This section has been written for homeowners who may be considering building an extension or making alterations. It outlines the main impacts that drainage may have on your project. If you decide to proceed with your project, your builder will need to look at Approved Document H for detailed advice on how to comply with Building Regulations.
What is drainage?
There are two systems of drainage that you need to think about: ’foul‘ and ‘surface water’. In general, these two systems should be kept separate.
Each of these has above-ground and underground elements.
Foul drainage carries the used water from toilets, sinks, basins, baths, showers, bidets, dishwashers and washing machines. The above-ground pipework is referred to as sanitary pipework; the underground pipework is referred to as foul drains and foul sewers.
Surface water drainage carries rainwater (and melted snow and ice) from hard surfaces. The above-ground system of gutters and rainwater pipes is referred to as roof drainage; the underground pipework is referred to as surface water drains and surface water sewers.
What is the difference between a drain and a sewer?
In general, a drain serves a single property whereas a sewer serves more than one property.
Private sewers are owned by the properties they serve. Public sewers are owned by the sewerage undertaker (whose address can be found on your sewerage bill). Building work on and around a sewer needs permission of the sewer owner.
Why do I need to think about underground drainage?
You may have to change your plans to suit the depth and location of the underground drain or sewer that you intend to connect to.
If you intend to build over or close to a public sewer, you will require written agreement from your sewerage undertaker, so you should consult the company at the earliest planning stage of your building work.
Building over an existing drain or sewer can damage pipes, so that they leak or block, potentially leading to odour nuisance, health problems and environmental damage. It also makes it more difficult, time consuming and expensive to clear blockages and repair or replace faulty drains. So if there is an existing drain below, or close to, your proposed extension, it may need to be moved or protected, which is likely to increase the cost of your project.
The route of the drain should avoid obstructions (eg. ponds or outbuildings) and keep away from foundations, so may need to be longer and have additional access chambers, rather than running in a straight line. Approved Document H gives guidance on additional measures needed where drains have to run close to foundations.
In order to carry the flow and to avoid blockages, the drain or sewer that you intend to connect to generally needs to be at least 0.8m lower than the ground floor level. If it is less than this, you should seek advice from a builder, architect or drainage engineer.
How can I find out the location of underground drains and sewers?
Maps of public sewers can be inspected free of charge at the offices of the sewerage undertaker or local authority. Private sewers and drains are not normally mapped and their location needs to be found in other ways, as described below.
Drain covers give an indication of drains below. By lifting the cover, it may be possible to see the direction, size and depth of pipes but do not enter the chamber (which can be filled with toxic gas) and ensure that the cover is replaced securely.
Locations of rainwater pipes, sanitary pipework stacks and external gullies can indicate where their underground drains are likely to run.
There are many firms which can carry out CCTV surveys that will indicate the condition of the drains as well as their location and depth.
You are strongly advised to seek advice from a builder, architect, drainage engineer or Local Authority Building Control before committing to or commencing work.
I’m thinking about increasing the size of my roof.
You may need to increase the size of your gutters and rainwater pipes, or add new rainwater pipes. Information on sizing gutters and rainwater pipes is given in Approved Document H.
Additional rainwater pipes can discharge onto the ground, or into new or existing underground pipework. If you decide to allow rainwater pipes to discharge onto the ground, you need to make sure the water will not damage foundations (eg. by encouraging it to spread out over a wide area) or flow onto neighbouring property (eg. by providing a slight lip at the boundary).
A larger roof area will increase the amount of surface water. It is preferable to keep the extra volume on site, in order to avoid increasing flood risk elsewhere. Rainwater can be kept on site by using a soakaway or some other way of allowing it to soak into the ground (referred to as infiltration), or stored and used for toilet flushing or garden watering (known as rainwater harvesting). Approved Document H gives advice on where to site soakaways, how large they should be and how they should be built.
Where it is impractical to use infiltration (eg. because of nearby foundations, impermeable or contaminated ground, or high groundwater), it is preferable to discharge it to a watercourse or, failing this, to a surface water sewer or, as a last resort, to a combined sewer. Surface water must not be discharged into a foul drain or sewer.
I’m considering a new patio or driveway.
In order to avoid increasing flood risk elsewhere, it is preferable for these to be sloped towards permeable ground or to be made of pervious materials. Pervious materials include both porous materials (eg. as reinforced grass or gravel, porous concrete or porous asphalt) and permeable materials (eg. clay bricks or concrete blocks, designed to allow water to flow through joints or voids). As well as minimising environmental impact, this avoids the cost of drainage.
Surface water from hardstandings must not be allowed to run onto the highway, where it could lead to accidents or cause a nuisance.
Where it is impractical to drain onto pervious ground or use a pervious paving, it is preferable to keep the extra surface water on site, in order to avoid increasing flood risk elsewhere. This can be achieved by using a soakaway or some other way of allowing it to soak into the ground (referred to as infiltration).
Approved Document H gives advice on sizing soakaways. Where it is impractical to use infiltration (eg. because of nearby foundations, impermeable or contaminated ground, or high groundwater), it is preferable to discharge it to a watercourse or, failing this, to a surface water sewer or, as a last resort, to a combined sewer. Surface water must not be discharged into a foul drain or sewer.
What size pipes do I need from kitchen and bathroom appliances?
Pipes need to be sized for the flow of water, to minimise the risk of blockage and to allow air movement. Advice on pipe sizes is given in Approved Document H.
Sanitary pipework should be designed with access hatches, or be capable of being dismantled, in order to deal with blockages.
Rodding eyes and access chambers should be used to enable all parts of the underground drainage to be cleared and to allow removal of blockages.
Do I need a ventilating pipe through the roof?
Sanitary pipework needs to be ventilated to avoid air from the pipework and drains from escaping into the building.
The normal way of doing this is to extend the pipework (known as the ventilating pipe) to outside the building, leaving the end open (but protected with a mesh to prevent birds getting in). To stop smells entering a building, the open end of the ventilating pipe should be at least three metres to the side of, or extended to 0.9m above, any opening into a building.
If the drainage is already ventilated, additional ground floor appliances (eg. a WC and washbasin) may be connected directly to the drain without a ventilating pipe.
It may be possible to use proprietary valves to avoid the need for ventilating pipes.
Disclaimer - Wales
This is an introductory guide and is not a definitive source of legal information. This guidance relates to the planning regime for Wales. Policy in England may differ. If in doubt contact your Local Planning Authority.