Is There a Public Sewer Under Your Garden Gym Site? Build-Over Agreements, the 3 Metre Rule and What Water Companies and Building Regulations Say in England

Many gardens, especially in towns, have a public sewer running beneath them. A garden gym, garage conversion or extension built over or close to one can need the water company’s permission as well as any planning or building control approval. This article draws on Thames Water’s build-over guidance, Severn Trent Water’s policy notes for domestic extensions and garages, and Approved Document H of the Building Regulations for England. It applies to England; water companies set their own procedures, so those of the company serving the address govern, and the rules in Scotland and Northern Ireland differ. It is general information, not advice.

Why the water company has a say

Thames Water explains that a build-over agreement is a contract between the owner and the sewage provider, made when building over or near a sewer. It protects the sewers and drains serving the owner and neighbours and ensures the company can reach them for maintenance or repair. The company says planning permission does not grant permission to build over a sewer or drain, that work should not start before an agreement is in place, and that liability for damage to sewers during construction lies with the owner.

Severn Trent makes the same point in stronger words. It states that building close to or over a public sewer without formal approval is illegal and can jeopardise a later sale. Thames Water adds that it holds legal powers under the Water Industry Act 1991 to maintain and renew public sewers and could seek an injunction to remove a building that restricts them. Without an agreement the council may also withhold the Building Regulations completion certificate.

The 3 metre guide

Thames Water says it is likely an application will be needed for building within 3 metres of a public sewer, or within 1 metre of a public lateral drain, which is the pipe serving one property but sitting outside its boundary. Severn Trent measures the 3.0 metres from the nearest point of the building, including foundations, to the centreline of the sewer. Approved Document H takes the same distance: its provisions apply to building over or within 3 metres of the centreline of a drain or sewer shown on the sewerage undertaker’s records, whether the sewer is public or not.

For a garden gym, the distance should therefore be checked against the foundations, not just the walls. This site’s guide to checking for buried services before digging foundations covers other utilities.

A new detached building is treated differently

Thames Water says a new property must apply to have the sewer diverted rather than for a build-over agreement, and gives new car ports or sheds that are not attached to the property as examples. Whether a garden gym of a particular size falls into that category is for the company to decide, which is a reason to ask before design work. Severn Trent’s notes are written for extensions and garages to existing homes and say its application is not intended for new property builds.

Both companies list situations that make approval harder or unlikely. Thames Water says it is unlikely to consent to building over strategic sewers and that a pressurised sewer cannot be built over. Severn Trent lists a formal easement preventing building over, a sewer deeper than 3 metres, a pressurised pumping main, strategic sewers and piling or ground stabilisation as complex cases, and says it will not permit building over a pumping main.

What the process involves

Thames Water sets three classes by sewer size and requires a location plan, a site plan, a drainage layout and a section drawing of foundations in relation to the sewer. Class 2 applications involve CCTV surveys before and after construction, and a retrospective application is possible but may not be approved if the pipework was not properly protected. Severn Trent likewise requires a post-construction CCTV survey before final approval and says the owner pays for any repair of damage. Both publish fee schedules by class, and Thames Water says its charges are updated each 1 April, so the current figure should be taken from the company.

On the building control side, Approved Document H says work over or near sewers should not overload or damage them, should not obstruct reasonable access to a manhole or inspection chamber, and should leave a satisfactory diversionary route. It says buildings should not normally be built over a manhole on a shared sewer, that the length of drain under a building should not exceed 6 metres without the owner’s permission, and that building over or within 3 metres of a drain or sewer more than 3 metres deep, or larger than 225mm in diameter, needs the owner’s permission. It adds that heavy materials should not be stored over drains or sewers during construction. Sewer record maps are held by the sewerage undertaker and by local authorities, and Thames Water says private sewers may not be mapped, in which case a builder or drainage specialist can survey.

FAQs

Does a small timber garden gym on slabs need an agreement?

The Thames Water and Severn Trent pages read do not describe an exemption for lightweight buildings; both focus on distance from the sewer. The water company serving the address should be asked.

What if the work is already built?

Thames Water says a retrospective application can be made, with a survey to check for damage, but approval is not guaranteed.

The bottom line

In England, a garden gym within about 3 metres of a public sewer, or built over one, may need a build-over agreement or a diversion, whatever the planning position. Approved Document H sets technical limits for building near drains and sewers. The sensible order is to locate the sewers, ask the water company serving the address whether the building falls within its rules, and obtain any written agreement before groundworks begin.

Sources

• Build over agreements (thameswater.co.uk)
• Building over sewers or public drains (thameswater.co.uk)
• Policy Guidance Notes and Requirements: Building over/Close to Public Sewers, Domestic Extensions and Garages (stwater.co.uk)
• Drainage and waste disposal: Approved Document H, requirement H4 Building over sewers (gov.uk)