Will a soakaway work on chalk? Drainage in Brighton & Hove

3 November 2026 · Blog

Building an extension or a new home in Brighton or Hove? You’ll need somewhere for the rainwater from the new roof and hard surfaces to go. On most sites the first option is a soakaway, and because most of the city sits on chalk, the answer is often yes. But chalk comes with rules of its own.

Why chalk is different
Chalk usually drains well, which is good news for soakaways. But concentrating water in one spot can soften the chalk, wash fine material away and, over time, open up voids. If that happens close to a building, it can undermine the foundations.

The 10 metre rule
In most soils, a soakaway needs to be at least 5 metres from any building. On chalk, we typically keep soakaways at least 10 metres from all foundations, including your neighbours’. On a typical Brighton terrace or semi with a modest garden, that can be hard to achieve, so it’s worth checking early, before the design is fixed.

Protecting the drinking water
The chalk under Brighton & Hove is also an important source of drinking water, and much of the city lies within groundwater Source Protection Zones. Clean roof water is generally fine, but runoff from drives and parking areas may need treatment before it soaks into the ground.

How we test and design a soakaway
1. Desk check: ground conditions, Source Protection Zones, sewer records and space on the plot.
2. Percolation test: a trial pit is filled with water and the drainage rate measured, following BRE 365.
3. Design: we size the soakaway for the roof and paved areas, with an allowance for climate change, and position it a safe distance from foundations.
4. Approval: we provide the calculations and drawings for Building Control and, where needed, for planning.

If a soakaway won’t work
• Permeable paving that lets water soak through over a wider area
• Attenuation, storing water in a tank or crates and releasing it slowly
• Rainwater harvesting for toilets and garden watering
• As a last resort, a limited connection to the sewer, agreed with Southern Water

Common questions

Do I need a percolation test for an extension?
Usually, yes, if you plan to use a soakaway. Building Control will want to see that it will work.

Can I use the old soakaway in my garden?
Only if it’s in good condition, far enough from the buildings and big enough for the extra roof area. Many older soakaways are too small or too close to the house.

Can I build over a sewer?
Not without Southern Water’s approval first. Build-over agreements explained →

Drainage and SuDS design in Brighton & Hove →
Structural engineers in Brighton & Hove →
SuDS for homeowners →
Get a drainage quote or call 020 3835 2633