What it is
A formal agreement with the water authority — Thames Water in London — giving consent for your building work to sit over or close to a public sewer. Separate from Building Control.
When you need one
Usually within three metres of a public sewer. The water authority decides based on the sewer's size, depth, age and material, plus the proposed loading.
Cost and time
Hundreds to low thousands of pounds depending on the sewer. Eight to twelve weeks is not unusual. Start it the day the design is firm — not the week before excavation.
Frequently asked
A retrospective agreement is possible but harder and more expensive. CCTV survey of the sewer is usually required.