Retrofitting SuDS on a constrained urban site

On a retrofit suds constrained site the baseline is not a greenfield run-off rate. It is whatever the existing yard, roof and car park already discharge, usually with no attenuation at all, through a connection that may or may not be recorded. Defra's non-statutory technical standards require peak run-off from previously developed land to be as close as reasonably practicable to the greenfield rate and never worse than the existing rate. On a tight urban plot those two halves of the sentence pull in different directions, and the design has to show which one it got closest to and why.

The honest version of that argument is more persuasive than an optimistic one.

Start from the existing discharge, not a greenfield rate

Everything rests on the evidenced existing rate, so build that evidence first.

Measure the existing impermeable area from a topographic survey rather than an aerial photograph, separating roof, yard, road and any permeable ground. Get the sewerage undertaker's asset records and read the invert levels, not just the alignments. Then confirm the connection exists and functions, because an old plan showing a drain is not proof of a live positive connection. A manhole survey and a dye or CCTV check settle it in a morning.

Where no positive connection can be evidenced, the undertaker and the lead local flood authority will usually hold you to a greenfield-based figure regardless of what the site looks like. That is the single most expensive discovery on a brownfield drainage job, and it is entirely avoidable by checking early.

In London the position is tighter again, because the London Plan sets its own drainage hierarchy and aims development at greenfield run-off rates. Check the local requirement before assuming the national standard is the ceiling.

Disconnection before storage

The least costly attenuation is the water you never collect. Before sizing a tank, work through what can simply be taken out of the piped system:

• Disconnect downpipes from the combined or surface water drain and route them to planters, rain gardens or permeable areas at ground level.
• Replace impermeable yard surfacing with permeable paving over a granular sub-base, which both stores and intercepts.
• Break up unnecessary hardstanding entirely, particularly narrow strips against buildings that serve no function.
• Redirect clean roof water away from any area of contaminated ground so that the treatment requirement falls only on the surfaces that need it.
• Reuse part of the roof water where there is a real year-round demand, though the credit for rainwater harvesting against attenuation has to be argued carefully rather than assumed.

Each of those reduces the volume that has to be stored underground, and underground storage is the expensive part of a retrofit scheme.

Storage under buildings, roads and podiums

When storage is unavoidable and there is no open ground, it goes under something. The usual options and their catches:

Two rules save trouble later. Keep every storage component accessible for inspection and cleaning, because a tank beneath a building with no access chamber is a tank that fails silently. And put a silt trap or filter upstream of anything that cannot be cleaned, since fines are what kill geocellular storage.

Green roofs and blue roofs where the ground is unavailable

Where there is no ground left, the roof is the remaining surface. The two systems do different jobs and are often confused.

A green roof holds the early rainfall in its substrate and loses it to evaporation and plant uptake. It delivers interception and water quality benefit and some volume reduction, but its attenuation capacity in a large event is modest. A blue roof stores water on the roof deck behind a controlled outlet, delivering real attenuation volume, with no vegetation required.

The constraints are structural and contractual rather than hydraulic. Additional dead load has to be carried, which on a retrofit over an existing structure needs checking before the design is fixed. The waterproofing warranty has to cover standing water, and roofing manufacturers are specific about that. Maintenance access has to exist, and on a retrofit that sometimes means a new roof hatch. A Green Infrastructure Plan can carry the amenity and biodiversity case for the vegetated option where the hydraulic case alone is thin.

Contamination, groundwater and shallow services

Infiltration is the first step in the hierarchy and the first thing ruled out on most brownfield land.

It is unacceptable where the ground is contaminated and infiltrating water would mobilise pollutants into groundwater, and the Environment Agency objects on that basis in source protection zones. It is impractical where groundwater sits within a metre or two of the surface, where made ground is variable, and where a neighbouring basement or a retaining structure would be affected. Test it properly and record the result, because "infiltration is not feasible" without a test is the sort of assertion that generates a holding objection.

Shallow services are the other urban constraint and the one that catches designers out. City centre footways and yards contain gas, water, electricity, telecoms and often abandoned apparatus at the exact depth a swale or a rain garden wants to occupy. Easements over utility apparatus prevent both planting and excavation. A utility survey before the drainage layout is drawn saves a redesign, and the plans should show apparatus and easements alongside the proposed features so a reviewer can see they do not clash.

Illustrative example: a former printworks yard of 0.28 hectares, entirely impermeable, with an evidenced 150mm connection to a combined sewer. Infiltration is ruled out by made ground and hydrocarbon contamination. The scheme disconnects the roofs to two rain gardens in a new courtyard, resurfaces the parking as permeable with a lined sub-base, and adds a geocellular tank under the access road with a vortex flow control. Discharge is reduced against the evidenced existing rate, the greenfield rate is not reached, and the strategy says so in one paragraph with the reason.

The betterment the undertaker will accept

Negotiation on brownfield turns on a percentage reduction against the evidenced existing rate. Many lead local flood authorities and sewerage undertakers publish the figure they expect in local guidance, and some express it as a target with a fallback where the site is constrained. Find the published figure before you design, and if there is none, ask.

Make the pre-development enquiry to the undertaker early. Their developer services team will tell you the capacity position, the point of connection and the rate they will accept, and their answer often changes the layout. A right to connect to a public sewer exists under the Water Industry Act 1991, but it is exercised on the undertaker's terms as to rate and location, so treating the connection as a given is a mistake.

Presenting a constrained solution honestly

The structure that works in a Drainage Strategy on a constrained site is a sequence a reviewer can audit rather than a claim of compliance.

1. The evidenced existing impermeable area and discharge rate, with the source of each figure.
2. Each step of the drainage hierarchy in order, with the reason for rejection recorded and tested where testing was possible.
3. The proposed components, the volume each provides and the resulting peak discharge for each design event.
4. The comparison against both the existing rate and the greenfield rate, stated plainly, including the gap.
5. Treatment and interception, assessed separately from attenuation.
6. Maintenance, with a responsible body, a schedule and access arrangements.
7. Exceedance routing, showing where water goes when the design event is exceeded, which then feeds the Flood Risk Assessment where one is required.

Say what you did not achieve. A strategy that admits it reached a 40 per cent reduction rather than a greenfield rate, and explains the constraints with evidence, tends to be accepted. One that claims greenfield compliance on a site where it is physically impossible invites a reviewer to check the arithmetic, and they will.

PlanScope drafts a Drainage Strategy structured to UK planning policy from your site data and guided answers, and flags site-specific figures such as existing discharge rates and infiltration results as still to be confirmed rather than filling them in. Drainage design needs a suitably qualified author, and for specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.