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.
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 to 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.