Most people planning an extension spend a lot of time thinking about the visible parts of the project – the kitchen or the feature glazing for example. However, some of the most important construction decisions happen underground when you design and install your extension's drainage system.
For most extension projects, there’s a risk that when diggers arrive and trenches are opened, an existing drain could suddenly appear exactly where the new structure needs to be built. Work then stalls before the first block has even been laid. This scenario is not unusual, but it is entirely manageable with a bit of early planning.
In this article, our build expert Mark Stevenson explains the steps you need to take to get your extension drainage right.
Article continues belowWhy drainage catches people out
The problem is that drainage sits largely out of sight, and once installed it disappears beneath patios and floors where nobody thinks about it again. Rear extensions typically sit exactly where drainage is already installed, as kitchens, bathrooms and their waste pipes tend to be located along the rear wall.
If neighbouring houses have a similar layout, the drainage may also pass through several gardens before reaching the street. This compounds the problem, and results in the extension foundations clashing with the drainage layout.
If a pipe needs diverting, the drainage system will need to be redesigned before installation can proceed. In some cases, approval is also needed from the local sewerage authority. None of this is unusual, but it does introduce additional time and drainage costs that self builders rarely anticipate.
Depending on the complexity of the drainage run, and whether approvals are required for diversion work, it’s quite common for this element alone to add around £2,000 to the cost of an extension project.
Drainage systems to know about
Every house has two types of drainage systems, which in older properties could be combined.
The first deals with foul water drainage – the wastewater from toilets, sinks, showers and washing machines. This water normally flows through underground pipes and eventually connects to the public sewer network, or some kind of cess pit or treatment plant.
The second system handles surface water drainage. This is rainwater collected from roofs, gutters, patios and driveway drainage. Depending on the location, it may discharge to a surface water sewer, a nearby watercourse, or into a soakaway within the property.
Combined systems are those where both the surface water and foul water share the same pipe. They were effectively phased out many decades ago, as the regulations gradually evolved to require separate systems for new buildings. This means that they’ll only be found on older properties.
Drainage regulations and inspections
The design and installation of drainage systems for extensions is governed by Approved Document Part H of the Building Regulations, which deals with drainage and waste disposal.
Whilst the document contains plenty of technical detail, the principles behind it are fairly straightforward. Pipes must be laid with a consistent gradient so that water flows naturally away from the building. Inspection chambers must be positioned so the system can be accessed if a blockage occurs. And surface water must be directed away from the structure so that rainwater doesn’t accumulate around the foundations.
Building control inspectors pay close attention to drainage installations because drainage problems can cause serious structural problems, contamination and even public health concerns.
Drainage is therefore a key inspection for inspectors. Before trenches are backfilled and the drain covered over, they’ll check pipe gradients, jointing and bedding materials, and confirm that inspection chambers are positioned correctly, with appropriate access for maintenance purposes. They’ll also examine the system for any issues such as leaks, using either an air pressure or a water test.
If the drainage is to be built over, this check is vital because once the system is buried beneath concrete or floor slabs, correcting any mistakes becomes significantly more difficult.
Working around existing drainage
A common complication for extension projects is when an existing drain sits directly beneath the proposed footprint. If that pipe serves only the property being extended, it can usually be diverted around the extension, before reconnecting to the existing system.
Things become more complicated when the drain serves neighbouring houses as well. In this situation the pipe is often classified as a public sewer, even if it runs through a private garden. Building over, or diverting a public sewer requires permission from the local water authority through something known as a build-over agreement.
Water companies such as Thames Water or Anglian Water will review the proposed extension and drainage design to ensure the sewer remains accessible for maintenance. In some cases, they allow the structure to be built over the pipe with additional protection. In others they require the drain to be diverted completely outside the building footprint. Applications can take several weeks to process, and the diversion work itself will take extra time to complete before it can be inspected and signed off.
Rainwater drainage when building an extension
The requirements for rainwater drainage have changed significantly in recent years, particularly around new building work.
Historically, most rainwater from roofs and paved areas simply flowed into the nearest sewer. But as towns have expanded and gardens paved over, these sewer systems have struggled to cope during heavy rainfall so simply buying guttering from sites like Amazon is no longer sufficient. As a result, planning policy and environmental guidance increasingly require Sustainable Drainage Systems, usually referred to as SuDS.
The idea behind sustainable drainage is rather than sending all rainwater straight into the sewer network, it is managed on site, wherever possible. Water is slowed down, stored temporarily, or allowed to soak naturally into the ground. Whilst SuDS is often associated with large housing developments, the principles are gradually influencing smaller projects, including extensions.
Soakaways and natural drainage
A common SuDS features is the soakaway. A soakaway is essentially an underground chamber designed to collect rainwater and allow it to disperse gradually into the surrounding soil. Modern soakaways are typically built using modular plastic crates wrapped in permeable membrane, creating a large void space that temporarily stores water during heavy rainfall.
The effectiveness of a soakaway depends largely on the ground conditions it's installed in. In sandy soils water disperses quickly, but in clay soils drainage can be much slower. In soils with poor percolation properties, soakaways may need to be larger to cope with heavy rainfall. Building Regulations also specify that soakaways should normally be located at least five metres away from building in order to prevent water saturation of foundations and structural movement.
Shop soakaways
These crates are suitable for landscaping but not driveways, but are easy to install and slot together with a 95% void ratio
These crates come in kit form with geotextile and tape. They easily slot together depending on the size required
These crates are larger and can also be joined together depending on the soakaway size needed. They are suitable for trafficked and loaded conditions
Levels and gradients
Drainage doesn’t only involve underground pipes. The way patios, driveways and landscaping are designed around the extension also plays an important role in site drainage.
External paving should always fall away from the building so that rainwater naturally drains towards the garden, rather than back towards the wall. It’s a small detail, but an important one as water that might collect around a threshold could find its way into the building.
Permeable paving can also help in some situations. Materials such as resin-bound gravel, which you can buy easily on Amazon, or permeable block paving allows water to drain through the surface, rather than running directly into gullies or drains. These materials aren’t essential on every project, but they are a great solution where site levels are challenging.
Designing your extension drainage
One of the simplest ways to avoid drainage problems is to investigate existing services before finalising the extension design. Drainage designs that look perfectly workable on drawings, sometimes become awkward once real site levels are considered.
A water and drainage search will normally reveal whether public sewers cross the property. Whilst these searches don’t show every pipe in perfect detail, they provide a useful starting point about where services may lie. It’s also essential to check that the installation on site matches the existing drainage survey drawings. This can be done by lifting existing manhole covers and observing the direction of pipe runs to establish how the drainage system is arranged.
Depending on what the investigation uncovers, small adjustments to the extension footprint can sometimes avoid the need for major diversion works. For example, moving a wall slightly or adjusting the position of foundations can keep the new structure clear of existing pipes.
If the design isn’t carefully planned, you could end up with construction problems and unacceptable compromises. For example, an inspection cover is right where you might want the kitchen island to go. It’s also worth noting that resolving these details during the design stage is far easier than dealing with them once excavation has begun.
Most drainage systems rely entirely on gravity. For this reason, every pipe run must be installed with a suitable gradient. Too little fall and waste water moves slowly, increasing the risk of blockages. Too steep and liquids run ahead of solids, which can leave material behind in the pipe.
Approved Document Part H provides recommended gradients depending on pipe diameter, but the practical challenge is often coordinating those falls with fixed floor levels and foundation depths.
When drainage systems are designed and installed properly, they tend to disappear into the background, wastewater flows away, rainwater disperses quickly, and the drainage system continues working without anyone giving it a second thought.
But when drainage is rushed, or poorly coordinated during construction, the drainage problems tend to appear long after the builders have left. Persistent damp around the building, slow drainage or recurring blockages are often signs that something underground hasn’t been installed quite correctly.
Fixing those problems later is rarely straightforward, so in my opinion, it’s always worth the extra investment to investigate early, design drainage carefully and coordinate installation properly with the site conditions, foundation positions and whatever your specific plot might present.
