Soakaways and Sustainable Drainage in London: A Planning Guide for Homeowners
With increasing pressure to reduce surface water runoff entering the London sewer network, soakaways and sustainable urban drainage systems are becoming relevant for more London homeowners and developers. This guide explains how soakaways work, when they are appropriate, and what planning requirements apply.
What Is a Soakaway and How Does It Work
A soakaway is an underground pit or chamber filled with coarse aggregate, rubble, or purpose-made plastic crates that receives surface water runoff and allows it to disperse gradually into the surrounding ground through the base and walls of the pit. Instead of discharging surface water directly into the sewer network or a watercourse, a soakaway holds the water temporarily and releases it at a rate that the ground can absorb, reducing peak flow rates and the volume of water entering the drainage system during heavy rain events.
Traditional soakaways for domestic use typically consist of a pit excavated to a minimum depth of one metre below invert level, filled with broken brick rubble or coarse gravel and covered with a geotextile membrane and backfill. Modern soakaway systems use purpose-built modular polypropylene crates that provide a higher void ratio than rubble fill, allowing a given volume of water to be stored in a smaller excavation footprint. The crates are wrapped in a geotextile membrane before backfilling to prevent fine soil particles from migrating into the void space and reducing its capacity over time.
When Soakaways Are Appropriate for London Properties
The suitability of a soakaway depends entirely on the permeability of the ground at the proposed installation location. London geology is variable. The clay-dominated soils of inner and outer London — particularly the London Clay formation that underlies much of the capital — have very low permeability and do not support soakaway drainage. Water infiltrates through clay so slowly that a soakaway in clay soil will fill and overflow during any significant rainfall event rather than dispersing the water effectively. Attempting to install a soakaway in impermeable London Clay is not only ineffective but can also cause waterlogging and structural problems in adjacent foundations.
Soakaways are more feasible in the areas of north and east London where terrace gravels and sand deposits overlie the clay, and in some outer London locations where more permeable subsoils are present at accessible depths. A soil percolation test — a simple site test that measures the rate at which water drains from an excavated hole — is mandatory before any soakaway is sized or installed. The test is described in BRE Digest 365 and takes approximately two to three hours to complete on site. The result is expressed as a percolation value and determines whether a soakaway is feasible and, if so, what size is needed to handle the design rainfall event for the catchment area.
Sustainable Drainage Systems and London Planning Policy
Sustainable urban drainage systems (SuDS) are a broader category of drainage approaches that aim to manage surface water closer to where it falls, reduce peak runoff rates, and improve water quality before discharge. In addition to soakaways, SuDS measures include permeable paving, green roofs, rain gardens, swales, and attenuation tanks. London planning policy, set out in the London Plan and individual borough local plans, requires that new developments and significant extensions manage surface water runoff in accordance with SuDS principles where feasible.
For householder planning applications in London — extensions, outbuildings, and driveways — the requirement to demonstrate SuDS compliance varies by borough and by the scale of the proposed impermeable surface area. Front garden hardstanding works that create more than five square metres of impermeable surface in a non-permitted development situation require planning permission, and planning officers may require the applicant to demonstrate how surface water will be managed. Permeable paving — block paving with open joints, gravel, resin-bound surfaces, or grass paving systems — allows rainfall to pass through the surface into the subsoil and is the most straightforward SuDS measure for residential driveways and paths.
Connecting to the Sewer as an Alternative
Where ground conditions make soakaway drainage impractical and a permeable surface is not feasible, surface water can be discharged to the sewer network with Thames Water approval. Surface water connections to the combined sewer require a build-over or drain connection agreement from Thames Water, and in some areas of London new surface water connections to the combined sewer are restricted or prohibited as part of the effort to reduce sewer flooding during storm events. The Thames Tideway Tunnel — the new super sewer beneath the River Thames — is designed to address the long-term capacity shortfall in the London combined sewer system, but reducing the volume of surface water entering the network in the first place remains a planning and environmental priority.
Prestige Engineers design and install soakaway systems and surface water drainage for London homeowners and developers, including percolation testing, soakaway sizing calculations, and full installation with appropriate geotextile membranes and inspection access. We advise on the feasibility of soakaway drainage at any proposed London site and provide alternative surface water management solutions where ground conditions exclude soakaway use.