Modifying the Drainage System for an Extension

Why drainage changes are often needed for an extension

Building an extension usually means altering your existing drainage system. The extent of the work depends on what you are adding and where it sits on your plot.

A simple rear extension might only need new guttering and downpipes to handle rainwater from the additional roof area. A kitchen or utility room extension will require new waste pipes connecting to the foul water system. A ground floor extension with a new toilet or bathroom involves more significant pipework. All of these changes must conform to Building Regulations Part H, and properties in conservation areas or flood risk zones may face additional planning requirements.

Ensuring proper drainage is critical to prevent structural and environmental issues. Poor planning can lead to blocked drains, slow drainage, bad smells, or water pooling where it should not. Mistakes can affect neighbouring properties or conflict with water company requirements. In areas prone to flooding, you may also need to consider the Flood and Water Management Act 2010, which sets out responsibilities for managing flood risk.

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Mapping your existing drains

Before making any changes, you need to understand what is already there. Above ground pipework is straightforward to trace. Underground drainage is less obvious.

There are several ways to work out how your drains run beneath the ground:

• Check with your local council. They may hold drainage plans from when the property was built, though not all councils retain these records, particularly for older properties. There is usually a small fee for copies, and available plans might not reflect modifications made after the original construction.
• Contact the local water company. They can provide maps showing public sewers in your area, which helps you understand where your private drains connect to the main system.
• Hire a drainage surveyor. A CCTV drain survey gives you accurate, current information about the layout, condition and depth of your drains. This costs more but removes guesswork.
• Lift manhole covers yourself. If you have inspection chambers in your garden, you can often trace the direction of pipes by looking at the channels inside. This gives you a rough idea but will not show everything.

Even for projects that appear straightforward, professional advice is worth seeking. Failure to comply with regulations can cause significant problems later, and drainage errors are often expensive to fix once construction is complete. A structural engineer or surveyor can also assess whether proposed drainage changes might affect your property's foundations.

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Surface water and foul water: understanding the difference

UK drainage systems typically keep two types of water separate. Surface water, also called rainwater, comes from roofs, driveways and paved areas. Foul water comes from toilets, sinks, baths and appliances like washing machines.

Surface water usually drains into soakaways, watercourses or dedicated surface water sewers. However, regulations specify when a soakaway can be used, and planning permission is sometimes required. You should also check whether Sustainable Drainage Systems (SuDS) regulations apply to your project, as these govern how rainwater runoff is managed to prevent localised flooding. Managing rainwater effectively is increasingly important given changing weather patterns and more frequent heavy rainfall events.

Foul water goes to the foul sewer and on to treatment works. Mixing the two types can overload the sewage system and may breach regulations.

Some older properties have combined systems where both types share the same drain. If your home has this arrangement, check with your water company about requirements for new connections.

For rainwater from a new extension roof, you may be able to use an existing soakaway if it has enough capacity. Otherwise, you might need a new soakaway or a connection to a surface water drain. Your building control officer can advise on what is acceptable for your site.

Permissions and regulations

Drainage work for an extension falls under Building Regulations. Your plans will need to show how foul and surface water will be handled, and the work must meet approved standards.

If your extension will be built over or near an existing public sewer, you will need a Build Over Agreement from your water company. This applies to sewers that the water company maintains, not your private drains. The water company will want to check that the construction will not damage the sewer or restrict future access for maintenance.

For new connections to public sewers, you usually need to apply to the water company. They will assess whether the sewer can handle the additional flow and set out any conditions.

Work on foul drainage should be reviewed by Building Control to ensure compliance with regulations. Connecting to public sewers is not suitable as a DIY project and requires proper approval. Even work on private drains benefits from professional involvement, as errors can be costly to rectify and may cause problems for you or your neighbours.

For current guidance on Building Regulations for drainage, check the Approved Documents on GOV.UK. Part H covers drainage and waste disposal.

If you are using a contractor for the drainage work, check they have relevant experience and appropriate insurance. Trade bodies like the Chartered Institute of Plumbing and Heating Engineering or the National Association of Drainage Contractors can help you find qualified professionals.

The Next Step

Now that you have read through the advice above, you might want to put it into practice. Our Planning Permission Checker lets you tell us about your planned extension and find out whether you need planning permission or can build under permitted development. Try it now →