Urban creep allowance in a drainage calculation

The urban creep allowance drainage calculation adds an uplift to the impermeable area of a residential development to account for householders paving over gardens, extending houses and adding hardstanding during the design life of the drainage system. It is applied on top of, not instead of, the climate change allowance, and the figure is set by the lead local flood authority rather than by national policy. Where an authority publishes a figure, 10 per cent of the impermeable area is the most commonly encountered uplift on residential schemes.

Miss it out and the drainage strategy gets returned at the first technical review. Apply it to the wrong development type and you oversize the attenuation and spend money you did not need to.

What urban creep accounts for

Drainage design assumes the development as built. Residents do not keep it that way. Over 30 or 50 years a housing estate accumulates extensions, conservatories, paved front gardens for parking, patios, sheds on slabs and driveways widened for a second car.

Each of those converts pervious area to impermeable area, and each sends run-off to a system sized for the original layout. Permitted development rights make most of it lawful without an application, so the local authority has no practical control over the aggregate effect. The 2008 changes requiring permeable surfacing or drainage to a pervious area for front gardens over 5 m² helped, and they did not stop the trend.

The allowance is an engineering response to that certainty. It is not a safety factor and it is not duplicated by the climate change allowance, which addresses rainfall intensity rather than catchment area.

How the allowance is applied

The uplift is applied to the impermeable area used in the run-off calculation for the developed case. In practice that means increasing the positively drained impermeable area by the stated percentage before calculating run-off rates and storage volumes.

The sequence in a typical calculation runs like this:

1. Establish the existing site condition and the greenfield or pre-development run-off rate.
2. Set the proposed impermeable area from the layout: roofs, roads, drives, paths and parking.
3. Apply the urban creep uplift to that impermeable area, using the authority's published figure.
4. Apply the climate change allowance to rainfall intensity for the appropriate epoch and the relevant allowance band.
5. Size attenuation for the critical duration event, usually the 1 in 100 year event with the climate change allowance applied, and check the 1 in 30 year no-flooding criterion.
6. Check exceedance routing for events beyond the design standard.

Steps three and four are separate operations on different variables. Applying both is standard; applying one and calling it conservative is not.

One detail that catches designers out: the uplift applies to the area that can plausibly change. A block of flats with a communal landscaped courtyard, no private gardens and no scope for householder paving does not creep in the way a street of houses does, which is why the figure varies by development type rather than being universal.

Where the figure comes from

There is no single national urban creep percentage in statute or in the National Planning Policy Framework. The figure comes from the lead local flood authority, which is the county council or unitary authority acting as a statutory consultee on surface water drainage for major development.

Authorities set it in local design guidance, in a SuDS design guide, or in the standing advice they issue to applicants. The common approach is a percentage uplift on impermeable area for residential development, with a lower or nil figure for commercial and industrial schemes where householder alterations do not arise.

Take the figure from the authority's current published guidance and cite it in the drainage report with the document title and date. A number without a source is the single most common reason a drainage strategy is queried, and it is avoidable.

Interaction with climate change allowances

The two allowances are often confused because both increase the design volume, and they act on different parts of the calculation.

Climate change allowances published by the Environment Agency uplift rainfall intensity, and are selected by river basin district, by epoch and by the vulnerability of the development. They are revised periodically, so the correct approach is to take the current allowances at the point of design and state the epoch used.

Urban creep uplifts the contributing area. A system can be correctly sized for future rainfall and still be undersized because the catchment grew.

Applying both is standard practice on residential schemes. The combined effect on storage volume is more than either alone, and on a constrained site it is the difference between a shallow basin and an underground tank. That is a reason to establish both figures at concept design rather than at technical approval, when the layout is fixed and the only remaining option is the expensive one.

Illustrative example. A 60 home scheme has a proposed impermeable area of 1.1 hectares. A 10 per cent urban creep allowance takes the design impermeable area to 1.21 hectares before any rainfall uplift is applied. The climate change allowance is then applied to rainfall intensity for the relevant epoch and allowance band. The storage volume rises on both counts, and the figures are illustrative of the method rather than a result for any particular site.

Development types where it does not apply

The allowance is aimed at development where occupiers can change the surfacing. That gives a reasonably clear set of exclusions, though you should confirm each with the authority.

Commercial, industrial and distribution schemes usually attract no urban creep allowance, because the site layout is controlled by the operator and changes go through the planning system. Flatted development with no private external space often attracts none, or a reduced figure. Schemes where the whole site is already impermeable in the developed case have nothing to creep into, which sounds obvious and is worth stating in the report.

Retrofit and redevelopment cases need care. Where the existing site is substantially impermeable and the proposal reduces that area, the baseline and the betterment calculation matter more than the creep allowance, and the authority will usually want to see the reduction evidenced from a measured survey rather than estimated.

Houses with garden curtilage, terraces with front parking and any scheme with private drives are where the allowance bites hardest.

Evidencing the figure used

Put the whole chain in the drainage report so a reviewer can follow it without recalculating anything: the measured impermeable area with a plan reference, the creep percentage with its source document and date, the uplifted area, the climate change allowance with its epoch and band, and the resulting storage volume.

State any assumption about which areas are capable of creep, and show them on a drawing if the site is mixed. A scheme with 40 houses and a block of flats should not apply a blanket figure across both without explaining it.

Where you are departing from the authority's figure, say so explicitly and give the reason. A reduced figure for a scheme with no private gardens is a reasonable argument. The same reduction applied silently to save tank volume reads as an error and invites a rejection. The same discipline applies when the drainage details come back to be signed off, which is the process described in approval of details or discharge of conditions.

Agreeing it with the flood authority

Raise the figure with the lead local flood authority before the layout is fixed, ideally at pre-application stage. Most authorities answer a short written question about their current allowance quickly, and the answer is worth having in writing.

Three things make agreement smoother. Ask about the creep figure and the design standard in the same enquiry, since they are set in the same guidance. State the development type precisely, because the answer differs between houses and flats. And ask whether the authority requires any particular rainfall data source or modelling software outputs, as some do.

Where the drainage strategy forms part of a wider submission, the Drainage Strategy and the Flood Risk Assessment need to use the same figures, and an inconsistency between the two is a routine cause of a holding objection. The full list of report types shows which other documents sit alongside them on a major residential scheme.

This is general guidance, not legal advice. Urban creep allowances, climate change allowances and design standards are set locally and revised periodically, so take the current figures from your lead local flood authority and the Environment Agency before relying on them.