Most people planning an extension spend a lot of time thinking about the visible parts of the project – for example the kitchen or feature glazing. However, some of the most important construction decisions happen underground when you design and install your extension. Drainage system.
For most extension projects, there is a risk that when the excavators arrive and the trench is opened, the existing drain suddenly appears right where the new structure needs to be built. Work stops before the first block is laid. This scenario is not uncommon, but it is completely manageable with a little advance planning.
In this article, our build expert Mark Stevenson explains the steps you need to take to get your extension drainage right.
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Why does drainage evict people?
The problem is that drainage often sits out of sight, and once installed it disappears under patios and floors where no one thinks about it again. Rear extensions usually sit exactly where drainage is already installed, as kitchen, bathroom and their waste pipes are located along the rear wall.
If neighboring houses have a similar layout, the drainage may pass through several gardens before reaching the street. This compounds the problem, and consequently Expansion foundation Conflict with drainage layout.
If the pipe needs to be diverted, the drainage system will need to be redesigned before installation can proceed. In some cases, approval from the local sewerage authority is also required. None of this is unusual, but it does take extra time and familiarity Drainage costs Which self builders rarely expect.
Depending on the complexity of the drainage run and whether clearances are required for the diversion work, it is quite common to add around this element alone. £2,000 for Cost of extension project
To know about drainage system
Every house has two types of drainage system, which can be combined in older properties.
Deal with the first Waste water disposal – Wastewater from toilets, sinks, showers and washing machines. This water usually flows through underground pipes and eventually connects to a public sewer network or some type cess pit or treatment plant.
Another system handles it Surface water disposal. This is the rainwater that flows through roofs, gutters, patios and Driveway drainage. Depending on location, it may discharge into surface water drains, nearby watercourses, or water. the pit within the property.
Combined systems are those where both surface water and wastewater share the same pipe. They were effectively phased out several decades ago, as regulations gradually evolved to require a separate system for new buildings. This means they will only be found on older properties.
Drainage regulations and inspections
The design and installation of the drainage system for the extension is governed by an approved document Part H of the Building Regulationswhich deals with drainage and waste disposal.
Although the document contains plenty of technical details, the principles behind it are fairly straightforward. Pipes should be laid with a uniform slope so that water flows away naturally from the building. Inspection chambers should be located so that the system can be accessed if an obstruction occurs. And surface water must be directed away from the structure so that rainwater does not accumulate around the foundation.
Building control inspectors pay close attention to drainage installations because drainage problems can cause serious structural problems, contamination, and public health concerns.
Drainage is therefore a key observation for inspectors. Before trenches are backfilled and drains are covered, they will check pipe gradients, joints and bedding materials and ensure that inspection chambers are properly located, with proper access for maintenance purposes. They will also check the system for any issues such as leaks, using either air pressure or water testing.
If the drainage is to be built over, this check is very important because once the system is buried under concrete or floor slabs, it becomes significantly more difficult to correct any mistakes.
Working around existing drainage
A common complication for extension projects is when an existing sewer sits directly below the proposed footprint. If that pipe only serves the property being extended, it can usually be routed around the extension, before reconnecting to the existing system.
Things get more complicated when the sewer also serves neighboring homes. In this situation the pipe is often classed as a public sewer, even though it runs through a private garden. Building or diverting a public sewer requires permission from the local water authority. Build-over agreement.
Water companies such as Thames Water or Anglian Water will review proposed extensions and drainage designs to ensure that sewers remain accessible for maintenance. In some cases, they allow building a structure on the pipe with additional protection. In others they require drains to be diverted completely outside the building footprint. Applications can take several weeks to be processed, and diversion work may take additional time to complete before it is inspected and signed off.
Drainage of rain water while constructing extension
The requirements for stormwater drainage have changed significantly in recent years, particularly around new building work.
Historically, most rainwater from roofs and paved areas was drained into nearby drains. But as towns have expanded and parks have been paved over, these drainage systems are struggling to cope during heavy rains. Buying guttering from sites like Amazon Not enough anymore. Consequently, there is an increasing need for planning policy and environmental guidance Sustainable drainage systemsCommonly known as SuDS.
The idea behind sustainable drainage is that rainwater is managed on-site wherever possible, rather than being sent directly into the sewer network. Water is slowed down, temporarily stored or allowed to dry naturally in the ground. While SuDS is mostly associated with large housing developments, the principles are slowly influencing smaller projects, including extensions.
Soakways and natural drainage
A typical SuDS soak away symptoms. A soakaway is essentially an underground chamber that collects rainwater and allows it to slowly disperse into the surrounding soil. Modern soakaways are typically constructed using modular plastic crates wrapped in a permeable membrane, creating a large void that temporarily stores water during heavy rains.
The effectiveness of a soakaway depends largely on the soil conditions in which it is installed. Water disperses quickly in sandy soils, but drainage can be very slow in clay soils. 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 meters away from the building to prevent water saturation of the foundation and structural movement.
Buy soakaways
Amazon
50 ton soakaway crates
These crates are suitable for landscaping but not driveways, but are easy to install and slot with a 95% void ratio.
B&Q
Spudulika 190 Liter Crate Kit
These crates come in kit form with geotextile and tape. They slot together easily depending on the size required
Amazon
Polyform 190 liter crates
These crates are large and can be joined together depending on the size of soakaway required. They are suitable for handling and load conditions
layers and gradients
Drainage does not just include underground pipes. The way patios, driveways and landscaping are designed around the extension also play an important role in site drainage.
External paving should always slope away from the building so that rainwater flows naturally towards the garden rather than towards the wall. It’s a small detail, but water that can collect around the threshold can find its way into the building.
Permeable pavement Can also help in some situations. stuff like that Resin-bound gravel, which you can easily buy on AmazonOr impervious block paving allows water to flow off the surface rather than directly into creeks or gutters. These materials are not essential for every project, but are an excellent solution where site levels are challenging.
Design your expansion drainage
An easy way to avoid drainage problems is to check existing services before finalizing the extension design. Drainage designs that look perfectly functional on drawings sometimes turn out to be clumsy when actual site levels are considered.
A water and drainage search will usually reveal whether a public sewer crosses the property. While these searches don’t show every pipe in full detail, they provide a useful starting point about where services might be. It is also necessary to check that the installation on the site matches the existing one Drainage Survey Drawings This can be done by lifting the existing manhole cover and observing the direction of the pipe run to establish how the drainage system is arranged.
Depending on what the investigation reveals, 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 a foundation can keep the new structure clear of existing pipes.
If the design is not carefully planned, you may encounter construction problems and unacceptable compromises. For example, an inspection cover is suitable where you want to go over the kitchen island. It is also worth noting that solving these details during the design stage is much easier than dealing with them once excavation has begun.
Most drainage systems rely entirely on gravity. For this reason, each pipe run must be installed with a proper slope. Fall too low and waste water moves slowly, increasing the risk of blockages. Liquids flow steeper than solids, which can leave material behind in the pipe.
The approved document Part H provides recommended gradients based on pipe diameter, but the practical challenge is often coordinating those falls with fixed floor levels and foundation depths.
When drainage systems are properly designed and installed, they disappear into the background, wastewater flows, rainwater dissipates quickly, and the drainage system continues to operate without anyone giving it a second thought.
But when drainage is rushed, or poorly coordinated during construction, then Drainage problems Visible long after the builders are gone. Continuously wet Around the building, slow drainage or frequent blockages are signs that something underground has not been installed properly.
Those problems are rarely easy to fix later, so in my opinion, it’s always worth the extra investment to investigate early, carefully design the drainage and properly coordinate the installation with site conditions, foundation conditions and whatever else your particular plot may present.
