Planning an extension? Check your drains before the foundations
Extensions get held up over drainage more often than over planning. Three checks at design stage prevent nearly all of it.
Check one: where the drain actually runs
Not where the plans say, and not where the builder assumes. Drain runs get altered over decades and rarely get recorded. The only reliable answer is a camera and a locator, which gives you the line and the depth marked on the ground.
Finding a drain with a digger bucket is expensive twice over: the repair, and the delay while everyone works out who pays for it.
Check two: is there a public sewer involved
Building over or close to a public sewer requires a build-over agreement with the water company, which for nearly all London addresses means Thames Water. This is the single most common reason extensions stall.
Since 2011 a great deal of pipework that owners assume is private actually transferred to the water company, so the presence of a sewer under a garden surprises people regularly. Sewer records are indicative rather than exact, which is why tracing on site matters as well.
We cover the process in building over a sewer.
Check three: can the existing system take it
An extension usually adds fittings, and often a second bathroom or a utility room. The question is whether the run it connects into has both the capacity and the fall to carry the extra.
The related trap is surface water. A new roof and a paved terrace both produce run-off, and pushing that into a foul drain overloads a system that was sized for one household. Separating surface water at design stage costs very little; retrofitting it later costs a great deal.
What the regulations expect
Below-ground drainage falls under Part H of the Building Regulations, with BS EN 752 covering drainage outside buildings and BS EN 12056 gravity systems inside them. New runs should be tested before backfilling, and that test is the step most often skipped.
Anything found while the trench is open is a small problem. The same thing found afterwards is an excavation.
Ask for an as-built record showing what went in and where the access points are. It saves money on every future job at the property.
The extension that builds over its own drain
Very common in London terraces. A side-return extension goes over the original run, and five years later a blockage needs reaching from under a kitchen floor.
It is not always avoidable, but it is always worth knowing in advance, because the answer is usually to reroute at build stage or to add an access point that will still be reachable. We see the consequences constantly in Fulham, Wandsworth and Balham.
More on drainage installation and what we design around.