Planning a home extension is one of the most significant investments a homeowner makes. Most people spend months considering the architect's drawings, the kitchen layout, the bi-fold doors. Relatively few give the same attention to what is underneath the ground — until the groundworks start and the digger driver hits a clay drain that nobody knew was there.
At Blocked Drains Liverpool we attend a steady stream of extension drainage jobs across Merseyside every year: pre-build surveys for homeowners and architects, build-over agreement inspections for United Utilities, and emergency callouts from builders who have uncovered an unexpected drain mid-dig. The same problems arise again and again, and almost all of them were avoidable with a survey and a conversation before the groundworks began.
What Building Regulations Part H Requires
Building regulations Approved Document Part H governs drainage in and around new construction, including extensions. The key requirements for most domestic extension projects are:
**H1: Foul water drainage.** Any new foul water outlets must drain to an appropriate sewer or cesspool. The gradient, pipe diameter, and access point positions must meet the standard.
**H4: Building over sewers.** Part H4 sets out the conditions under which you can build over or near a public sewer, and requires you to consult the sewerage undertaker — United Utilities for virtually all of Merseyside — before doing so. Your local building control officer will check these requirements at foundation inspection stage.
The 3-Metre Rule Explained
The 3-metre threshold is a trigger for the build-over agreement process, not an absolute prohibition. United Utilities' build-over policy allows construction within 3 metres — and in some cases directly over a public sewer — provided the design meets their requirements for access, loading, and pipe protection.
Violating the 3-metre rule without a build-over agreement is not just a regulatory problem — it is a civil liability. If your extension causes damage to the public sewer, United Utilities can require you to remediate at your own cost.
What Happens If You Build Without a Build-Over Agreement
Several times each year we attend Merseyside properties where a previous extension was built without the required build-over agreement. At the serious end: the extension has loaded a shallow clay sewer that was already cracked, the sewer has deformed or partially collapsed under the load, sewage is percolating into the ground beneath the extension floor slab, and the remediation involves a partial dig-out from within the property. We have attended several of these in the terrace streets around Walton and Bootle where rear extensions were added without building regs in the 1980s and 1990s.
On Victorian terraced streets throughout Merseyside, the back-yard drainage runs that serve two or more terraced properties often became public sewers under the 2011 transfer. Many homeowners do not know their shared back-entry drain is now a public sewer.
Getting the Drainage Plan from United Utilities
Before instructing a CCTV drain survey, obtain a drainage record from United Utilities via their Asset Register portal. This shows the location, size, and ownership of public sewers in the vicinity of your property. Treat this as a starting point, not a definitive picture — the only way to confirm the position of a drain in the ground is to prove it physically with a CCTV camera and sonde locator.
What a Pre-Build CCTV Survey Reveals
An independent pre-build drain survey establishes the exact position and depth of drain runs under or near the proposed extension footprint, the condition of existing pipework, and the location of manholes — all of which must remain accessible after the extension is built.
On Victorian terraces with rear extensions, it is common to find a shared back-entry gully drain running within 1 to 2 metres of the rear wall line — directly in the path of a typical rear extension footprint. We find this regularly on Bootle and Kirkdale streets. The solution is usually a diversion — bringing the drain run around the outside of the extension footprint — but that diversion needs to be designed, built to spec, and approved by United Utilities before the foundations go in.
Common Mistakes on Merseyside Permitted Development Jobs
**Assuming permitted development means no drainage obligations.** It does not. Part H applies. The build-over rule applies.
**Building without checking the back-entry drain.** On Merseyside terraced streets, the back-entry shared drain is one of the most common obstacles to a straightforward rear extension. It often runs exactly where the new back wall footings want to go.
**Not retaining manhole access.** Manholes must remain accessible after the extension is built. We have attended properties in Huyton and Knowsley where a manhole has been buried under a concrete base slab, creating a building regulations failure.
**Assuming the drain is private.** Post-2011, many shared back-entry drains in Merseyside terraces are public sewers. If you assume it is private and build over it without a build-over agreement, you may find out it is public when United Utilities sends an inspection team.
If you are planning a Merseyside extension and have not yet established the position, depth, and ownership of the drains in the footprint, commission a survey before groundworks begin. Request a quote online from Blocked Drains Liverpool to arrange a pre-build drain survey. We cover all of Merseyside and can usually attend promptly.
