Drainage strategies for planning applications in London

What is a drainage strategy and when do you need one?

A drainage strategy is a report explaining how a development will deal with rainwater and wastewater: where it goes, how much there is, and how the design avoids making flooding worse. Councils commonly ask for one with planning applications for new dwellings, conversions creating new units, basements and larger extensions, and it is effectively always required for major developments. If it is not submitted with the application, it often reappears as a planning condition that must be discharged before work starts.

Requirements vary between boroughs and depend on the site, for example whether it lies in a flood zone or an area prone to surface water flooding. The council's validation checklist, or the planning officer, will confirm what is needed for your application.

Surface water and foul water

A strategy normally covers two separate flows. Surface water is rain falling on roofs, paving and other hard surfaces. Foul water is wastewater from toilets, kitchens and bathrooms. They are treated differently, and in newer systems they run in separate pipes. The volumes behave differently too: foul flows are fairly steady, while surface water arrives in short, intense bursts during storms, which is what the strategy has to manage.

Much of inner London is served by older combined sewers that carry both, and these can be overwhelmed in heavy rain. That is why councils focus on surface water: every extra square metre of roof or paving sends more rain into the system unless something slows it down. Our surface water drainage strategy and foul drainage strategy can be prepared separately or together, depending on what the council has asked for.

The drainage hierarchy

The London Plan sets out a drainage hierarchy, an order of preference for where surface water should go. In broad terms it runs from most to least preferred as follows:

The strategy has to show how the design works down this list, and explain why each higher option is not practical before moving to the next. Policy also encourages developments to restrict their runoff towards the rate the site would produce if it were undeveloped, with larger schemes held most closely to that aim.

• Store rainwater for reuse, for example in water butts or harvesting systems
• Let it soak into the ground close to where it falls
• Hold it back in green features such as green roofs, rain gardens or ponds
• Discharge it to a watercourse
• Controlled discharge to a surface water sewer or drain
• Controlled discharge to a combined sewer, as a last resort

What the strategy contains

The level of detail matches the scale of the scheme. A single new house needs a concise report with supporting calculations, while a block of flats needs fuller calculations and a drawn layout. A typical strategy covers:

The runoff calculations go through a three-step quality control process, with final sign-off by our Head of Engineering, because errors in storage volumes tend to surface only when an officer checks them.

• The site, its existing drainage and where it currently discharges
• Hard and permeable areas before and after development
• Runoff calculations for a range of storm events, with an allowance for climate change
• The proposed discharge route and, where needed, a restricted discharge rate
• Attenuation storage volumes and how they will be provided
• Infiltration test results where soakaways are proposed
• Foul flows and the proposed connection to the sewer
• An outline layout and arrangements for maintenance

Why soakaways often do not work on London Clay

Infiltration sits high in the hierarchy, but London Clay drains very slowly, so soakaways in clay are often ineffective. They can also cause harm: water held in the ground near a building can soften the clay around its foundations, which is why soakaways are kept well away from buildings. Where a site sits on gravels, as parts of west and south-west London do, infiltration can work well. Councils will not normally accept soakaways on assumed ground conditions, so soakaway and infiltration testing settles the question.

Where infiltration is not viable, the usual approach is attenuation: storing water temporarily and releasing it slowly through a flow control. Common methods include:

Blue and green roofs add significant weight, so the roof structure must be designed for them from the outset. A sustainable drainage (SuDS) strategy brings these features together and sets out how they will be looked after.

• Below-ground storage tanks or modular crates
• Permeable paving with a storage layer beneath
• Blue roofs that hold water on a flat roof and release it slowly
• Green roofs, which slow runoff and reduce the volume reaching the drains

Sewer connections and Thames Water

New connections to the public sewer need Thames Water's agreement, and for larger schemes it is sensible to check sewer capacity early. A restricted discharge rate agreed in the strategy will need to be reflected in the connection application, so the two should be consistent.

A CCTV survey of the existing drains is often worthwhile at strategy stage. It confirms where the pipes run, whether surface and foul water are already separated and what condition the system is in. If the new building will sit over or close to a public sewer, that is a separate approval; see our guide to building over a public sewer.

Planning conditions and getting it right first time

Drainage is one of the most common subjects of pre-commencement conditions. Discharging them needs detailed information that matches the approved strategy, so a strategy written with realistic storage volumes, discharge rates and layouts saves time later. A vague strategy that promises soakaways on clay is likely to unravel at condition stage. Equally, a strategy that ignores the space needed for tanks, chambers and pipe falls can clash with foundations, basements or trees once the detailed design begins, so it should be developed alongside the structural and landscape layout.

Every job starts with a fixed fee confirmed in writing before any work starts, and our typical turnaround is 10 working days, confirmed in writing with the fee. We can also prepare the maintenance plan and condition discharge information when the time comes.