. Ask for the condition to be framed around submission of the design and evidence that the application has been made, rather than around the grant of the other consent, and ask for the trigger to be a defined stage rather than any development at all.

Whether a start on site counts as lawful implementation of the permission is a separate question again, and the tests are set out in a material operation and lawful implementation. A start made in breach of a pre-start condition is a poor foundation for an argument that the permission has been kept alive.

Remember also that these consents are regulatory, not proprietary. A right to discharge into a neighbour's ditch is a private law question, and the distinction between private rights and the planning regime is the same one described in covenants and planning conditions.

Finding the assets before you design

Do this at the sketch stage. It costs almost nothing and it changes layouts.

Request the sewer asset plan from the sewerage undertaker for the site and the adjoining land, which shows public sewers, lateral drains and manhole positions. Check the Environment Agency's main river map to establish whether any watercourse on or near the site is a main river. Ask the lead local flood authority for its register of structures and features that affect flood risk, which it is required to maintain, and for any records of consented works nearby. Then walk the site and look: a manhole cover in a lawn, a rodding eye against a boundary wall, a dry ditch under brambles.

Where the desk records and the site do not agree, trust the site and commission a drainage survey with CCTV and tracing. Asset plans are indicative and manhole positions in particular are often approximate.

Put the findings on the proposed site plan before the foundation design is set. Moving an extension 1.5 metres at sketch stage costs nothing. Moving it after the build over agreement is refused costs a redesign and a resubmission. Browse the 66 report types by category to see which drainage and flood documents the application will need, and enter your postcode on PlanScope to see what the site triggers.

This is general guidance, not legal advice. Undertaker criteria and local authority procedures vary, so check the position with your sewerage undertaker, the lead local flood authority or a qualified professional before relying on it.

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