SuDS for Homeowners: Extensions, Driveways and Gardens

Sustainable drainage isn’t just for big developments. If you’re building an extension, paving a driveway or improving your garden, where the rainwater goes matters for Building Control, for planning and for your neighbours. Here’s a practical guide from the drainage engineers at Kelp Studio.

Extensions and new roofs
A new extension adds roof area, and that rainwater has to go somewhere. Building Regulations Part H sets an order of preference:
1. A soakaway or other infiltration into the ground, where the soil allows
2. A watercourse, such as a stream or ditch
3. A sewer, only if neither of the above is practical
Building Control will usually want to see that you’ve considered a soakaway first. If the extension is near a public sewer, you may also need a build-over agreement from the water company.

Soakaways
A soakaway is a pit filled with rubble or plastic crates that lets rainwater soak slowly into the ground. To size one properly we carry out a percolation test to BRE 365, digging a trial pit, filling it with water and measuring how quickly it drains. Soakaways are usually placed at least 5m from the house and don’t work well on heavy clay or where groundwater is high, so testing first avoids an expensive mistake.

Driveways and front gardens
In England, paving more than 5 square metres of a front garden with a hard surface that drains onto the road needs planning permission. Permeable options, such as permeable block paving, gravel or porous asphalt, or directing water to a lawn or border, avoid this and reduce flooding.

Simple SuDS for any home
• Water butts: collect roof water for the garden and slow the flow into drains.
• Rain gardens: planted dips that take water from a downpipe and let it soak away.
• Green roofs: planted roofs on extensions and garden rooms hold back rainfall and improve insulation. Check the structure can carry the extra weight.
• Permeable paving: lets water drain through patios and paths.
• Planting instead of paving: every square metre of soil and planting helps.

What if the ground won’t drain?
On clay or where groundwater is high, rainwater can be stored in an underground tank or crates and released slowly to a sewer through a flow control, at a rate agreed with the water company or council.

Common questions

Do I need a drainage design for my extension?
Often, yes. Building Control will ask how the new roof water is disposed of, and many councils ask for a drainage plan with planning applications. We can provide a percolation test, soakaway design and drainage layout.

Can I connect my new extension’s rainwater to the existing drains?
Sometimes, but Part H expects a soakaway to be considered first. Connecting surface water to a foul-only sewer isn’t allowed.

My extension is near a sewer. What do I need?
Building over or near a public sewer usually needs a build-over agreement from the water company. We prepare the drawings and handle the application.

Ask us about your project or call 020 3835 2633

SuDS and drainage design →
SuDS explained: a plain-English glossary →
Is my property at risk of flooding? →