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.