Rear extensions, kitchen extensions and garage conversions often end up over or close to a drain. Finding out early avoids delays, extra costs and problems later.
Private drain or public sewer?
This makes a big difference:
- Private drains serve only your property. Building control will want to know how the drain will be protected and how access will be kept.
- Public sewers include most drains shared with neighbours, as many were transferred to water companies in 2011. Building over or near them needs the water company’s agreement. See who is responsible for a shared drain.
In terraced and semi-detached houses, the drain behind the house often serves neighbours too, which can make it a public sewer even though it is in your garden.
Build over agreements
Southern Water, which provides sewerage for most of Hampshire, requires an application if you plan to build over a public sewer or within 3 metres of one. Wessex Water covers Bournemouth, Poole, Christchurch and Salisbury and has its own process. Check the current requirements, fees and timescales on the relevant water company’s website before work starts.
The water company may approve the plans, ask for changes, or in some cases require the sewer to be diverted.
Building Regulations
Building Regulations (Part H) cover building over or near drains and sewers. Your building control body will check that:
- the drain is protected from the weight of the new structure;
- the drain can still be accessed for maintenance;
- inspection chambers are not lost, or are moved or replaced appropriately; and
- the drain is in good enough condition to be built over.
Why a survey first makes sense
A CCTV drain survey before building:
- shows exactly where the drain runs and how deep it is;
- confirms whether it is a private drain or a shared sewer;
- reveals defects such as cracks, roots or displaced joints. Repairing a drain before it is built over is far cheaper than after; see drain repairs;
- gives you a record of its condition before work starts, useful if anything goes wrong later.
During the build
- Make sure builders know where the drains and chambers are.
- Keep debris, concrete and mortar out of the drains. Building waste is a common cause of blockages after extensions.
- Protect chambers from heavy machinery.
After the build
A post-construction survey shows that the drain was not damaged during the work and confirms it is clear. See also drain surveys before buying a house and how to find your drains.