Drainage guide
Do I need a build-over agreement for my extension?
You need a build-over agreement if you build over, or within 3 metres of, a public sewer — or within 1 metre of a public lateral drain (Building Regulations Approved Document H4). Planning permission does not cover this; the water company requires a CCTV condition survey before it will grant the agreement.
Typical cost: £200–£500 for a domestic build-over agreement, plus £250–£500 for the CCTV condition survey.
What is the 3-metre rule?
Building Regulations Part H4 requires water-company approval to build over, or within 3 metres of, a public sewer — or within 1 metre of a public lateral drain (Thames Water). Planning permission does not grant permission to build over a sewer.
Self-certify or formal agreement — which do I need?
For small domestic sewers (160mm or under) a self-certification route is often available. Larger or deeper sewers need a formal build-over agreement, which can take from a few weeks to several months (Thames Water Class 3: 3–9 months).
What does the water company need from me?
A CCTV condition survey of the sewer before construction, and again after completion, in the format the company requires. This confirms the sewer is sound before you build over it and undamaged afterwards.
Nearly half of public sewers in the Thames Valley region do not appear on records because they were privately owned before 2011 (FastPlansUK) — so check before you build.
What happens if the survey finds a defect?
A defect usually has to be repaired before the agreement is granted. Getting the survey done early means the repair is quoted and completed without stalling your build — one enquiry can cover both the survey and any remedial work.
Which water company handles my area?
Build-over agreements route to your regional sewerage company — Thames Water, Severn Trent, United Utilities, Yorkshire Water, Anglian, Wessex, Southern, South West, Northumbrian, Welsh Water or Scottish Water. Each publishes its own fee schedule and process.
UK drainage in numbers
The scale of the problem
Sources: Water UK, Ofwat and the ABI (2024–2025). Figures are UK-wide and rounded.
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