Proving the existing run-off rate on a brownfield site

On previously developed land, the brownfield existing runoff rate is the number the whole drainage strategy turns on, and it is the number the lead local flood authority will test hardest. The national non-statutory technical standards for sustainable drainage say that for redevelopment the peak run-off rate should be as close as reasonably practicable to the greenfield rate and should never exceed the rate discharged before redevelopment. Both halves of that sentence matter. The existing rate sets your ceiling. The greenfield rate sets the target you are being pushed towards.

Where the argument goes wrong is in the word "existing". A developer measures every square metre of tarmac on site. The flood authority asks which of it was actually connected to a drain.

Why the existing rate is contested

An inflated existing rate lets you discharge more, which means a smaller attenuation tank, which is worth real money on a tight site. Flood authorities know that, so they start from scepticism, and a strategy that asserts a rate without evidence gets a holding objection.

The contest usually turns on three claims. That the site was fully hard surfaced. That the hard surfacing drained to a positive system rather than soaking away. And that the connection ran to the point of discharge you are now proposing to use.

Each of those is provable, and each needs different evidence.

Evidence of the impermeable area

Start with what exists now and work backwards.

• A topographical survey with hard surface types identified, not just levels
• A drainage survey: manhole covers lifted, inverts recorded, connectivity traced, with CCTV where the system is buried or blocked
• Dated aerial imagery, with the date shown on the extract you submit
• Historical Ordnance Survey mapping showing building footprints and yard areas over time
• The sewerage undertaker's public sewer records and asset plans
• Any as built drainage drawings from the previous occupier or from the council's own file
• Photographs taken before demolition, with dates

Where the site has already been cleared, the record you did not take is gone. That is the single most common own goal on brownfield sites: demolition first, drainage survey never, and then an argument with the flood authority that costs more than the survey would have.

Present the impermeable area as a plan with areas scheduled, colour coded by whether each area is connected, unconnected or permeable, and with a total in square metres that ties to the calculation. A schedule that does not add up to the plan will be picked apart.

Connected against unconnected hard surfacing

This is the technical heart of it. A run-off rate is a rate of discharge from the site to a receiving system. Hard surfacing that drains to a soakaway, to a soft verge or over the edge into a ditch is not contributing to the discharge you are claiming, whatever its area.

So the schedule needs three categories. Positively drained impermeable area, meaning connected by pipe or channel to a sewer, watercourse or other outfall. Impermeable but not positively drained, such as a yard falling to a gravel strip. And permeable or soft area.

Only the first category feeds the existing rate calculation. Where the connection is uncertain, say so and use the conservative figure, because an assumption you have flagged is defensible and one you have hidden is not.

Roof areas are usually the easiest to prove, since downpipes and gullies are visible. Yards are the hardest, particularly older industrial sites where drainage was informal and partly unrecorded.

The assumptions behind a positively drained rate

Once the connected area is fixed, the rate is calculated. The conventional approach on a brownfield site is the modified rational method, applying a run-off coefficient to the connected impermeable area for the return period in question, with the rainfall intensity from the appropriate source.

Three assumptions get scrutinised. The run-off coefficient, where a value of 1.0 on impermeable surfaces is conservative in the flood authority's favour and anything lower needs justification. The capacity of the existing outfall, because a rate calculated from area is meaningless if the existing 150 millimetre connection could never have carried it. And whether the existing system was surcharged in practice, which is where the CCTV survey earns its fee.

State each assumption in a short table in the Drainage Strategy with the source and the sensitivity. A reviewer who can see the assumption will argue about the value. A reviewer who cannot see it will reject the method.

How betterment is expressed and where it lands

Betterment is the reduction from the existing rate that the authority requires. It is expressed as a percentage of the existing positively drained rate, and the figure is a matter of local policy rather than national standards.

Many lead local flood authorities publish an expectation of a minimum reduction on brownfield redevelopment, with a stronger expectation where the receiving sewer is known to be under pressure or the catchment is designated as a critical drainage area. Read the authority's own published guidance and the local plan policy before fixing the design, and check whether the figure is applied to the 1 in 1 year event, the 1 in 100 year event plus climate change, or both.

Where a percentage reduction from a large existing rate still leaves a discharge above greenfield, expect the authority to ask why greenfield is not reasonably practicable. The answers it accepts are physical: no space for infiltration because of contamination or groundwater, no available outfall at the required level, ground conditions that fail infiltration testing. The answer it does not accept is that attenuation storage is expensive.

Where greenfield rates are insisted upon

Some authorities require greenfield rates on brownfield sites as a matter of policy, and some require them where the site is being wholly redeveloped rather than partially. The national standards allow that position, because "as close as reasonably practicable to greenfield" is the starting instruction and the existing rate is only a cap.

If greenfield is the requirement, the calculation changes: the estimation method for the undeveloped rate is used on the site area, usually producing a very low figure for a small site, with a minimum practicable discharge rate applied to avoid a design nobody can maintain. Infiltration becomes the first option to test, with percolation testing to a recognised method, and the results included rather than summarised.

Where the receiving water is an ordinary watercourse rather than a sewer, the works themselves need consent, which runs on its own timetable as set out in the note on land drainage consent for works to an ordinary watercourse.

The documents that settle the argument

• Existing impermeable area plan with a scheduled breakdown by connection status
• Drainage survey report, including manhole schedule, inverts and CCTV where relevant
• Dated historical imagery and mapping extracts
• Sewerage undertaker asset plan and any pre-application enquiry response
• Calculation sheets for the existing rate, the proposed rate and the storage volume, with inputs listed
• Infiltration testing results, or a reasoned statement of why infiltration is not viable
• The authority's published betterment requirement, quoted with its source

Surface water risk is read alongside all of this, and overstating the mapped risk is as unhelpful as understating it, as covered in reading the surface water flood risk map without overstating it. Where the site also falls in Flood Zone 2 or 3, the Flood Risk Assessment carries the sequential and exception test work and the drainage strategy sits under it.

Illustrative example: a 0.45 hectare former builder's yard has 3,200 m² of hard surfacing, of which the drainage survey proves 2,100 m² was connected to a 225 millimetre surface water sewer, with the remainder falling to a gravelled area. The existing rate is calculated on 2,100 m² rather than 3,200 m², the authority requires a 30 per cent reduction, and the difference between the two area figures changes the attenuation volume by roughly a third. Figures are illustrative.

PlanScope drafts the Drainage Strategy and Flood Risk Assessment structure from your site details, flags the inputs it does not have rather than inventing them, and leaves the hydraulic calculations to the drainage engineer who signs them. Enter your postcode on PlanScope to see which reports the site is likely to need.

This is general guidance, not 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.