Can I Extend Over or Near a Sewer?
One of the most overlooked parts of planning a home extension is understanding whether there are existing public sewers or drains running within or near the proposed build area.
For many homeowners, this only becomes apparent later in the process once drainage surveys, technical drawings or foundation designs begin to develop. However, identifying drainage constraints early on can help avoid delays, redesign work and unexpected costs later in the project.
The good news is that extending near or even over a sewer is often still possible — it simply needs to be approached properly.
What Is a Public Sewer?
A public sewer is a drainage pipe maintained by the local water authority, commonly carrying wastewater or surface water from multiple properties.
Public sewers can often run:
Along rear gardens
Between neighbouring properties
Beneath extensions
Across shared access routes
Many homeowners are unaware a public sewer exists until beginning extension plans or technical investigations.
How Do I Find Out If There Is a Sewer on My Property?
One of the most common ways is by obtaining a sewer records search from the local water authority.
For properties across Worthing and much of Sussex, this is often through Southern Water.
Drainage information may also become apparent through:
Existing manholes
Drainage surveys
Measured surveys
CCTV drainage inspections
Technical design stages
Understanding drainage routes early on is often extremely helpful when developing extension layouts.
Can I Build Over a Sewer?
In many cases, yes.
Homeowners can often build near or over public sewers, although approval from the relevant water authority may be required depending on:
The size of the sewer
Its depth and location
The proposed foundations
Access requirements
Structural protection measures
This is commonly known as a build-over agreement.
What Is a Build-Over Agreement?
A build-over agreement is formal approval from the water authority allowing construction near or over a public sewer.
The process typically involves submitting:
Technical drawings
Foundation information
Drainage layouts
Structural details
The purpose is to ensure the sewer remains protected and accessible where necessary.
Will a Sewer Affect the Extension Design?
Potentially, yes.
Drainage positions can sometimes influence:
Foundation layouts
Structural arrangements
Extension positioning
Drainage rerouting
Construction methodology
However, this does not necessarily prevent development altogether.
Carefully coordinated technical design can often resolve drainage constraints successfully.
What About Existing Manholes?
Existing inspection chambers or manholes within the proposed extension area often require careful consideration.
Depending on the project, these may:
Remain externally accessible
Be relocated
Be replaced internally with sealed inspection chambers
Drainage coordination is therefore usually an important part of the Building Regulations stage.
Is Drainage Considered During Planning Permission?
Planning applications themselves often focus primarily on the overall proposal and planning considerations rather than detailed drainage design.
Drainage coordination is usually addressed more fully during the technical design and Building Regulations stage.
However, understanding sewer positions early on can still be extremely valuable during the design process itself.
Why Early Technical Coordination Matters
One of the biggest advantages of understanding drainage constraints early on is avoiding unnecessary redesign work later during technical stages or construction.
Residential projects often involve several overlapping considerations including:
Structural coordination
Drainage
Foundations
Approaching these elements together early in the process often leads to smoother overall project delivery.
Navigating Residential Projects Clearly
Drainage, sewers and technical constraints can initially feel intimidating for homeowners beginning a residential project.
At Downland Studio, projects are approached collaboratively and practically, helping homeowners across Worthing, West Sussex and the South Coast navigate residential extensions and alterations clearly from early ideas through to technical design and construction-ready drawings.
If you’re considering extending or altering your home and would like some honest advice before getting started, feel free to get in touch for an informal conversation.