The short answer
SUDS (Sustainable Drainage Systems) is the approach of managing rainwater close to where it falls — letting it soak into the ground, storing it, or releasing it slowly — instead of piping it straight to a sewer. It exists to reduce flooding and pollution, and it shapes your groundworks because surface water now follows a hierarchy: infiltrate into the ground first (a soakaway), then to a watercourse, and only as a last resort to a surface water sewer. In practice this means features like soakaways, permeable paving, attenuation crates, swales or rain gardens rather than a simple connection. For an extension, SUDS can add cost and dig — a soakaway needs space and a percolation test — but it is now the expected way to deal with roof and paving runoff under Part H and local planning policy. It affects what you build, how much you dig, and what the local authority will approve.
SUDS is less a single product than a way of thinking about rainwater. For groundworks it changes where the water goes and what you have to install. Here is the practical impact.
SUDS in practice
- What it isManage rainwater on-site
- AimCut flooding & pollution
- HierarchyGround → watercourse → sewer
- Common featuresSoakaway, permeable paving, attenuation
- AffectsDig, cost, planning approval
The surface water hierarchy
SUDS turns the old habit of piping rainwater to the nearest sewer on its head. The expected order of preference is:
- Infiltration into the ground (soakaway, permeable paving) — first choice.
- Discharge to a watercourse — where infiltration is not feasible.
- Surface water sewer — last resort, often with restrictions on the rate.
This is why your designer cannot simply assume roof water goes to the sewer: they have to demonstrate why each higher option is or is not feasible, usually starting with a percolation test for infiltration.
The features and their groundworks impact
Different SUDS features mean different digging, space and cost.
| Feature | What it does | Groundworks impact |
|---|---|---|
| Soakaway / infiltration crate | Soaks water into ground | Pit dig, 5m+ from building, percolation test |
| Permeable paving | Lets rain through the surface | Special sub-base build-up |
| Attenuation tank/crate | Stores then releases slowly | Large buried unit + flow control |
| Swale / rain garden | Surface channel/planted basin | Landscaping, gentle gradients |
Indicative features and impacts. The right one depends on ground and space. Source: GOV.UK SUDS technical standards.
How SUDS affects approval and cost
For most domestic extensions, SUDS is handled through the drainage design under Part H and any local planning conditions; larger or major developments can trigger formal SUDS approval and adoption arrangements. The cost impact is real but proportionate: a soakaway or permeable paving build-up costs more than a bare pipe to the sewer, and attenuation with flow controls costs more again. Against that, managing water on-site can avoid a difficult or refused sewer connection and reduce flood risk to your own property. The practical message for groundworks is to establish your surface water strategy — and test the ground — before finalising levels and the dig.
SUDS and your driveway: the rule people miss
One part of SUDS catches homeowners out more than any other: front gardens and driveways. Because hard, impermeable surfaces shed rainwater straight onto the road and into already-stretched drains, there are rules about paving over your front garden. In England, if you create or replace a driveway of more than five square metres using impermeable materials that drain to the road, you generally need planning permission. If instead you use a permeable or porous surface, or direct the runoff to a permeable area or soakaway on your own land, permission is usually not required. This is SUDS in everyday form — keeping rainwater on your property rather than overloading the public system.
- Permeable paving: blocks, gravel or porous asphalt over a free-draining sub-base let rain soak through where it falls.
- Drain to a soakaway or border: a solid surface can still avoid permission if its runoff is directed to a soakaway, lawn or planted border rather than the road.
- Larger schemes: on bigger projects, attenuation and swales spread the same principle across the whole site.
For groundworks this matters because the surface water strategy should be set before levels and surfaces are finalised. Designing a soakaway or permeable build-up in from the start is far cheaper than discovering, after an extension and new driveway are laid, that the runoff has nowhere acceptable to go. Whether through Part H drainage design or a planning condition, the consistent message is the same: deal with rainwater on-site first, and treat a sewer connection as the last resort rather than the default.
The four SUDS aims and what they mean for a small project
SUDS is usually explained through its four design aims — quantity, quality, amenity and biodiversity — and even on a domestic extension these explain why the local authority asks for more than a pipe to a drain.
- Water quantity: control how much and how fast rainwater leaves the site, so the extension does not increase flood risk downstream. On a small job this is the soakaway or attenuation that holds back a downpour and releases it slowly.
- Water quality: let the system filter pollutants — grit, oil, leaves — before the water reaches the ground or a watercourse. Permeable paving and silt traps do this on a driveway scale.
- Amenity: on larger schemes, features like rain gardens and swales double as usable, attractive landscape rather than hidden pipework.
- Biodiversity: planted SUDS features create habitat, which is why swales and basins are favoured over buried tanks where space allows.
For a single extension, the first two aims dominate — managing the rate and volume of runoff and keeping it clean — while amenity and biodiversity come into their own on bigger developments. The practical consequence is that your surface water cannot simply be piped away as fast as possible; the design has to show it is held back and, ideally, cleaned and infiltrated on-site. This is why a soakaway plus a percolation test, or a permeable driveway build-up, has become the normal expectation rather than an optional extra. Setting the SUDS strategy at design stage — deciding where runoff goes and proving the ground can take it — keeps both the groundworks and the planning approval straightforward, and avoids retrofitting drainage into a finished site where there is no longer room for it.
Frequently asked questions
Is SUDS a legal requirement for my extension?
Surface water must be managed in line with Part H's hierarchy and any local planning policy, which strongly favours on-site management. Formal SUDS approval mainly affects larger developments, but the principles apply to extensions too.
What is the simplest SUDS feature for a house?
A soakaway for roof and paving runoff is the most common domestic SUDS feature, provided a percolation test shows the ground drains well enough and there is space at least 5m from buildings.
Does SUDS cost more than a normal drain?
Usually a little more, because features like soakaways, permeable paving or attenuation cost more than a simple pipe. But they can avoid a difficult sewer connection and reduce flood risk, which has its own value.
Sources & further reading
- GOV.UK — sustainable drainage systems: non-statutory technical standards
- Planning Portal — Approved Document H (drainage)
- LABC — surface water and SUDS
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.