From 13 July 2026, the ground rules for building over or near public sewers in Southern Water’s region have changed.

We have been contacted by Southern Water to advise that, following a comprehensive review of its management of Build Over Agreements, Self-Declaration processes and Building Regulations (BREGS) consultations, it has implemented a number of significant procedural changes. For years, the Build Over Self-Declaration process offered a quick, low-friction route for householders and developers proposing modest works near public sewers. That route closed on Monday 13 July 2026.

The trigger, according to Southern Water, was an increase in operational incidents associated with structures constructed over or close to public sewers. Whatever your view of water company performance in other areas — and there is plenty to say on that subject — the underlying logic here is sound. A collapsed or inaccessible sewer beneath a completed extension is an expensive, disruptive problem that is almost always cheaper to prevent than to remedy.

The risks are not theoretical. A new structure that fails to protect the sewer beneath it can impose additional loading that leads to the pipe collapsing — and with it the prospect of structural damage to the property above, interrupted drainage for neighbouring properties, and an increased risk of internal sewage flooding. Southern Water’s revised process is designed to check, before works start, that the proposed construction will neither load nor damage the sewer during or after the build, and that 24-hour access to all public sewers and manholes is preserved for routine inspection, maintenance and repair.

What has actually changed?

Self-declaration is gone for anything over a sewer. If any part of a proposed structure sits over any length of public sewer — mapped or unmapped — a Full Build Over Agreement (also referred to as a Sewer Protection Agreement) is now required. There is no de minimis threshold. Scale is irrelevant.

A new “Build Near-To” application replaces it for close-proximity works. This lighter-touch route is available only where all of the following are satisfied:

• the structure keeps at least 500mm clear of the sewer or manhole wall;
• the sewer is classified as non-critical;
• nothing is built over the sewer itself; and
• no alteration to Southern Water’s assets is needed — no diversions, no manhole removals or repositioning, no pipework replacement.

Fail any one of those tests and you are into a full agreement – this may affect you if you are proposing an extension, annexe, garage and conservatory, infill between existing extended properties, underpinning near a sewer, and, significantly, non-conventional foundation systems including auger piles and screw piles.

The powers behind the paperwork

It is worth being clear about why these agreements matter, because the consequences of ignoring them are more serious than many property owners realise.

Public sewers are vested in the sewerage undertaker, which has a statutory duty under the Water Industry Act 1991 to maintain them — and statutory rights of access to do so, even where the sewer runs beneath private land. Where a structure has been built over a public sewer without an agreement in place, the undertaker retains the right to enter the property and access its asset, even if that means damaging or, in extreme circumstances, demolishing the structure that sits over it. Critically, the undertaker is under no obligation to reinstate the property to its previous condition or to compensate the owner for damage caused in gaining access. In practice, water companies will look for less drastic ways in first — but the legal risk sits squarely with the owner.

The absence of an agreement casts a long shadow at resale, too. Conveyancers routinely pick up unauthorised build-overs through drainage and water searches, and buyers and lenders will typically demand indemnity insurance, a retrospective agreement, or a CCTV survey and comfort letter before proceeding. What seemed a shortcut at construction stage becomes a recurring cost and delay every time the property changes hands.

Where building control fits in

Proposals over or close to public sewers are no longer suitable for approval on a self-declared basis. Where a sewer may be affected, the applicant must be directed to Southern Water for the appropriate approval before construction proceeds. Southern Water has been explicit that this cannot be agreed directly with building control bodies: the acceptability of works affecting sewerage infrastructure belongs with the sewerage undertaker. As part of a full application, Southern Water will normally require a CCTV survey to establish the sewer’s condition, location, depth, material, diameter and any defects. The survey is carried out by Southern Water’s contractors and included in the application cost. This is a sensible discipline: knowing what is actually in the ground before foundations are designed, rather than discovering it at the excavation stage, protects the developer as much as the asset.

To achieve a building control completion certificate — the “sign off” for your works — we would expect to see evidence that permission for the build over or build near-to is in place, or at the very least confirmation that the water authority has been contacted. But a word of caution: even where a completion certificate has been issued, it provides no protection against action by the water authority. The certificate confirms compliance with the Building Regulations; it does not extinguish the undertaker’s statutory rights over its asset, which — as set out above — can extend to demolition of the offending structure.

What this means in practice

The first step for any project is to determine whether there are public sewer pipes or manhole chambers on the land, and whether they fall within 3 metres of the proposed new structure and its footings. If they do, before reaching for an application form, consider if you can modify the plan so that the building sits at least 3 metres from the sewer; this is often the cheapest and easiest route, and one that avoids the agreement process altogether.

For architects and designers, the message is to deal with sewer constraints at RIBA Stage 2, not Stage 5. Getting the sewer question answered before design freeze is now, more than ever, the difference between a smooth project and a stalled one. Obtain the asset records early, check for unmapped sewers on older sites, undertake a CCTV survey and if your scheme relies on a diversion or a piled foundation near a public sewer, build the agreement lead time into your programme. The standard consultation response for sites with no mapped sewers remains unchanged — but it carries the familiar caveat that if a sewer turns up during construction, works must stop until the appropriate application is made. The practical test will be turnaround times. A more rigorous process is only defensible if Southern Water resources it properly. A full agreement requirement with a CCTV survey attached must not become a three-month bottleneck on a domestic extension.

We will be monitoring how the new regime performs in practice on our projects across Kent, Sussex and Hampshire. Expect other undertakers to watch this closely, too. Where one statutory undertaker tightens its regime in response to asset failures, others tend to follow.