Drainage survey (foul and surface water drainage assessment)
Drainage survey
Above and below ground drainage assessed together: layout, condition, ownership and adoption, and where responsibility for a shared run actually sits.
Last reviewed 31 August 2026
What does a drainage survey cover?
A drainage survey looks at how a property gets rid of foul and surface water. It covers above-ground pipework, gullies, inspection chambers and their condition, the apparent layout of the below-ground system, evidence of blockage or surcharge, and any private treatment arrangements such as a septic tank. It also addresses the question owners most often get wrong: which parts of the run you are responsible for.
At a glance
- Two systems
- Foul drainage and surface water — sometimes combined, sometimes separate.
- Ownership
- Most shared and downstream pipework transferred to sewerage companies in 2011.
- Off-mains
- Septic tanks and treatment plants carry their own regulatory obligations.
- Relationship to CCTV
- This is the assessment; camera survey is the instrument that confirms it.
Who owns which pipe
This is the source of most drainage disputes. Broadly, the pipework within the boundary of a property that serves that property alone remains the owner's responsibility. Pipework that serves more than one property, and pipework beyond the boundary, largely transferred to the statutory sewerage companies in 2011.
The practical consequence is significant. A collapsed shared run in the road is normally the water company's problem; the same collapse in a single-property run under your own garden is yours. Establishing which you are looking at is often worth more than the condition report itself.
What the above-ground inspection finds
Rainwater and soil pipework is a common and cheap source of building damage. Blocked or misaligned gutters saturate walls, defective hopper heads discharge behind render, and soil stacks with failed joints wet masonry continuously.
Where a damp or defect survey has flagged persistent external wetting, drainage above ground is very often the actual cause, and it usually costs a fraction of what the damp remedy would have.
- Gutters, downpipes, hoppers and their discharge points
- Gullies, back inlets and rodding access
- Soil and vent pipework and visible joints
- Surface water disposal, soakaways and evidence of ponding
Off-mains drainage
Where a property is not connected to a public sewer, it will use a septic tank, a small sewage treatment plant or, occasionally, a cesspool. These carry regulatory obligations that differ between England, Wales, Scotland and Northern Ireland, and rules about discharges to surface water have tightened.
For a buyer this is a due diligence item as much as a condition item: the location and capacity of the system, how it discharges, when it was last emptied and whether the arrangement currently complies all matter, and none of them appear on a standard survey by default.
Who this is for
A good fit if
- Buyers of properties with shared drainage, off-mains systems or a history of flooding
- Owners with recurring blockages, slow drainage or smells
- Anyone planning an extension over or near a drain run
- Owners in dispute with a neighbour or water company about responsibility
Not the right choice if
- Clearing a blockage — that is a drainage contractor's job, often the same day
- Producing a legal determination of ownership; that is for your solicitor and the water company
- Structural assessment of subsidence, even where a leaking drain is suspected
Working out what you actually need
Drainage questions divide neatly into condition, ownership and capacity — and each has a different answer.
1.Describe the symptom precisely
Recurring blockage, standing water, smell and slow flow point to quite different causes.
2.Establish the layout and who owns each section
There is no point surveying pipework your sewerage company is responsible for maintaining.
3.Add a camera survey where a below-ground defect is likely
Assessment identifies where to look; the camera confirms what is there.
CCTV drain surveys4.Escalate to a structural view if movement is involved
A long-term leak near foundations can be a subsidence question rather than a drainage one.
Crack and movement surveys
When to bring in a specialist
- Clearance, repair, relining and excavation: a drainage contractor.
- Ownership determination and adoption records: your sewerage company and your solicitor.
- Structural calculations, load paths and remedial design: a structural engineer. A surveyor describes and diagnoses; it does not design.
Common questions
Limitations and sources
- Below-ground pipework is assessed from accessible chambers and visible evidence only unless a camera survey is separately commissioned.
- Drainage layouts are frequently undocumented, and no survey can guarantee every run has been identified.
- Regulatory duties for off-mains drainage differ across England, Wales, Scotland and Northern Ireland; confirm the position for your jurisdiction.
- Nothing on this page is a professional opinion about a specific property. KJ Surveyors has not inspected your building, and general guidance cannot replace an inspection and report.
Source: UK Government · checked August 2026
Source: Ofwat · checked August 2026
Source: Environment Agency · checked August 2026
Source: RICS · checked August 2026
Last reviewed 31 August 2026.
Where to go next
- CCTV drain surveys — the camera-based confirmation of below-ground condition.
- damp surveys — where defective rainwater goods are wetting the building.
- our building survey work — the broader inspection this often sits within.
Drainage problem or unknown layout?
Tell KJ the symptom, the property type and whether the drainage is shared or off-mains. KJ will explain what needs surveying and who owns it.
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KJ gives general guidance only and has not inspected your property. A professional opinion follows an inspection by a surveyor.