Build over agreements and sewer diversions in your budget

A build over agreement cost is small on its own and large in its consequences. The undertaker's application fee and the camera survey are modest items. The redesigned foundation, the eight weeks waiting for a decision, and the diversion you discover you need after the slab is poured are not.

On a single plot infill, a public sewer running diagonally under the garden is one of the three things most likely to move the programme by a quarter. So find out where the sewers are before you buy the plot, not after planning permission.

When a build over or build near agreement is needed

Since the 2011 transfer of private sewers in England, almost any shared drain serving more than one property is a public sewer, and the pipe under your garden is probably the water company's responsibility even though nobody has ever seen a manhole.

Approval from the sewerage undertaker is required where you intend to build over a public sewer, or within a stated distance of it. Three metres is the common trigger, although undertakers set their own requirements by pipe size and depth, so take the figure from your undertaker's current developer guidance rather than from a general rule. Requirement H4 of the Building Regulations 2010 deals with building over sewers, and the building control body will want the agreement in place before it signs off the foundations.

Three situations that routinely catch self-builders:

• A lateral drain from a neighbouring property crossing your plot. It is public, and it counts.
• A sewer under a proposed single storey extension where the existing house already sits partly over the line. Previous tolerance is not consent.
• Piled foundations near but not over a sewer, where the pile positions and the construction vibration are the issue rather than the finished building.

Order a sewer records search at the same time as your topographical survey. The asset plans are not always accurate, which is exactly why the camera survey exists.

The application process with the sewerage undertaker

Each undertaker runs its own process, but the structure is consistent:

1. Submit the application with a site layout, foundation details, sections showing the relationship to the pipe, and the sewer records extract.
2. Provide a condition survey of the pipe, normally a closed circuit television survey, carried out before work starts.
3. Agree protective measures: lintelling across the pipe, a minimum clearance, no manhole or inspection chamber inside the building, and in some cases a concrete surround or a replacement length of pipe.
4. Accept the standard terms, which bind you and successors in title and reserve the undertaker's right of access.
5. Carry out a second survey after construction to show the pipe is undamaged.

The survey before and the survey after are the parts people skip, and they are the parts that protect you. Without a pre-works condition record, any defect found later is your problem to disprove.

Timescales are usually measured in weeks rather than months for a straightforward build over. Build the determination period into the construction programme and apply as soon as the foundation design is fixed, which is typically well before you need the approval.

Diversion under the Water Industry Act and what it costs

Where the pipe cannot be built over, the alternative is to move it. Section 185 of the Water Industry Act 1991 gives a landowner the right to require the undertaker to alter or remove a pipe, subject to conditions, with the person making the request paying the reasonable costs.

Those costs cover design, approvals, the physical works, reinstatement and the undertaker's own project management. The undertaker publishes charging arrangements annually, approved by the economic regulator, and they change each April, so price the diversion from the current arrangements rather than from a figure quoted on another project. Ask for the quotation to separate design from construction, because the design stage often has to be paid for before the construction cost is known.

Two cost drivers dominate. Depth, because a deep sewer in poor ground may need a shaft rather than an open trench. And the receiving point, because a diversion has to discharge into a position with adequate capacity and gradient, and the nearest manhole is not always suitable.

CCTV survey and pre-condition evidence

Commission the camera survey early, because it answers more than the condition question. It establishes the true line and depth of the pipe, which the asset plan may show incorrectly by several metres. It reveals whether the pipe is a public sewer or a private drain, which changes who you are dealing with. And it identifies existing defects, roots and partial collapses that would otherwise be attributed to your works.

Ask the surveyor for a drawing with levels tied to your topographical survey, alongside the video and the defect schedule. The drainage engineer needs the invert levels to design the connection anyway, and a Drainage Strategy that works from surveyed levels rather than assumed ones avoids a round of comments from the drainage officer.

Timescales and their effect on the construction programme

The sequence that works is: records search at feasibility, camera survey before the foundation design is finalised, build over application as soon as the design is fixed, approval before groundworks start on site. The sequence that fails is: start on site, find the pipe, stop.

For a diversion, assume the design and approval stages alone occupy several months before any excavation, and that the undertaker's own programme is set by its workload rather than by yours. If the diversion is on the critical path, look hard at whether the layout can be adjusted to avoid it, because a 2 metre shift in the building position is almost always less costly than moving a sewer.

An illustrative example. A plot with a 225 millimetre foul sewer crossing the rear third. Option one builds over it with a reinforced concrete lintel, a 300 millimetre clearance and a short replacement pipe length, agreed in about six weeks. Option two diverts the sewer around the new footprint, requiring design, approval and a road closure to reconnect at the manhole in the street. The first adds a line to the groundworks package. The second adds a season. This example is illustrative and your undertaker's requirements will differ.

Where a foundation redesign is the less costly answer

Quite often it is. Deepening a trench fill foundation to pass below the pipe with the required clearance, bridging with a reinforced concrete beam, or switching to a piled and beam solution with piles positioned clear of the easement can all remove the need for a diversion.

Run the comparison properly. Price the foundation variant including any additional structural design, the extra concrete and the excavation and disposal, then price the diversion including design fees and the programme cost of the delay. Bring the structural engineer and the drainage engineer into the same conversation, because the answer usually involves a trade between them that neither will propose alone.

And check the planning drawings still work. Raising floor level by 300 millimetres to clear a sewer can push the ridge above the approved height, which means a section 73 application and another eight weeks.

Sequencing the agreement against the planning application

You do not need a build over agreement to obtain planning permission, and the council cannot refuse permission because one has not yet been granted. What the council can do is condition the drainage details, and a condition requiring an approved drainage strategy before work starts will not be discharged until the sewer position is resolved.

So the practical sequence is to deal with the sewer in parallel with the planning application rather than after it. Include the sewer records extract and a note on the proposed route in the Foul Sewage Assessment submitted with the application, and say which undertaker you are dealing with and at what stage. That one paragraph removes a common reason for a drainage holding objection.

Where highway works are also needed to reconnect in the street, those costs sit under a separate regime, and our note on section 278 and section 38 highway costs covers them. The same budgeting logic applies to obligations, as set out in section 106 costs beyond the contribution.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For the document side of a self-build budget, PlanScope pricing is one free report, £50 per report, or £150 per month on the Professional tier.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.