4th Floor
Silverstream House
45 Fitzroy Street
London
W1T 6EB
Silverstream House
45 Fitzroy Street
London
W1T 6EB
Structural Surveys
Structural Design
Civil Engineering
SuDS & Drainage Design
How We Work
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Contact
4th Floor, Silverstream House, 45 Fitzroy Street, London, W1T 6EB
E: info@kelpstudio.co.uk
T: 020 3835 2633
M: 07561 417319
Soakaway Design & Calculations
Soakaway design for extensions, new homes, garden rooms and driveways across London, Surrey, Sussex, Kent and Essex. As civil engineers, we size the soakaway from your infiltration test results and provide the calculations, drawings and specification that Building Control, the planning officer and your builder need, typically within 2 working days.Get a quote or call 020 3835 2633
What you get
• Soakaway calculations based on BRE 365 infiltration test results
• A drawing showing the soakaway’s position, size, depth and construction, whether crates or a rubble-filled pit, with inspection access and any overflow
• A specification your builder can price and build from
• Answers to any questions from Building Control or the planning officer
Guide price and turnaround
• Soakaway design: typically £400 – £800 inc. VAT, depending on the size and complexity of the site. We confirm a fixed fee before you instruct us.
• Calculations and drawings: typically within 2 working days of instruction or our site visit, once infiltration test results are available.
Infiltration testing
A soakaway has to be designed from a test of how quickly the ground drains, carried out to BRE 365 in a trial pit on your site. We don’t carry out the testing ourselves, but we can recommend a specialist testing contractor we always use, who can also carry out CCTV drainage surveys. We then design the soakaway from their results.
When do you need a soakaway design?
• Building an extension, garden room or new home that adds roof area
• A planning condition asks for a surface water drainage strategy or SuDS
• Building Control asks for soakaway calculations
• Paving a front garden or driveway, where water needs somewhere to go
Building Regulations Part H expects rainwater to go to a soakaway or other infiltration system first, where the ground allows, before a watercourse or a sewer.
Where can a soakaway go?
• At least 5 m from buildings, so the water doesn’t soften the ground under foundations
• On chalk, we typically keep soakaways 10 m from all foundations, as chalk can soften and form cavities when it gets wet
• Not in made ground, near unstable slopes or where the water table is high
• With care near drinking water sources: much of the chalk in Sussex, Kent and Surrey lies within Source Protection Zones
The ground across our area
• London and Essex: mostly clay, which drains very slowly, so soakaways often aren’t possible and an alternative is needed. Drainage design in London →
• Brighton and the South Downs: chalk, which usually suits soakaways, with the 10 m rule above. Soakaways on chalk →
• Sussex and Kent Weald: a mix of clays and sandstones, so results vary from site to site. Sussex → · Kent →
• Surrey: chalk on the North Downs, with clays and sands elsewhere. Surrey →
What if the infiltration test fails?
It’s common on clay, and there’s usually a good alternative. We design:
• Attenuation tanks or crates that store water and release it slowly to a sewer, with the water company’s agreement
• Permeable paving, rain gardens and planters
• Water butts and rainwater harvesting to cut the volume that needs to go anywhere
• A connection to a nearby watercourse, with consent where needed
SuDS and drainage design →
Our drainage work
• Rotherfield Close, Brighton: structural design and below-ground drainage for a new-build house, with a Section 106 sewer connection application to Southern Water.
• Combermere Road, Hastings: structural design and below-ground drainage specification for a new-build house, plus help discharging the planning conditions.
Civil engineering →
Common questions
How far does a soakaway need to be from my house?
Usually at least 5 m from any building, and we typically allow 10 m on chalk. On a tight site we’ll look at where it can fit, or at an alternative.
What’s the difference between a percolation test and a BRE 365 test?
Both measure how quickly water drains into the ground from a trial pit. BRE 365 is the method used to design surface water soakaways. Percolation tests to British Standards are mainly used for septic tank and treatment plant drainage fields.
Can I just connect my rainwater to the sewer?
Only if infiltration isn’t practical. Building Regulations and most councils expect you to try a soakaway first, and the water company has to agree to a new connection.
Do you design drainage fields for septic tanks and treatment plants?
Yes. We size the drainage field from the percolation test and specify the system, and the manufacturer designs and installs the unit. More on off-mains drainage →
12+ years’ experience · £1,000,000 PI insurance · Calculations typically within 2 working days
SuDS and drainage design →
Flood risk assessments →
Drainage design in Brighton & Hove →
Get a quote or call 020 3835 2633
• At least 5 m from buildings, so the water doesn’t soften the ground under foundations
• On chalk, we typically keep soakaways 10 m from all foundations, as chalk can soften and form cavities when it gets wet
• Not in made ground, near unstable slopes or where the water table is high
• With care near drinking water sources: much of the chalk in Sussex, Kent and Surrey lies within Source Protection Zones
The ground across our area
• London and Essex: mostly clay, which drains very slowly, so soakaways often aren’t possible and an alternative is needed. Drainage design in London →
• Brighton and the South Downs: chalk, which usually suits soakaways, with the 10 m rule above. Soakaways on chalk →
• Sussex and Kent Weald: a mix of clays and sandstones, so results vary from site to site. Sussex → · Kent →
• Surrey: chalk on the North Downs, with clays and sands elsewhere. Surrey →
What if the infiltration test fails?
It’s common on clay, and there’s usually a good alternative. We design:
• Attenuation tanks or crates that store water and release it slowly to a sewer, with the water company’s agreement
• Permeable paving, rain gardens and planters
• Water butts and rainwater harvesting to cut the volume that needs to go anywhere
• A connection to a nearby watercourse, with consent where needed
SuDS and drainage design →
Our drainage work
• Rotherfield Close, Brighton: structural design and below-ground drainage for a new-build house, with a Section 106 sewer connection application to Southern Water.
• Combermere Road, Hastings: structural design and below-ground drainage specification for a new-build house, plus help discharging the planning conditions.
Civil engineering →
Common questions
How far does a soakaway need to be from my house?
Usually at least 5 m from any building, and we typically allow 10 m on chalk. On a tight site we’ll look at where it can fit, or at an alternative.
What’s the difference between a percolation test and a BRE 365 test?
Both measure how quickly water drains into the ground from a trial pit. BRE 365 is the method used to design surface water soakaways. Percolation tests to British Standards are mainly used for septic tank and treatment plant drainage fields.
Can I just connect my rainwater to the sewer?
Only if infiltration isn’t practical. Building Regulations and most councils expect you to try a soakaway first, and the water company has to agree to a new connection.
Do you design drainage fields for septic tanks and treatment plants?
Yes. We size the drainage field from the percolation test and specify the system, and the manufacturer designs and installs the unit. More on off-mains drainage →
12+ years’ experience · £1,000,000 PI insurance · Calculations typically within 2 working days
SuDS and drainage design →
Flood risk assessments →
Drainage design in Brighton & Hove →
Get a quote or call 020 3835 2633