architects FROM LONDON Drainage Design
Drainage Design for Extensions, Renovations and Residential Projects
↓
RIBA Chartered
Award winning
London · Vienna · Rome
Featured in The Times
Panel at Grand Designs Live
Drainage Design intro
Drainage Design for Extensions, Renovations and Residential Projects
Drainage is easy to overlook at the design stage, but it can have a major impact on how a project is planned, approved and built.
Extensions, renovations and other residential works can affect existing drains, rainwater routes, inspection chambers and connection points.
Tholos can help coordinate drainage design with the architectural layout so that the project works technically as well as visually.
Drainage Design explained
What Is Drainage Design?
Drainage design is the process of planning how foul water and surface water are safely removed from a property.
Depending on the project, this may include:
- foul drainage
- surface water drainage
- rainwater disposal
- new drainage runs
- altered drainage runs
- inspection chambers
- manholes
- connection points
- soakaways
- drainage around extensions
The drainage strategy needs to work with the existing property, the proposed design and the relevant Building Regulations requirements.
Drainage Design explained
Why Drainage Design Matters
Poor drainage planning can cause problems such as:
- clashes with foundations
- inaccessible manholes
- difficult pipe routes
- poor falls
- flooding
- delays during construction
- additional excavation
- unexpected cost
It is much easier to coordinate drainage during the design stage than to solve these problems once construction has started.
Why Drainage Design Matters
Poor drainage planning can cause problems such as:
- clashes with foundations
- inaccessible manholes
- difficult pipe routes
- poor falls
- flooding
- delays during construction
- additional excavation
- unexpected cost
It is much easier to coordinate drainage during the design stage than to solve these problems once construction has started.
Drainage Design for Extensions
Extensions often affect existing drainage because new foundations and floors may cross existing drainage routes.
The design may need to consider:
- existing drains under the proposed extension
- relocation of manholes
- new foul drainage
- rainwater drainage
- connection to existing systems
- access for maintenance
- foundation conflicts
This should be understood before excavation begins.
Drainage Design for Renovations
Renovations may also affect drainage, particularly where kitchens, bathrooms or utility spaces are moved.
This can involve:
- new waste routes
- altered soil stacks
- new connections
- changes in floor levels
- rerouting drainage
- coordination with existing services
The drainage design should support the new layout rather than being treated as an afterthought.
Drainage Design explained
Foul Drainage
Foul drainage carries wastewater from:
- toilets
- sinks
- showers
- baths
- kitchens
- utility rooms
The design needs to provide suitable falls, connections and access for maintenance.
Where layouts change significantly, the new drainage route may need careful coordination with the structure and existing building.
Surface Water Drainage
Surface water drainage deals with rainwater from:
- roofs
- gutters
- downpipes
- paved areas
- hard landscaping
Depending on the site, surface water may be managed through:
- existing drains
- soakaways
- attenuation
- other approved methods
The correct solution depends on the property, ground conditions and existing drainage arrangement.
Rainwater Design
Extensions and roof alterations increase or change the amount and location of rainwater runoff.
The design may therefore need to consider:
- gutter positions
- downpipe locations
- roof falls
- connection points
- discharge routes
- soakaways
These elements should be coordinated with the appearance of the building as well as the technical requirements.
Drainage Design
Existing Drainage Surveys
Before altering drainage, it can be useful to understand what is already there.
Existing drainage may not match old drawings or assumptions.
Depending on the project, investigation may include:
- locating manholes
- tracing drainage routes
- identifying connection points
- checking levels
- CCTV drainage surveys
This can be especially useful where the proposed extension sits over or close to existing drains.
Drainage and Foundations
Foundations and drainage can easily conflict.
For example:
- a new footing may cross an existing drain
- drainage may pass too close to foundations
- a manhole may fall inside a proposed room
- drainage levels may conflict with floor levels
Architectural, structural and drainage information should therefore be coordinated early.
Drainage Design explained
Drainage and Structural Calculations
Drainage routes can also affect structural design.
Where pipes pass through or near structural elements, the engineer may need to consider:
- foundation depth
- trench positions
- load transfer
- openings
- local strengthening
Drainage Design for Building Regulations
Drainage is one of the technical areas considered under Building Regulations.
The design may need to demonstrate:
- suitable pipe routes
- appropriate gradients
- access for clearing blockages
- correct connections
- safe discharge of wastewater
- suitable rainwater disposal
The level of detail required depends on the work.
Drainage Design and Full Plans Applications
For a Full Plans application, drainage information may form part of the technical submission.
This can include:
- drainage layouts
- pipe sizes
- connection points
- manholes
- rainwater disposal
- notes and details
Drainage Design and Building Notice
Where a Building Notice route is used, drainage still needs to comply with the Building Regulations.
Building Control may ask for additional information during construction.
This can include drawings or clarification of drainage routes.
Building Control Inspections for Drainage
Building Control may inspect drainage before it is covered.
This can include:
- pipe runs
- bedding
- connections
- falls
- inspection chambers
- drainage testing
It is important that inspections happen before trenches are filled or floors are closed.
Manholes and Inspection Chambers
Drainage systems need suitable access points for maintenance and cleaning.
The location of manholes and inspection chambers should be considered carefully.
Problems can arise where:
- a chamber sits inside a proposed extension
- access is blocked
- floor levels conflict
- landscaping covers access points
The layout should make future maintenance practical.
Drainage Design
Building Over or Near Existing Drains
Extensions can sometimes be proposed over or close to existing drains.
This may require:
- design changes
- drainage relocation
- structural coordination
- additional approvals
- consultation with the relevant drainage or sewer authority
The correct approach depends on the ownership and location of the drain.
Shared Drains and Sewers
Some properties share drainage with neighbours.
This can affect what alterations are possible and what approvals may be needed.
Before changing or building near a shared drain, it is important to establish:
- ownership
- route
- access
- connection points
- any additional requirements
Soakaways
A soakaway allows surface water to disperse into the ground.
It may be suitable where ground conditions allow.
The design can depend on:
- soil permeability
- site size
- distance from buildings
- groundwater
- drainage area
A soakaway should not simply be positioned wherever space is available without considering these factors.
Drainage Design explained
Drainage on Sloping Sites
Sloping sites can make drainage more complex.
The design may need to consider:
- levels
- retaining structures
- gravity flow
- discharge points
- surface water movement
Accurate site and level information can therefore be important.
Drainage and Ground Levels
Small changes in external ground levels can affect drainage performance.
The design should consider:
- finished floor level
- external paving
- garden levels
- threshold levels
- drainage falls
- water movement around the building
Poor coordination can create water ingress or difficult drainage routes.
Drainage Design explained
Drainage and Technical Drawings
Drainage should be shown clearly within the wider technical drawing package.
This helps coordinate:
- pipe routes
- pipe routes
- foundations
- walls
- structural elements
- external works
Accurate site and level information can therefore be important.
Drainage Design for Builder Pricing
Drainage can represent a significant cost, particularly where:
- existing drains need relocation
- deep excavation is required
- new chambers are needed
- connections are complicated
- the project involves shared drains
Providing clear drainage information before tendering can reduce allowances and assumptions.
Drainage Design explained
What If Existing Drainage Is Different From the Drawings?
This is common in older properties.
If site conditions differ from the assumed layout, the design may need to change.
The project team may need to:
- survey the drainage
- revise the route
- adjust foundations
- update drawings
- coordinate with Building Control
This is why drainage should be investigated early where it is likely to affect the design.
Drainage Problems Discovered During Construction
If drainage problems are discovered once work has started, they should be addressed before construction progresses too far.
Typical issues can include:
- damaged pipes
- unexpected routes
- insufficient falls
- hidden chambers
- connections in poor condition
A revised solution may be needed.
Drainage Design
Drainage and Completion Certificates
Building Control may need to be satisfied that drainage work has been completed correctly before issuing a Completion Certificate.
Keeping records of inspections and any relevant testing can help avoid delays at the end of the project.
Drainage and Fire Safety
Drainage and fire safety are separate design areas, but both form part of the wider Building Regulations process.
They should be coordinated alongside structure, insulation and ventilation rather than considered independently.
Drainage Design explained
Already Have Drainage Information?
If another architect, engineer or contractor has already prepared drainage information, Tholos can coordinate it with the architectural design.
We can help check whether:
- the drainage route fits the proposed layout
- foundations conflict with drains
- access remains practical
- the information is suitable for Building Regulations or construction
Need Drainage Design for Your Project?
Tell us what work you are planning and what information you already have about the existing drainage.
Tholos can help coordinate the drainage strategy with the architectural and structural design and prepare the project for Building Regulations and construction.
Client experience
"
Lorem ipsum dolor sit amet, consectetur adipiscing elit. We had a vision for our home but no idea how to realise it. Tholos Interiors took complete ownership of the project — every detail was considered, every decision explained. The result exceeded everything we had imagined.
– Lawrence Ipsum, Homeowner · London –
Frequently Asked Questions
Can I build over an existing drain?
Sometimes, but this depends on the drain, its ownership, location and any additional approvals required.
Do I need a drainage survey?
Not always, but it can be useful where the existing drainage route is uncertain or likely to conflict with the proposed work.
Is drainage part of Building Regulations?
Yes. Drainage is one of the technical areas considered under Building Regulations.
Can drainage affect foundation design?
Yes. Drainage and foundations need to be coordinated carefully to avoid conflicts.
Do builders need drainage drawings?
Clear drainage information can help builders price and construct the work more accurately.
Can Tholos coordinate drainage with the architectural design?
Yes. We can coordinate drainage with the layout, structure and wider technical drawing package.
Disclaimer
This article is part of a series created by our dynamic marketing team for informational purposes only. While we strive to review all content thoroughly, occasional oversights may occur, even after publication. Therefore, we cannot accept liability for any interpretations or use of the information provided. For the most up-to-date information or to report any discrepancies, please contact us at hello@tholos-architects.com.
Start your project