Building and Extensions
Extension Drain Survey: Avoid a Costly Mistake Before You Build
If you are planning an extension, a CCTV drain survey before work starts tells you where the drains run, what condition they are in and whether you need a build-over agreement. Skipping it is one of the most common, and most expensive, mistakes homeowners make, because putting things right under a finished floor costs far more than checking first.
Need a drainage engineer? Call Drain 247 on 0208 387 5591. Answered 24/7, no call-out fee, and the price is agreed before we start. See drain mapping for extensions and new drain installations.
Why Drains Matter Before an Extension
Sewers and drains often run under back gardens, exactly where rear extensions go. If a pipe is under the footprint, your designer needs to know its position and depth to design the foundations, and your sewerage company may need to approve the work.
Do You Need a Build-Over Agreement?
Thames Water, which runs the sewers across much of North West London and Hertfordshire, says you are likely to need a build-over agreement if you are building within 3 metres of a public sewer, or within 1 metre of a public lateral drain. It also points out that:
planning permission does not give you permission to build over a sewer or drain, and permitted development does not either;
liability for any damage caused to the sewers or drains during construction lies with you;
retrospective agreements may not be approved if the pipework was not properly protected, and without an agreement you may have issues selling your property later.
Thames Water’s current fee for a Class 1 application (sewers up to 175mm) is £130. Its technical specification also says no manholes or inspection chambers will be built over or sit inside the new structure, so a chamber in the way usually has to be moved. Our build-over agreements guide explains the process.
Three Costly Problems a Survey Helps You Avoid
1. a Refused or Delayed Sign-Off
Building control and your sewerage company will want to know the position and condition of drains near the works. Finding a sewer halfway through the dig means redesigns and delays.
2. a Damaged Pipe Under the New Floor
Old clay and pitch fibre pipes can crack under diggers and new foundations. Without “before” footage it is hard to show whether a pipe was already damaged or broken during the build, and repairs under a finished extension mean breaking out floors.
3. Problems When You Sell
A buyer’s solicitor will ask whether you built over a public sewer and whether you had consent. A missing agreement can hold up the sale.
Your Pre-Build Drainage Checklist
CCTV survey and drain mapping of every run near the extension, with depths. See CCTV drain surveys and drain mapping.
Share the results with your architect or structural engineer before foundations are designed.
Apply for a build-over agreement if the works are within the distances above.
Plan any manhole relocation or repair before the build. Our drain repair and drain installation teams can do this.
Survey again after the build to show the pipes were not damaged.
For the Hertfordshire rules in more detail, read extension drain rules in Hertfordshire.
Frequently Asked Questions
How Much Does a Pre-Extension Drain Survey Cost?
Our domestic CCTV surveys are from £150, with the price agreed before we start. Mapping is quoted to the property.
When Should I Book It?
Before the foundations are designed, ideally at the planning stage, so the drawings reflect what is really underground.
Can I Build Over My Own Private Drain?
A build-over agreement is for public sewers and lateral drains. Building over a private drain still needs building control approval, and access must be kept.
Book Your Pre-Extension Survey
Call Drain 247 on 0208 387 5591. We survey and map drains across Watford, St Albans, Harpenden and North West London.
