Walkthrough: a rear extension over a public sewer
A 4 metre rear extension with a 150mm public sewer running diagonally under the kitchen. This illustrative walkthrough follows the asset search, the build over agreement and the foundation redesign that followed.
A single manhole cover in the middle of a back garden is enough to add three months to a householder extension. The illustrative case below follows a 4 metre rear extension on a 1960s semi where a 150mm public foul sewer ran diagonally under the proposed kitchen, and the planning application and the build over agreement had to be run as two separate jobs. The figures and dates are illustrative rather than taken from a real project, but the sequence is the one that plays out on these schemes.
The owners had a builder's price and a set of drawings before anyone looked at a drainage record. The manhole was visible, lifted, and dismissed as "ours" because it sat inside the garden boundary. It was not theirs. Since the transfer of private sewers and lateral drains to the water and sewerage undertakers in October 2011, a shared drain that serves more than one property is normally a public sewer, whoever's land it crosses.
What the records showed
A pre-development enquiry to the sewerage undertaker produced an asset plan within a fortnight. The sewer ran from the neighbour's inspection chamber, diagonally across the rear garden, to a second chamber in the adjoining garden. Depth to invert at the near chamber was about 1.4 metres. The proposed extension footprint covered roughly 2.5 metres of its length and clipped the near chamber by about 400mm.
Two numbers mattered from that point. The 3 metre protection distance either side of the pipe, which undertakers apply as standard for sewers up to 375mm in diameter and inside which they expect to be consulted, and the position of the chamber, because building over a manhole is the one thing an undertaker will almost never agree to. Ordering those records early is the whole trick, and the sequence for doing it is set out in the piece on ordering sewer records through a pre-development enquiry.
Three options, priced against each other
The design team put three routes to the owners.
- Build over the sewer under an agreement with the undertaker, with a bridging foundation and the chamber relocated outside the footprint.
- Divert the sewer around the extension under section 185 of the Water Industry Act 1991, which lets an owner require the undertaker to move a public sewer, at the owner's cost and subject to the undertaker being satisfied the diversion is reasonable.
- Redesign the extension to a 3 metre depth with a wrap around the chamber, keeping the structure clear of the protection zone.
The diversion was the cleanest engineering answer and the slowest and most expensive one, because it needed a new pipe run, new connections at both ends, work in the neighbour's garden with their consent, and a design approved by the undertaker before anything was priced. The redesign lost a metre of kitchen and the owners did not want it.
They chose the build over. On the great majority of householder jobs that is the right call, and it is worth understanding what the agreement actually asks for before assuming it is a formality. The mechanics are covered in detail in the article on building over or near a public sewer and the agreement that allows it.
The foundation design the undertaker wanted
Approved Document H deals with building over existing sewers, and the principle behind every undertaker's standard detail is the same. The new structure must not load the pipe, and the pipe must remain accessible for repair.
What that meant here:
- Foundations taken down so that a line drawn at 45 degrees upwards from the invert of the sewer passed below the underside of the footing, rather than through it. This is a load spread line and has nothing to do with the 45 degree line used for daylight and amenity.
- A reinforced concrete lintel bridging the pipe where the rear wall crossed it, bearing on pads outside the protection zone.
- The near chamber rebuilt roughly 1.8 metres further along the run, outside the footprint, with the redundant section grouted and capped.
- A rodding eye brought up inside the extension was refused. The undertaker wanted external access at both ends of the covered length.
- No internal manhole and no soil pipe connection into the covered section.
The structural engineer's fee for the bridging detail and the deeper foundation design was a small fraction of what the diversion quote would have been. The groundworks were not: deeper footings on a diagonal line through the footprint meant more excavation, more concrete and two days of hand digging around the live pipe.
Running the agreement alongside the planning application
Here is the part householders get wrong. Planning permission and a build over agreement are unrelated consents. The council does not check the sewer, does not consult the undertaker on a householder application as a matter of course, and will grant permission over a public sewer without comment. And a planning permission gives you no right whatever to build over the pipe.
So the two ran in parallel. The householder application went in first, because the extension was 4 metres deep on a semi and therefore beyond the permitted development limit for a single storey rear extension without the prior approval procedure. Eight weeks later it was approved. The build over application went to the undertaker a fortnight after the planning submission, once the foundation sections existed, and came back with comments requiring the chamber relocation and a revised lintel detail. Those revisions, the resubmission and the final approval took about seven weeks in total.
The build over agreement also had to be in place before the Building Regulations full plans approval was useful, because building control will not sign off a foundation over a public sewer without the undertaker's position being settled. Three consents, two of them sequential.
| Consent | Who decides | Runs when |
|---|---|---|
| Householder planning permission | Local planning authority | First, around eight weeks for a minor application |
| Build over agreement | Sewerage undertaker | In parallel, but needs foundation sections |
| Building Regulations approval | Building control body | After the undertaker's position is settled |
What the drawings had to show
The planning drawings were ordinary. The build over submission was not, and the commonest cause of a rejection is a drawing set that shows the extension without showing the pipe.
What went in:
- A site plan with the sewer line, chamber positions and the 3 metre protection zone drawn and dimensioned.
- Existing and proposed foundation plans.
- A long section along the sewer showing cover depth, invert levels and the new footing.
- A cross section through the bridging lintel, with bearing dimensions.
- A construction method note covering how the excavation would be supported next to the live pipe and how surcharge during the works would be avoided.
For the planning submission itself, a householder application needs no Planning Statement in most authorities, and this one did not include one. Where the sewer forces an unusual footprint, a short note explaining why the extension steps around a chamber saves the case officer guessing, and the full list of what a constrained site like this typically needs is set out at the report index.
Where the time went
From first drawing to a start on site was about five months on this illustrative job. Roughly two weeks for the asset records, eight weeks for the planning decision, nine weeks for the build over agreement including one round of revisions, and the remainder in structural redesign and re-pricing. Had the records been ordered before the first set of drawings, the two consents would have overlapped almost completely and the same scheme would have been on site inside three months.
The money was mostly not in fees. It was in a groundworks package repriced after the foundation design changed, and in a builder who had quoted for strip footings at 900mm.
Order the asset plan before you pay anyone to draw anything. It is the least expensive piece of information in the whole process.
This is general guidance, not legal advice; check the position with your local planning authority, your 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. To see which documents your own site is likely to need, enter your postcode on PlanScope.
Frequently asked questions
Does planning permission let me build over a public sewer?
No. The two consents are unrelated. A local planning authority will usually grant a householder permission without consulting the sewerage undertaker, and that permission gives no right to build over or near the pipe. You need a separate build over agreement from the undertaker, and building control will want the undertaker's position settled first.
How do I find out whether a sewer crosses my garden?
Order an asset location search or pre-development enquiry from your sewerage undertaker, which produces a plan of public sewers, lateral drains and chamber positions. A visible manhole inside your boundary does not mean the pipe is private. Shared drains serving more than one property generally became public sewers on transfer in October 2011.
How close to a public sewer can I build?
Undertakers typically apply a protection distance of 3 metres either side of a sewer up to 375mm in diameter, inside which they expect to be consulted and to approve the foundation design. Larger sewers carry wider distances. Building directly over a manhole is almost always refused, so the chamber usually has to be relocated.
Can I make the water company move the sewer?
Section 185 of the Water Industry Act 1991 lets an owner require a sewerage undertaker to divert a public sewer, at the owner's cost, where the undertaker is satisfied the diversion is reasonable and practicable. It is the slowest and most expensive option, needs a design approved before pricing, and often needs a neighbour's consent for works on their land.
What foundation design will the undertaker accept?
Something that keeps load off the pipe and keeps the pipe accessible. In practice that means footings deep enough that a 45 degree load spread line from the sewer invert passes below them, a reinforced concrete lintel bridging the pipe where a wall crosses it, external access at both ends of the covered length, and no new connections into the covered section.
How long does a build over agreement take?
Undertakers publish their own target timescales for build over applications, and a first time submission with complete foundation sections is far quicker than one that needs revisions. Allow several weeks, and more if a chamber has to be relocated or a detail redesigned. Check your undertaker's developer services pages for the current timescale and charges.
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.