Drainage evidence for a change of use with no new build

On a change of use with no new floorspace, the change of use drainage evidence you need is usually one page, and occasionally a great deal more. The test is not whether you are building anything. It is whether the new use puts more into the foul sewer, puts something different into it, or changes what runs off the site. A shop becoming an office changes almost nothing. A shop becoming a restaurant changes the foul loading, the fat content and the need for a grease separator, and a shop becoming flats can engage a habitats assessment in the wrong catchment.

Work out which of those you are, then write only what that requires.

Why a change of use raises drainage questions at all

Planning authorities ask about drainage on change of use applications for three reasons, and they are separable.

Foul capacity, because the public sewer and the treatment works downstream were sized for the old pattern of use. Effluent quality, because some uses discharge things a domestic sewer was not designed to carry. And surface water, which on a genuine no-build change of use is normally unaffected but occasionally is not, where the application also involves hardstanding, an altered roof, or a site in a critical drainage area.

Answer all three explicitly, even where two of the answers are "unchanged". An officer reading a statement that addresses only the point you thought was relevant will assume the others were not considered.

Foul loading against the previous use

This is the calculation that carries most applications.

Estimate the foul flow for the lawful existing use and for the proposed use using published loading rates, and present the two side by side with the assumptions stated: floor area or seat numbers, occupancy, hours, and the rate per unit you have applied with its source. The comparison, not the absolute number, is what matters.

Where the loading rises materially, make a pre-development enquiry to the sewerage undertaker before you apply. It is a paid service with a response time measured in weeks, and its output is the evidence the authority actually wants: a statement from the undertaker that capacity exists, or that it exists subject to works. A Foul Sewage Assessment is the document that gathers this together where the change is more than trivial.

Where there is no public sewer, the question changes completely and becomes one about a package treatment plant or septic tank and its discharge, which our note on foul drainage with no mains sewer covers.

Grease, trade effluent and the right to connect

Three legal points that save arguments.

Section 106 of the Water Industry Act 1991 gives the owner or occupier of premises a right to connect drains to a public sewer, subject to the undertaker's requirements about the mode of construction and the point of connection. That right is not unlimited, and it does not extend to discharges the undertaker may lawfully refuse.

Section 111 of the same Act prohibits discharging into a public sewer any matter likely to injure the sewer, interfere with the flow, or affect the treatment process. Fats, oils and greases from a commercial kitchen fall squarely within that, which is why grease management is not optional and why Approved Document H addresses grease separation for hot food premises.

Trade effluent, meaning liquid discharged from a trade premises other than domestic sewage and surface water, needs a consent from the sewerage undertaker under the trade effluent provisions of the 1991 Act. A commercial laundry, a vehicle wash, a food production unit and some clinical uses all need one. Apply for it in parallel, not afterwards.

A practical drafting point for the planning submission. A planning condition that simply requires you to obtain a trade effluent consent duplicates another regulatory regime and is hard to justify against the tests for conditions in national policy. A condition requiring the installation of a grease separator to an approved specification before first use of the premises is a different matter, and is both common and reasonable.

Surface water where nothing is being built

On a true no-build change of use, the impermeable area is unchanged, and the honest statement is that the existing surface water arrangements remain and no increase in run-off arises. Say it in one sentence, with the impermeable area figure to support it.

Three situations break that.

Where the application includes external works, such as a new bin or cycle store, a servicing area, or a change from grass to hardstanding for parking. Then the additional impermeable area has to be quantified and dealt with, usually by permeable construction or a small soakaway.

Where the site is in a critical drainage area or the authority's own local plan policy requires betterment on any application. Some authorities take the opportunity of any application to ask for improvements to an existing unrestricted discharge, and whether that is reasonable depends on the policy wording.

And where the proposal introduces a more vulnerable use into a flood risk area, in which case the flood risk sequential and exception tests may be engaged even with no construction at all. That is a flood matter rather than a drainage matter, and it needs care not to overstate the mapped position, as our note on reading the surface water flood risk map sets out.

The catchment issue nobody expects

A change of use to residential in a nutrient neutrality catchment can be the hardest drainage question on a no-build scheme.

Where a protected habitat site downstream is in unfavourable condition because of nitrogen or phosphorus, Natural England advice in a number of catchments means new residential development has to demonstrate nutrient neutrality before permission can be granted, under the Conservation of Habitats and Species Regulations 2017. An office to residential conversion generates a new wastewater loading where the previous use generated little, so the scheme has to find mitigation, usually by buying credits from a scheme in the same catchment.

Check the catchment position at the outset, because it changes both the cost and the programme, and no amount of drainage design removes the requirement.

Evidence that satisfies the officer

For most no-build changes of use, this is the whole submission:

1. A statement of the lawful existing use and the proposed use.
2. A foul loading comparison with the rates and assumptions named.
3. The point of connection and the drainage arrangement, taken from a drainage survey or the existing record where one exists.
4. Where loading rises, the sewerage undertaker's response to a pre-development enquiry.
5. Where food, laundry, vehicle or process water is involved, the grease or pre-treatment proposal and the trade effluent position.
6. One paragraph on surface water, with the impermeable area figure.
7. Where residential is proposed, a check on nutrient and water neutrality requirements in the catchment.

That is two to four pages on a small unit. If the authority's validation checklist asks for a full Drainage Strategy on a change of use with no construction, say what is unchanged and why, and ask the case officer to confirm the reduced scope in writing before you spend money on modelling that has nothing to model.

Conditions sometimes imposed

Expect one or more of these, and check the wording carefully:

• Installation of a grease separator or other pre-treatment to an approved specification before first use, and maintained thereafter.
• A scheme of foul drainage to be submitted and approved where the connection route is changing.
• Restriction on hours of operation, which sometimes appears as a drainage condition where the concern is peak flow into a constrained sewer.
• A condition requiring the works shown on the approved drainage drawing, which is the one to read most carefully because it fixes you to a drawing you may want to change.

One honest uncertainty. Practice on change of use drainage varies more between authorities than almost anything else in validation, and two neighbouring councils will ask for different documents for the same proposal. The published local validation list is the authority on what that council wants, and a pre-application enquiry is the reliable way to settle it.

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

Enter your postcode on PlanScope to see which drainage documents your change of use is likely to need.

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