Charges

If your business pays for surface water drainage, how do you know whether the charge is correct? This is where things become more technical.

There is no single surface water drainage tariff covering every UK business. The charging methodology depends on factors including the wastewater wholesaler serving the property and its applicable charging scheme.

For many commercial properties, one of the most important factors is site area.

Understanding Site-Area Charging

Ofwat confirms that a number of water companies use site-area-based charging for surface water drainage.

In simple terms:

Chargeable Site Area → Charging Band/Tariff → Annual Drainage Charge

This means a larger commercial property can potentially pay considerably more than a smaller one.

But there is an important distinction:

The total size of a property isn’t necessarily the same as its chargeable drainage area.

That is where businesses need to look more closely.

How to Check Your Surface Water Charge

Start with your water bill.

Identify charges described as:

• Surface Water Drainage;
• SWD;
• Site Area Drainage;
• Property Drainage; or
• Highway Drainage.

Then establish which wastewater wholesaler serves the property and the charging methodology being applied.

Next, establish the actual site area.

This can involve information from:

• site plans;
• lease plans;
• architectural drawings;
• drainage plans;
• mapping;
• aerial imagery; and
• physical measurements.

You can then start examining how individual areas of the property drain.

Calculate the Drainage Footprint

A useful way of approaching a commercial site is to divide it into separate areas.

For example:

Area	Size	Drainage
Warehouse roof	5,000 m²	Public sewer
Car park	3,000 m²	Soakaway
Service yard	2,000 m²	Public sewer
Landscaped area	2,000 m²	Natural drainage

The total site is 12,000 m².

But that doesn’t automatically mean all 12,000 m² should be treated identically for surface water charging.

The important technical question is:

Which areas ultimately contribute surface water to the public sewer?

Follow the Water

An impermeable surface does not automatically mean that its rainwater enters the public sewer.

A car park, for example, might discharge into an infiltration system.

Likewise, having private drainage doesn’t necessarily mean the water avoids the public sewer.

Consider:

Car Park → Private Drain → Public Sewer

The water still ultimately reaches the public network.

Compare that with:

Car Park → Drainage Channel → Soakaway → Ground

The drainage destination is fundamentally different.

For this reason, a proper assessment needs to establish the final destination of the surface water, rather than simply identifying drains on the property.

Common Reasons Charges Can Be Wrong

Problems can arise where:

• the recorded site boundary is incorrect;
• the recorded area is wrong;
• land has been sold or divided;
• a site has been redeveloped;
• soakaways haven’t been accounted for;
• drainage arrangements have changed;
• naturally draining areas have been incorrectly assessed; or
• historical information no longer reflects the current property.

Ofwat specifically recommends checking whether the estimated site area is correct and whether areas that drain naturally have been included.

Could You Be in the Wrong Charging Band?

This can be particularly important under banded site-area charging.

Suppose a business is currently assessed on a 15,000 m² drainage area.

A technical review establishes that a significant part of the property drains through qualifying alternative arrangements and the corrected chargeable area is materially lower. If that correction moves the property into a different charging band, the annual cost could fall.

The potential saving should therefore be calculated using the specific wholesaler’s charging methodology, rather than applying a generic national £/m² figure.

Surface water drainage is one of the most commonly misunderstood charges on a business water bill.

Unlike metered water charges, it isn’t necessarily related to how much water your business consumes. Instead, surface water drainage (SWD) is the charge associated with rainwater from your property entering the public sewer network.

For businesses with large roofs, car parks, loading yards and other hard surfaces, this can become a significant annual cost.

What Is Surface Water Drainage?

When rain falls onto a commercial property, it has to go somewhere.

Rain falling onto roofs, car parks and hardstanding may enter drains that eventually connect to the public sewer system. The wastewater wholesaler has to provide infrastructure capable of receiving and managing that water.

This creates the surface water drainage charge.

A simple example is:

Rain → Roof → Gutter → Drain → Public Sewer

However, not every property drains this way.

Rainwater could instead enter a:

• soakaway;
• infiltration system;
• Sustainable Drainage System (SuDS);
• watercourse;
• rainwater harvesting system; or
• other private drainage arrangement.

Where water does not ultimately enter the public sewer, this can potentially affect how the property should be charged.

Ofwat advises businesses to check whether areas that drain naturally have been included within their chargeable site area.

Who Is Most Affected?

Surface water drainage deserves particular attention where a business occupies a large physical site, regardless of how much mains water it consumes.

Examples include:

• warehouses and distribution centres;
• manufacturing sites;
• retail parks and supermarkets;
• industrial estates;
• schools and universities;
• hospitals;
• hotels and leisure facilities;
• vehicle depots; and
• large offices with extensive car parking.

Consider a warehouse using relatively little mains water but occupying a 20,000 m² site.

Its water consumption may be modest, but thousands of square metres of roof, yard and parking could generate significant quantities of surface water.

This is why businesses should not assess their water costs purely by looking at consumption.

Surface Water Drainage vs Highway Drainage

These charges are easily confused.

Surface water drainage relates to rainwater from your property entering the public sewer.

Highway drainage relates to rainwater from public roads and highways entering the sewer network.

Reducing or removing surface water drainage from a property does not therefore automatically remove highway drainage charges.

Why Should Businesses Check Their Charges?

The information used to calculate a property’s drainage charges may not always reflect what is happening on the ground today. Properties are extended. Land is sold. Car parks are resurfaced. Drainage systems are altered. Soakaways are installed. Buildings are demolished. If the charging information isn’t updated accordingly, the business could potentially continue paying based on an outdated assessment.

For larger businesses and multi-site organisations, even relatively small inaccuracies can accumulate into substantial unnecessary expenditure. The first question is therefore simple:

Where does the rainwater from your property actually go?

If the answer doesn’t match the assumptions behind your water bill, the charges may be worth investigating.