A Building Regs’ Guide to Building Over Public Sewers
Building control officer Paul Hymers looks at whether building over a public sewer could scupper your plans — and add to costs
There’s a couple of major points to consider when it comes to Requirement H4 of the Building Regs, particularly as this could put a stop to your development plans. The first regards the presence of existing sewers, the second, connecting your new home to mains drainage.
The whole of Part H Drainage and Waste Disposal concerns itself with foul water drainage, wastewater treatment systems and cesspools, rainwater drainage, separate systems of drainage and solid waste storage, but for the purposes of this feature, we’ll be focusing on building over or near sewers, and connecting to mains sewers.
How to Establish Whether a Public Sewer Crosses Your Site This is one of the key questions to answer before building a home — and should, in fact, be addressed before buying your chosen plot. It could also impact if you’re extending your existing property.
If a public sewer crosses your site, you could have a major problem, as most water authorities will not entertain an application to build over it. Just because a plot has been granted planning permission, it doesn’t necessarily mean it can be built upon. A recent case was delayed for eight months by a major sewer on site which required a build over agreement, a sewer diversion and one corner of the house nearest to the
sewer being cut off and redesigned, all before work could start on site. And if you’ve ever wondered why you see the same plot routinely pop up at auction, a good place to start is by looking at the public sewer records. Some plots run beneath the auction hammer for years before finally (often unsuspecting) buyers and their architects find a solution to the problem.
The water authorities tend
“When it comes to new builds, a public sewer crossing the site can be a major problem”
to prefer a solution that moves the building away from the drain, but that of course means going back to planning with the new plans and this isn’t always possible or acceptable.
It shouldn’t be too difficult to find out whether you have a public sewer on your plot. The public sewer map records are in the public domain — you can find them at your local authority offices, websites or libraries. And of course, you can contact your local water authority and ask.
It’s a good idea to look out for manholes when assessing a plot. Even if there are no manholes present, a buried pipe may cross it nonetheless. This is really where checking the sewer plans becomes important.
When You Might Need a ‘ Build Over’ Agreement You’ll need to apply to your local water authority for a ‘ build over’ agreement if you are building over or within 3m of a sewer serving just one other property (known as a lateral drain). You don’t need your local water authority’s approval if you’re building outside these distances, and in the case of minor sewers (defined as less than 3m deep and up to 225mm pipe diameter), the Building Regulations enable the protective measures to be agreed as part of the building control process with your appointed local building control body.
Some water authorities have an agreed protocol with the building control bodies in their region to offer customers a faster, more efficient service
when they come to build. The protocol gives building control bodies the authority to approve potential build overs of minor sewers on their behalf.
The Potential Outcome of a ‘ Build Over’ Agreement
Of course, the preferable (and often cheapest) solution for the self-builder is to literally build over the sewer. However, for the water authorities this is less than ideal — particularly if they need access to repair or maintain it in the future. As such, it is more usual to negotiate a diversion design for the sewer, which will take it around the build.
This can often take time — sometimes months. Water authorities are not in competition with anyone and have nothing to gain from the process beyond trying to protect their own drains. A detailed engineering scheme must be produced and approved to their satisfaction before an agreement can be entered into. Even then, their own engineers will need to inspect the diversion works on site as they take place.
Invariably, if you are diverting a sewer, the drainage materials need to be compatible with that of the existing sewer, which may be old. In this case, brick-built manholes and clay pipes might be needed instead of PVCU materials. Of course, the depth and the gradient of the sewer will determine whether a diversion is possible at all, since extending the run out and around the build lengthens the pipework between two fixed points.
It’s interesting to see what happens with the deeper tunnel drains that are well beneath any chance of being affected directly by traditional foundations of the new build. Often many metres underground, these sewers are without hope of being diverted.
The older Victorian brickbuilt sewers may also be considered vulnerable and at risk from vibration damage caused by mechanical excavation or vibration of the ground above.
In these cases, a change in the design of the foundations to a shallow raft type may be needed. As well as dispersing the building’s weight over the whole footprint of the slab, rafts will also reduce the disturbance to the ground during construction.
Whatever the proposed solution, a build over agreement is not guaranteed. Often water authorities state on their websites that while they try to accommodate building over sewers for most extensions and improvements,
that obligation doesn’t extend to new buildings.
Connecting to Mains Sewers If you’re planning to connect your new home to a public sewer, you’ll need the local water authority’s consent before you go ahead. The water authority will need to check the connection will work as planned, and that it won’t potentially cause problems like sewer flooding or pollution, although they will usually have been consulted on these issues through the planning process.
The fees vary from one authority to another, and also by the nature of the connection proposed. For instance, for road connections you’ll need to apply to the highway authority, to agree any measures needed, such as traffic lights, permits, lane closures and other issues. This can take months to be authorised, as well as adding to the cost of your work.
While you have a legal right to be able to connect to a public sewer, this doesn’t apply to private sewer connections outside of your land. These depend on the agreement of the neighbouring landowners, and drainage easements are essential even when planning permission has been granted for a private sewer connection outside of your plot. As such, this will come at a cost.
Often planners get around this issue by imposing a Grampian condition. Negatively worded, these conditions prohibit the development authorised by the planning permission or other aspects linked to the planning permission (e.g. the occupation of the dwelling) until a specified action has been taken, such as the agreement for connection to drainage on neighbouring private land. Where this is not possible, a septic tank or sewerage treatment plant might be an alternative.
Sewerage Treatment Plants and Septic Tanks
A final word about sewerage treatment plants and septic tanks. Before deciding on the type of storage or sewer treatment, the Environment Agency should be consulted. In defined water aquifer zones, where rainwater is collected or rivers are close by, septic tanks may be prohibited or restricted in size. Mini-treatment plants that cleanse the discharged wastewater may be necessary. In heavy soils, like clay, the irrigation drainage from septic tanks may also be ineffectual.
Next month : Building Regs Guide to Water Efficiency Design (Part G) Explained
A traditional one brick thick manhole constructed from semi-engineering bricks is usually required for new public sewer inspection chambers, rather than the PVCU prefabricated types.
Useful, but Not Always Accurate
The public sewer maps can show pipe runs, diameters, inspection chambers, invert levels and cover levels for both foul and surface water public sewers — but not always accurately, which is why you should always investigate on site.