Drainage for a change of use with no new hard surface

A change of use drainage requirement often lands as a standard condition or a validation request, copied across from a housing scheme. Where the proposal is a change of use with no external alterations, the honest answer is short: the impermeable area does not change, so the surface water run-off rate and volume do not change either, and there is nothing to attenuate.

That is not the end of the drainage conversation. It moves it to the part that matters, which is foul flow. A shop becoming a restaurant, or offices becoming flats, can double or triple the foul load from the same roof area, and that is a real constraint that a surface water condition does nothing about.

Why drainage is raised at all

Three mechanisms put it on the table. Local plan policies require sustainable drainage on new development and are written with construction in mind. Validation checklists list a drainage strategy against broad application types. And the drainage officer works from a standard consultation response that assumes there is something to build.

None of those is a reason to ignore the request. The way to deal with it is to answer the question the policy is actually asking, which is whether the development increases flood risk elsewhere. Where it plainly does not, say so with evidence and offer the foul analysis instead.

Where run-off rates are unchanged

The arithmetic is simple and worth setting out on one page.

1. Measure the existing impermeable area: roof, yard, car park, footway within the site, in square metres.
2. Measure the proposed impermeable area on the same basis.
3. State the difference. Where the answer is nil, the run-off calculation is unchanged and no further surface water assessment is needed.
4. Show the existing drainage layout, with the outfall, the connection point and whether the system is combined or separate.
5. Note anything that changes the character of the discharge even if the area is the same, such as a new external plant area that needs a bunded surface, or a vehicle wash.

That fifth point is where honest cases fall apart. A change of use that includes resurfacing a yard, adding an extraction plant compound, forming a bin store on a grassed strip or laying a new hardstanding for deliveries does increase the impermeable area, and the submission should deal with the increase rather than claiming none.

Where the increase is small, the sensible offer is proportionate mitigation: permeable surfacing for the new area, a downpipe disconnection to a planted area, or a water butt and rain garden arrangement. Attenuation credit for the retained roof is sometimes available where the scheme includes rainwater reuse, and attenuation credit is worth understanding before offering a volume.

Foul flows and the real issue

Compare the uses on flow rather than on floor area. A retail unit generates staff and customer sanitary flow. A restaurant of the same size adds kitchen flow, hot water, dishwashing and grease. A conversion of offices to flats changes the pattern from weekday daytime peaks to morning and evening peaks with much higher per capita volumes.

Set out the comparison in a Foul Sewage Assessment with the assumptions stated: the number of fittings, the occupancy, the hours of operation and the peak flow estimate. Where the use is food and drink, deal with grease separation explicitly, with the separator type, its sizing basis and the maintenance regime, because environmental health and the undertaker will both ask.

Then ask the undertaker. A pre-development enquiry gives you a written position on capacity at the connection point, and it can take several weeks, which is why it should go in at the same time as the planning application. If the response identifies a constraint, the planning answer is a Grampian condition that ties the start of the use to the necessary works rather than a refusal.

Evidence that existing drainage is adequate

The pack for this is short and specific:

• A drainage record plan showing the existing system, from the undertaker's sewer records and your own survey.
• A CCTV survey where the building has been empty or the system is old, which also finds the misconnections that would otherwise become your problem.
• The undertaker's written response to a capacity enquiry.
• The foul load comparison between the existing and proposed use.
• Evidence of past flooding at the property, or its absence, and any record the undertaker holds of properties at risk of sewer flooding in the area.
• Photographs of manholes, gullies and the discharge point, with lids lifted where safe.

A CCTV survey is the underrated item. On a building that has stood vacant for two years, a collapsed drain or a surface water connection into a foul run is a reasonably common finding, and it is far better to know before you commit to a fit-out programme.

Where betterment is nonetheless sought

Many local plans ask for a reduction in run-off on redevelopment sites, sometimes towards greenfield rates where reasonably practicable. That policy is aimed at development that rebuilds the site, and the words "where reasonably practicable" carry weight on a change of use inside an existing shell.

There are still low cost opportunities worth offering, because they build goodwill and they are usually conditioned anyway:

1. Disconnect one or two downpipes into planted areas or butts.
2. Replace any impermeable surfacing you are relaigning anyway with permeable construction.
3. Add a green roof to a single storey flat roof element where the structure allows.
4. Remove redundant hardstanding and plant it.

Offer them as enhancement rather than as mitigation for an increase that does not exist. The distinction keeps the planning logic straight and avoids setting a precedent that the scheme has an impact to compensate for.

Conditions that are reasonable

National guidance sets six tests for conditions. They must be necessary, relevant to planning, relevant to the development permitted, enforceable, precise and reasonable in all other respects.

Apply them to the usual suspects. A condition requiring details of grease management before the use starts is necessary, relevant and precise. A condition requiring foul connection details agreed with the undertaker is reasonable where capacity is uncertain. A condition requiring a surface water drainage scheme with attenuation for a one in 100 year event plus climate change, on a change of use with no external works, is not necessary because there is nothing to attenuate, and it is not enforceable in any meaningful way.

Illustrative example. A vacant Class E(a) shop unit of 180 square metres becoming a café. No external alterations other than a flue and a small bin enclosure on existing hardstanding. The submission shows nil change in impermeable area, a foul flow comparison showing an increase driven by the kitchen, a grease separator specification, and the undertaker's confirmation that capacity is available at the existing connection. The reasonable condition package is grease management details, a flue and odour condition, and hours of use. A surface water attenuation condition has nothing to do.

Resisting requirements that are not

Do it politely and in writing, once, with the analysis attached. A short note to the case officer that sets out the area schedule, quotes the six tests and offers an alternative condition wording is more effective than an argument at committee.

Two points to make. First, the drainage question the policy asks is about increases in flood risk elsewhere, and a nil increase answers it. Second, the building will in any event have to satisfy the drainage requirements of Approved Document H for the works being carried out, and the sewerage undertaker has its own statutory regime under the Water Industry Act 1991. Planning does not need to duplicate either.

One matter deserves its own check. Where the proposal moves the building to a more vulnerable use in a flood zone, a Flood Risk Assessment is needed regardless of drainage, because the vulnerability classification and possibly the exception test are engaged. A shop becoming flats in Flood Zone 3 is that case, and drainage is the least of it.

Where external works are extensive enough to need a full Drainage Strategy, prepare one properly, using the same discipline as drainage evidence for a new dwelling scaled to the site.

PlanScope drafts the Foul Sewage Assessment and Drainage Strategy narratives from the site data and your answers, with flags where site-specific figures such as a capacity response are still to be inserted. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

Enter the property postcode on PlanScope to see whether flood risk applies to the site and which drainage documents a change of use is likely to need.