Extending your home offers a way to grow your living space and potentially increase your property’s value. But an extension, like any other build, must be designed for drainage first and foremost.
So, what do you need to consider before you start construction?
We’ll start with the basics. There are two types of drainage to factor in: foul water and surface water.
Foul water drainage
Introducing additional bedrooms as part of the extension is in effect increasing the amount of people occupying the site and therefore the amount of waste being generated.
However, if the extension is not relating to an increase in people, and the extension is just for additional ‘space’ (e.g. larger kitchen, new study, new utility room, new downstairs toilet etc.) this may not necessarily increase the amount of waste generated; and the existing system may be sufficient. Remember: people generate waste, not the number of appliances when sizing an appropriate treatment system.
We’d advise a thorough assessment of how much wastewater your household currently produces and whether your system can manage the additional flow from the extension.
Surface water drainage
Introducing an impermeable area as part of the extension could contribute to an increase in surface water run-off and therefore could cause additional flood risk, not just to the current dwelling, but to the wider hydrological catchment (e.g. flash floods, erosion etc.).
If the extension increases the footprint of the previous development by 10% or more, a suitable drainage strategy should be reviewed.
Considerations
The existing septic tank/ sewage treatment plant
There are requirements for the size and capacity of septic tanks and treatment plants based on the number of people in your home. If there is an increase in waste flows generated by the extension, the capacity of the current septic tank or treatment plant, or where applicable, the drainage field (soakaway), may not have sufficient capacity. In this case, the current plant may need to be replaced or an additional septic tank/treatment plant installed. This is similar to an existing drainage field, which would have also been sized based on a certain population and may need to be either increased in size (difficult to achieve) or a new drainage field installed, or the existing soakaway replaced.
You will also need to consider the age and condition of your current wastewater system. Older systems may not efficiently handle increased loads and might need upgrading or replacing. Modern package sewage treatment plants are remarkably energy efficient and ecofriendly, treating wastewater more effectively and with less environmental impact – and may prove more cost effective in the long term.
Your drainage network
What is the extent of the existing drainage network supporting the current house? Will the extension interfere with the existing drainage arrangement and how should this be managed during construction? WCI can survey your properties drainage system where required to gain a better understanding of your existing network.
If the existing drainage network is combined (foul and surface water mixed) it’s best to split the flows. When surface water run-off enters a foul drainage system it impacts the treatment capacity of both private and public sewage treatment systems. We see all too regularly in the media the devasting impact these overspills can have when they enter our waterways.
Topography & Permeability
The slope and layout of your property will affect its drainage. Will your extension require a pumped discharge to ensure that flows reach their designated outfall?
Then there’s the soil permeability. When there is no public sewer or water course to discharge treated effluent or surface water to, a discharge to a drainage field or soakaway may be your only options. It is advised that percolation/infiltration testing is completed to ensure feasibility in this scenario.
Space Constraints
The installation of a suitable foul and surface water drainage system to support the extension should consider the space required. Building regulations state that septic tanks and treatment plants shall be 7m from a building, a drainage field 15m from a building and a surface water soakaway 5m from a building. These distances can be quite constraining and should be considered as part of the extension works.
Environmental Permitting
Environmental Permitting Regulations can impact the proposed extension if you’re looking to vary an existing permit due to an increase in flows and if you’re looking to make a new discharge to either a non-British Standard soakaway or a seasonally dry ditch, amongst others.
Conclusion
A well-designed drainage strategy should be the basis of any building project. WCI has a wealth of experience working with self-builders to ensure projects not only comply with local regulations and but are designed for long-term sustainability. If you’d like advice on your drainage design, get in touch at enquiries@wci.co.uk or call 01984 623404.
