Soakaways and Sustainable Drainage for London Home Extensions

A soakaway is a pit filled with rubble or a proprietary plastic crate system, into which surface water drains and then slowly percolates into the surrounding ground. Soakaways work well in permeable soils such as chalk, sand, and gravel, where water infiltrates quickly enough for the soakaway to empty before the next significant rainfall event. In London, the dominant soil type is London Clay — a dense, low-permeability clay that was deposited across most of Greater London during the Eocene epoch. London Clay has an extremely low hydraulic conductivity (water movement rate), which means soakaways in London Clay fill up quickly and then cannot drain fast enough to recover capacity before the next rain. In practice, a soakaway in London Clay either becomes waterlogged and ceases to function, or discharges water into the surrounding ground so slowly that it provides no meaningful attenuation during a rainfall event. Building Regulations (Part H — Drainage and Waste Disposal) require that soakaways be proven to work by infiltration testing before approval. BRE Digest 365 describes the testing methodology: a trial pit is excavated to the proposed depth of the soakaway, flooded with water, and the rate of water level fall is measured over a series of test cycles. Where the measured infiltration rate is too low (a coefficient of permeability below 1 × 10-6 m/s is generally considered too slow for effective soakaways), Building Control will not approve a soakaway-based drainage scheme. In the vast majority of London locations, infiltration tests confirm that soakaways are not feasible, and an alternative drainage route must be found.

The National Planning Policy Framework (NPPF) and associated planning policy guidance require that surface water drainage from new development is managed sustainably, following a hierarchy: 1. Infiltration to ground (soakaway or rain garden) — first preference, but only where infiltration testing confirms the ground is permeable enough. 2. Discharge to a watercourse (stream, ditch, or river) — where a watercourse is available within a reasonable distance and connection is feasible. 3. Discharge to a surface water sewer — where a public surface water sewer exists in the highway. 4. Discharge to a combined sewer — last resort only, and subject to Thames Water approval. In London, most residential extensions end up at step 3 or 4 of this hierarchy because: step 1 fails infiltration testing; step 2 is not available (no accessible watercourse near the property); and step 3 is only available where a separate surface water sewer runs in the highway (many London streets have combined sewers only). Thames Water operates the public sewer network in most of London. Connecting new surface water drainage to the combined sewer requires Thames Water's approval, and they may require attenuation (slowing down the discharge rate) to protect the sewer from overloading. Planning authorities in London increasingly require a SuDS drainage strategy as either a pre-application consideration or a pre-commencement planning condition, particularly for extensions that create more than a modest increase in impermeable surface area.

Where infiltration is not feasible and a direct sewer connection is the only route for surface water, planning conditions often require SuDS mitigation measures to reduce the rate and volume of runoff compared to an uncontrolled discharge. Practical measures that can be incorporated into a London home extension include: Permeable paving — using permeable block paving, gravel, or resin-bound gravel for any associated hard standing (driveway, patio, path) in place of impermeable concrete or tarmac. Water that infiltrates through permeable paving into a sub-base layer drains slowly rather than running off immediately. Permeable paving cost: £50-£120 per m² depending on product. Green roofs — a sedum or wildflower green roof on a flat-roofed extension holds approximately 40-60mm of rainfall in the growing medium before discharging, significantly attenuating peak runoff rates. Green roofs also provide biodiversity net gain (a planning requirement in London from April 2024) and improve thermal performance. Green roof cost: £80-£160 per m² depending on system depth and vegetation type. Water butts and rainwater harvesting — a water butt (or larger underground rainwater harvesting tank) on the extension downpipe stores roof runoff for garden irrigation, reducing the volume of water discharged to the sewer. A 200-litre water butt costs approximately £30-£80; a 1,500-litre underground tank costs £1,500-£3,000 installed. Rain gardens — a planted depression in the garden that receives roof or patio runoff and allows it to infiltrate slowly over several hours after a rainfall event. Even in London clay, a well-designed rain garden with an amended soil mix can accept and infiltrate reasonable volumes of water, particularly if overflow is directed to a soakaway in a gravel layer above the clay. Rain garden cost: £500-£3,000 depending on size and planting specification.

Thames Water has a statutory role in approving certain drainage works in London, separate from the planning authority and Building Control. Thames Water build-over approval (also called sewer build-over agreement or Section 185 agreement) is required when a proposed extension is built over, or within three metres of, a public sewer. This is a common issue for rear extensions in London, where the public sewer often runs along the rear boundary of the properties. Thames Water's technical standards for building over sewers require: no load-bearing foundation within 500mm of the sewer centreline in most cases; a protective concrete slab or bridging arrangement where the extension crosses directly over the sewer; inspection chambers (manholes) to remain accessible or be relocated to a position accessible without entering the extension; and structural integrity of the sewer to be maintained throughout construction. A Thames Water build-over survey (typically carried out by CCTV drainage camera, cost £200-£500) is usually required to assess the existing sewer condition before approval is granted. The build-over application itself is submitted online to Thames Water and typically takes 6-8 weeks for a response. For new drainage connections (connecting the extension's surface or foul water drainage into the public sewer), Thames Water must be notified under the Water Industry Act 1991. For significant new connections or modifications, a formal application is required. Thames Water has the right to refuse connections that would overload the public sewer.

A drainage strategy is a technical document prepared by a drainage engineer or civil engineer that sets out how surface water and foul water from a proposed development will be managed, discharged, and maintained. For large residential developments (more than 10 houses) and commercial development, a full drainage strategy including hydraulic modelling is typically required by the planning authority. For individual home extensions, a drainage strategy is typically required when: the extension is significant in size and creates a substantial increase in impermeable surface area; the planning authority has imposed a pre-commencement drainage condition; the property is in a flood risk zone (1b, 2, or 3); or there is a build-over agreement issue requiring technical resolution with Thames Water. For householder extensions where drainage is straightforward (connecting to an existing gully or drainage channel that flows to a functioning sewer), a full drainage strategy may not be needed — the drainage details can be shown on the building drawings and approved by Building Control as part of the Building Regulations process. If you receive a planning condition requiring a drainage strategy, a drainage engineer (consulting engineer or specialist drainage consultant) can prepare this document. Cost: typically £1,500-£4,000 depending on complexity and whether hydraulic calculations are required.