Build over agreement or a sewer diversion
A public sewer crossing a site is a layout constraint before it is a legal one. Deciding between a build over agreement and a diversion at the wrong stage is expensive.
A public sewer crossing your site leaves two routes. Either you build over or close to it with the sewerage undertaker's written agreement, keeping the pipe where it is, or you pay to move it. The first is quick, inexpensive and heavily conditioned. The second costs real money, takes months, and is sometimes the only option available.
The decision belongs at layout stage, which is the point most schemes get wrong. By the time a reserved matters drawing is fixed and the house types are chosen, a sewer that has to be diverted will reshape the plot layout, and that reshaping happens after the planning permission is granted.
When a build over agreement is available
Sewerage undertakers publish their own criteria, and the detail varies between companies, but the pattern is consistent. An agreement is normally available where the pipe is shallow, of sound condition, of modest diameter, accessible for future maintenance, and where the proposed structure can be designed to protect it and to avoid loading it.
Typical conditions on an agreement include a minimum clearance between the foundation and the pipe, foundations designed to transfer load below the pipe or spanned across it with lintels, no manholes inside the building, retained access for maintenance, a pre-works and post-works condition survey of the pipe by closed circuit television, and a restriction on the depth of pipe that may be built over at all.
Where the sewer runs under a garden, a drive or a road rather than under a building, consent for building near it is usually straightforward. Where it runs under the footprint of a house, expect scrutiny.
Approved Document H of the Building Regulations sets the technical framework for building over or near a drain or sewer, and the undertaker will be consulted through the building control process as well as granting its own agreement. The two are separate approvals, and satisfying one does not satisfy the other.
What a diversion involves instead
Section 185 of the Water Industry Act 1991 gives the owner or occupier of premises the right to require a sewerage undertaker to move a public sewer, where doing so is reasonably practicable and will not prejudice the undertaker's ability to perform its functions.
The word requisition matters. This is a statutory right rather than a favour, but it is a right exercised at the applicant's expense. The undertaker designs or approves the diversion, carries out or supervises the work, and recovers its costs, and it will normally want security or payment in advance.
Three things drive the programme. The design, which the undertaker must accept. The land, because the new alignment needs an easement and the consent of anyone whose land it crosses. And the work itself, which is open cut excavation with traffic management if the alignment runs in a highway.
Allow months rather than weeks, and start the conversation before the planning application rather than after permission.
Foundation and access conditions
The technical conditions are where a build over agreement either works or does not.
| Issue | Typical requirement under an agreement |
|---|---|
| Clearance | A stated minimum horizontal distance from the foundation face to the pipe |
| Loading | Foundations taken below the pipe invert, or a reinforced lintel spanning the line |
| Depth limit | Undertakers commonly refuse to allow building over pipes below a stated depth |
| Manholes | No chamber inside a building, and access retained in the open |
| Condition | Closed circuit television survey before and after works, with any defects repaired |
| Diameter and type | Larger diameters, rising mains and combined sewers treated far more cautiously |
Read those conditions against the house type before committing. A standard foundation detail for a semi-detached pair will often not satisfy a clearance requirement, and redesigning the foundation on one plot is a smaller problem than discovering it on site.
Where a manhole sits inside a proposed building, the answer is almost always to move the building. Undertakers rarely agree to a chamber inside a habitable structure, and the alternatives are expensive.
Cost and programme compared
The comparison is stark enough that it usually settles the question.
A build over agreement involves an application fee, a condition survey, and design work on the affected foundations. Determination is a matter of weeks in most companies.
A diversion involves survey and design, a legal agreement, easements, possibly a road closure, and construction, all at the developer's cost. On a small site that cost can exceed the value of the plot the diversion releases, which is a calculation worth doing before the layout is fixed.
The programme difference is the one that hurts. A diversion sitting on the critical path between permission and start on site delays revenue, and the delay is measured in months. Where the diversion is needed, front-load it: begin the undertaker's process while the planning application is under consideration.
Who bears the cost under each
Under a build over agreement the developer pays the application fee, the survey and the design, and the pipe stays in the undertaker's ownership and maintenance. If the undertaker later needs to excavate to repair it, the agreement will usually say what happens to any building work that has to be disturbed, and that clause is worth reading rather than skimming.
Under a section 185 diversion the person requiring the diversion bears the cost, including the undertaker's own costs. Once complete the diverted sewer remains a public sewer in the undertaker's ownership along its new line.
Neither route transfers ownership of the pipe to the developer, and neither removes the easement. A new easement over the diverted alignment will bind the site in the same way the old one did, which is a point for the site layout rather than for the lawyers.
Approved Document H requirements
Approved Document H deals with drainage and waste disposal, and its provisions on building over or near a sewer cover clearance, load transfer, protection of the pipe and retained access. Building control will consult the undertaker, and an inconsistency between the planning drawings, the building control submission and the undertaker's agreement is a common cause of delay on site.
Two points to align early. The finished floor level and foundation depths shown on the planning drawings need to be consistent with whatever the undertaker eventually agrees. And the drainage layout submitted for planning should show the public sewer and its easement, because a case officer who cannot see the constraint cannot see that you have designed around it.
The Drainage Strategy is where that comes together, and the Foul Sewage Assessment is where the point of connection and the capacity position are set out. A Utilities Assessment covers the wider services picture where several apparatus owners are involved.
Deciding at layout stage
A short routine at feasibility avoids nearly all of this.
- Obtain the sewer records from the undertaker for the site and a margin around it.
- Mark every pipe, its diameter, its depth and its easement on the layout drawing.
- Test the layout against the undertaker's published build over criteria before house types are selected.
- Where a pipe cannot be built over, decide between moving the building and moving the pipe, and cost both.
- Start the undertaker's process for whichever route you choose at the same time as the planning application.
Where the answer emerges after permission has been granted and the layout has to change, a section 73 application or a non-material amendment may be the route to reflect it, and our comparison of section 73 against section 96A sets out which mechanism suits which change.
This article is general guidance, not legal advice; check the position with your sewerage undertaker, your building control body 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.
Frequently asked questions
What is a build over agreement?
Written agreement from the sewerage undertaker allowing a structure to be built over or close to a public sewer, subject to conditions. Those normally cover clearance from foundations, load transfer below or across the pipe, a depth limit, no manholes inside buildings, retained maintenance access, and a closed circuit television condition survey before and after works.
When is a sewer diversion needed instead?
Where the undertaker will not agree to building over, typically because the pipe is deep, in poor condition, of large diameter, a rising main or a combined sewer, or where a manhole would end up inside a building. Section 185 of the Water Industry Act 1991 gives a right to requisition a diversion where it is reasonably practicable, at the applicant's cost.
Who pays for a sewer diversion?
The person requiring it, including the undertaker's own design, supervision and administrative costs, and usually with security or payment in advance. On a small site that cost can exceed the value of the plot the diversion releases, so cost both options against moving the building before the layout is fixed.
How long does each route take?
A build over agreement is generally a matter of weeks once the application, condition survey and foundation details are submitted. A diversion involves design acceptance, easements, possibly a road closure and construction, so allow months. Start the undertaker's process while the planning application is under consideration rather than after permission.
Does a build over agreement replace building control approval?
No. They are separate. Approved Document H of the Building Regulations sets the technical requirements for building over or near a drain or sewer, and building control will consult the undertaker as part of its own process. Keep the planning drawings, the building control submission and the undertaker's agreement consistent with one another.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.