On a small site, choosing SuDS usually comes down to three workhorse techniques: soakaways, permeable paving and attenuation. Getting the choice right — and evidencing it correctly — is what turns a drainage strategy from a planning risk into a straightforward approval. This guide explains how each works, when to use it, and how the 2026 national standards for sustainable drainage now shape the decision in England.
Sustainable drainage systems (SuDS) manage rainwater close to where it falls, mimicking natural drainage rather than piping everything straight to a sewer. For householder extensions, new dwellings and small infill plots, a proportionate SuDS approach is now expected as part of the planning submission, not an afterthought.
Why SuDS matter more in 2026
Surface water is the flood risk most small applications underestimate. In June 2026 the government introduced new national standards requiring SuDS to be designed for changing climatic conditions while delivering wider benefits — flood prevention, reduced storm overflows, water reuse, improved water quality, amenity and biodiversity. Notably, Schedule 3 of the Flood and Water Management Act 2010 has still not been commenced in England; the approach remains delivered through the planning system and local plan policy rather than a separate SuDS Approval Body. We covered the wider picture in SuDS and your FRA in 2026: national standards and Schedule 3.
The practical upshot: even a modest scheme is expected to show that runoff is managed on site, ideally without increasing the rate or volume leaving the plot. That is where the choice of technique comes in.
Soakaways: infiltrate where the ground allows
A soakaway lets surface water infiltrate into the ground rather than discharge to a sewer or watercourse. It is the drainage hierarchy's first preference — and often the simplest solution on a small site — but only where the ground can actually take the water.
- Best for: sites with permeable subsoils (sands, gravels) and a water table comfortably below the base of the soakaway.
- The evidence you need: an infiltration test to BRE Digest 365, giving a soil infiltration rate that sizes the soakaway for the design storm.
- Watch out for: clay soils, high groundwater, contaminated land or proximity to foundations — any of which can rule a soakaway out.
Where infiltration works, a soakaway can discharge runoff to zero, which is the strongest possible position in a drainage strategy.
Permeable paving: drainage that doubles as a surface
Permeable paving replaces impermeable driveways, patios and parking with a surface that lets water pass through into a sub-base that stores and slowly releases it. For small residential sites it is often the most elegant option because it uses space you were paving anyway.
- Best for: driveways, courtyards and parking areas on new dwellings and extensions — especially useful where front gardens are being hard-surfaced.
- Added benefit: it can provide both attenuation storage and a degree of water-quality treatment as water filters through the layers.
- Watch out for: maintenance — the surface must be kept clear of silt to keep working, and this should be written into the maintenance plan.
Permeable paving also helps with a point many applicants miss: replacing a traditional impermeable driveway with a permeable one can remove the need for separate planning permission that a non-permeable front-garden surface might otherwise trigger.
Attenuation: store now, release slowly
Where infiltration is not viable, attenuation is the fallback. The principle is simple — store peak rainfall temporarily, then release it to the sewer or watercourse at a controlled, restricted rate. Attenuation can be provided by underground crates or tanks, oversized pipes, permeable paving sub-bases, or above-ground features such as rain gardens and basins.
- Best for: sites where the ground cannot infiltrate but a discharge point exists.
- The key figure: the discharge rate must be limited — ideally to the greenfield runoff rate, or as close to it as the site allows. Greenfield runoff rates matter because they represent what the land would have shed before development.
- Watch out for: flow-control devices need to be specified and maintained, and storage must be sized against climate-change allowances.
How to choose: the drainage hierarchy
The decision follows the SuDS drainage hierarchy, which prefers, in order: discharge into the ground (infiltration), then to a surface watercourse, then to a surface-water sewer, and only as a last resort to a combined sewer. In practice on a small site:
- Test the ground first — if it infiltrates, a soakaway (often combined with permeable paving) is usually the cleanest answer;
- If infiltration fails, use attenuation with a restricted discharge rate;
- Combine techniques where helpful — permeable paving over an attenuation sub-base is a very common small-site solution.
All of this should be designed with reference to CIRIA C753, the SuDS Manual, and set within a flood risk assessment where one is required. For the difference between the two documents, see our explainer on what a flood risk assessment is and why planning requires one.
How Fortress Associates can help
Fortress Associates produces automated, site-specific flood risk assessments for planning applications in England, using live Environment Agency data and following the EA's FRA template (March 2025). Each report includes a SuDS drainage strategy to CIRIA C753 and the Non-Statutory Technical Standards for SuDS, tailored to whether your site suits infiltration, permeable paving or attenuation. The report is free and ready in minutes — ideal for householder extensions, new dwellings and small-scale applications. Start on our flood risk assessment page or get in touch through our contact page.
Sources & further reading
Need help with a UK planning project?
Fixed-fee daylight reports and Building Regulations drawings — delivered in 4–5 working days. No advance payment.
Request a free quote