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.