Soakaway vs mains connection for surface water: which is better?

Rainwater from roofs and paving has to go somewhere, and the two routes are into the ground or into the sewer. UK rules and your soil decide which is allowed. Here is how a soakaway and a mains connection compare.

How each option works

A soakaway is a sub-surface structure — traditionally a rubble-filled pit, now more often modular plastic crates wrapped in geotextile — that collects rainwater from roofs and hard surfaces and lets it percolate gradually into the surrounding soil. It manages surface water on your own land, recharging groundwater and easing pressure on the sewer network. For it to work, the ground must absorb water at a reasonable rate and the soakaway must be sized for the area it drains and sited a safe distance from buildings.

A mains connection takes the same surface water and pipes it into the public sewer — either a dedicated surface-water sewer or, on older combined systems, the same sewer that carries foul drainage. It is simple in concept and works regardless of ground conditions, but it adds flow to the sewer network, which is exactly what current policy tries to reduce, and connecting surface water to a sewer is increasingly restricted and may require the sewerage company's permission.

Indicative comparison for guidance. Rules vary by area and connection consent.

The drainage hierarchy and the rules

UK guidance does not treat the two as equal choices. Part H of the Building Regulations sets out a surface-water drainage hierarchy: disposal should be considered first by infiltration to the ground (a soakaway), then to a watercourse, and only as a last resort to a sewer. Sustainable drainage thinking has reinforced this, because piping rainwater into sewers contributes to overload and to combined-sewer overflows during heavy rain. So for new builds, extensions and significant paving, a soakaway or other on-site solution is the expected approach unless it can be shown the ground cannot take it.

Connecting surface water to the public sewer is increasingly controlled. The sewerage company may need to consent, and on combined systems they often discourage adding more surface water. There is also a financial dimension: properties whose surface water drains to the public sewer are typically charged a surface-water drainage element in their water bill, whereas a property draining entirely to a soakaway can often apply to have that charge removed.

Which suits your property

Ground conditions are the practical deciding factor. Where the soil is free-draining — sand or gravel — and the water table is well below the soakaway, a soakaway works well, keeps surface water on site, avoids the sewer charge, and aligns with the preferred drainage hierarchy. A percolation test demonstrates the ground drains fast enough and informs the size needed for the roof and paved area being served.

Where the ground is heavy clay, the water table is high, or there is insufficient space at a safe distance from buildings and boundaries, a soakaway may not cope, and an alternative is needed — a watercourse connection where one is available, attenuation and slow release, or, as a last resort, a sewer connection if the company permits it. A mains connection removes the dependence on ground conditions but adds to the sewer load, usually carries the ongoing surface-water charge, and may simply not be allowed for new surface water in some areas. In short, test the ground first: a soakaway is the preferred and usually cheaper-to-run option where the soil allows it, and a mains connection is the fallback where it does not.

Sizing, siting and maintaining a soakaway

A soakaway only works if it is sized and sited correctly. The size depends on the area of roof and hard surface draining into it and on how quickly the ground absorbs water, which is why the percolation test result feeds directly into the design — a slow-draining soil needs a larger soakaway to cope with the same rainfall. Undersizing is a common cause of failure, where the structure fills faster than it can empty during heavy rain and surface water backs up. Modular crate soakaways have largely replaced rubble-filled pits because they hold more water in less space and are easier to size to a calculation.

Siting rules protect buildings and neighbours. A soakaway must be a safe distance from the property and from boundaries so that water dispersing into the ground does not undermine foundations or flow onto adjoining land, and it should not be placed where it could affect a neighbour's property or a watercourse inappropriately. The Building Regulations and good practice set out minimum distances, and on some ground types — for example where there is a risk to foundations or where contamination could reach groundwater — a soakaway may not be permitted at all, making an alternative necessary.

Soakaways also need occasional thought over their life. Silt and fine debris can gradually reduce how well the ground around them accepts water, so keeping gutters and gullies clear of leaves and grit, and fitting silt traps where appropriate, helps a soakaway keep working. A soakaway that has stopped coping — standing water, slow dispersal, or flooding in heavy rain — may be undersized, silted up, or sited in ground that no longer drains as assumed, and may need investigation or replacement. By contrast a mains connection needs little maintenance by the householder but carries the ongoing charge and adds to sewer load, which is the trade-off for its independence from ground conditions.

Frequently asked questions

Sources & further reading

• gov.uk — Building Regulations Approved Document H (drainage)
• Water UK — surface water and drainage

Figures on this page are typical UK ranges drawn from published cost guides and are intended as guidance, not a quotation.