Modelling urban creep on a redevelopment

Urban creep is the allowance a surface water calculation makes for householders paving things over after they move in. Patios, extensions, sheds, widened driveways, artificial grass laid on a membrane. None of it appears on the approved layout, all of it adds impermeable area, and the drainage system has to cope with it for the whole design life.

It is applied to the contributing area, not to the rainfall. That distinction matters because urban creep and the climate change allowance are frequently confused, and applying one instead of the other undersizes the system.

What the allowance is for

The design case for a residential Drainage Strategy is not the site as built on day one. It is the site as it will exist in 30 or 60 years, after two or three changes of ownership.

Evidence from developed catchments has consistently shown impermeable area increasing over time within fixed plot boundaries, most of it in private curtilage rather than on adopted land. Front gardens become parking. Rear gardens gain patios and garden rooms. Side returns are filled in.

The allowance converts that expectation into a number. It is expressed as a percentage uplift applied to the impermeable area of the proposed development, and the size of the uplift is graduated by housing density: largest for low density housing with generous private gardens, smaller for higher density housing with less curtilage to pave, and normally nil for flats where the external space is communal and managed.

The graduated table sits in the sewer adoption design guidance and is reproduced in most lead local flood authority standards. Use the local one where it exists, because the figures differ and the authority will apply its own.

Where in the calculation it belongs

Urban creep applies to the proposed impermeable area only. It does not apply to the existing baseline used to calculate betterment on previously developed land, because the existing site is being measured as it is, not as it might become.

The order of operations that most reviewers expect runs like this.

1. Establish the existing impermeable area from survey, not from an aerial photograph.
2. Establish the proposed impermeable area by parcel: roofs, private drives, private paths, adopted highway, adopted footway, communal hardstanding.
3. Apply the creep uplift to the private curtilage parcels only.
4. Apply the climate change allowance to rainfall depth or intensity, separately, using the current Environment Agency allowance for the river basin district and the relevant epoch.
5. Run the network and storage calculation with both applied together.

The most common error is applying creep to the total site area including adopted highway and communal landscaping, which overstates the volume and produces a basin nobody can fit. The second most common is omitting it entirely and discovering the gap at technical approval.

The development types where it is left out

Creep is a residential curtilage phenomenon, so several development types attract little or none.

Where a scheme is mixed, split it. A single blended percentage applied to a site with 40 houses and two blocks of flats is not wrong by much, but it is impossible for a reviewer to check, and unverifiable numbers get queried.

What it does to storage volume

Increasing the contributing area raises the volume of runoff to be stored, but not in direct proportion, because the flow control device continues to discharge throughout the event.

The effect is larger than people expect on constrained sites for a different reason. Storage volume is fixed by the critical duration event, and adding contributing area can shift which duration is critical, which in turn changes the shape of the required storage. On a site where the attenuation is in an underground tank with a fixed footprint, that can mean a deeper tank and a lower invert level, which then interacts with the outfall level and the groundwater table.

Run the calculation with creep included from the first iteration. Retrofitting it at technical approval is how schemes end up with attenuation below the outfall soffit and a pumped solution nobody wanted.

Evidencing the figure you chose

State the allowance explicitly and show its derivation. A drainage strategy that presents a single impermeable area figure with no breakdown cannot be checked, and a reviewer who cannot check it will ask for the calculation files.

Include an impermeable area schedule by parcel, in square metres, showing the as-proposed figure and the figure after creep. Name the source of the percentage: the lead local flood authority's standard, the sewer adoption design guidance table, or the local plan supplementary guidance. State the housing density used to select the band, calculated as dwellings per hectare over the residential parcel rather than over the whole red line, because including a large area of open space depresses the density and produces a smaller allowance than the authority intends.

Where the authority's standard is silent, say what you have assumed and why. An assumption stated openly is negotiable. An assumption buried in a spreadsheet is a holding objection.

Permeable paving claims and the creep that undoes them

This is where the two issues collide. A scheme claims permeable block paving on private driveways and treats that area as non-contributing. Urban creep then predicts that some of those driveways will be resurfaced in impermeable material by future owners, which removes the benefit claimed.

Most lead local flood authorities resolve this by refusing to credit permeable surfacing in private curtilage at all, treating private drives as impermeable from the outset. Others accept the credit where the surfacing is secured by condition and a maintenance obligation. Find out which applies before you rely on the credit.

There is a planning control in the background that helps. Class F of Part 1 of Schedule 2 to the General Permitted Development Order 2015 allows a hard surface within the curtilage of a dwellinghouse, but where that surface lies between the principal elevation and the highway it must either use porous materials or direct run-off to a permeable area within the curtilage. Paving a front garden in impermeable material without that provision is not permitted development. Enforcement is patchy, which is precisely the reason the creep allowance exists.

A similar argument arises over storage credits for reuse, and the question of whether rainwater harvesting counts as attenuation is worth settling on the same terms: what is secured, by what mechanism, and for how long.

Checks a reviewer should run

Six checks catch most of the problems.

• Is a creep allowance applied at all, and is the percentage stated in the text rather than only in the model output?
• Is it applied to private curtilage parcels only, and not to adopted highway, communal landscape or the existing baseline?
• Is the housing density used to select the band calculated over the residential parcel rather than the whole site?
• Are creep and the climate change allowance both applied, to area and to rainfall respectively, rather than one standing in for the other?
• Does the impermeable area schedule reconcile with the site layout drawing, parcel by parcel?
• If permeable surfacing is credited, what secures it, and does the local standard accept the credit?

For a single dwelling the arithmetic is small but the principle is identical, and the proportionate approach is set out in the notes on drainage evidence for a single new dwelling. Where the site is also in a flood zone, the creep figure should be consistent between the drainage strategy and the Flood Risk Assessment, because reviewers do compare them and inconsistent numbers undermine both.

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This is general guidance, not legal advice. Check the position with your lead local flood 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.