White Horse Reservoir Plan Grows as Costs Climb
Thames Water has revised the plan for the White Horse mega-reservoir near Abingdon, Oxfordshire, keeping storage at 150bn litres over 6.7 sq km but expanding the construction area. The Guardian
The water area stays at 6.7 sq km (2.6 sq miles). The wider site grows to 38 sq km to allow extra building work, relocation of nearby solar farms and rerouting of local rivers and streams. Thames Water said the scheme needs additional access points for construction and environmental mitigation work such as new habitat creation. The scheme was formerly known as the South East Strategic Reservoir Option.
The estimated cost has risen from £2.2bn to between £5.5bn and £7.5bn. Customers of Thames Water, Affinity Water and Southern Water would pay for the reservoir through their bills over the next 100 years. The design requires embankment walls up to 25 metres high. That would make it the largest fully bounded reservoir in Europe, meaning water is held in by built walls on all sides rather than by damming a valley.
The timeline is long. The first public consultation on the plans closed in January, and a further consultation started in the week before 14 September 2026 and is expected to end the next month. Thames Water aims to submit a planning application by late 2026. If approved, main works construction would start in 2032 and water would be available by 2040 for 15 million people.
The reservoir would work as seasonal storage. It would be filled in winter with water taken from the River Thames. BBC That abstraction and storage model, taking river water in winter and holding it, sits at the centre of the regional water resource plan. The aim is to balance winter availability against summer and drought-period demand across the three company supply areas.
The idea goes back two decades. Plans for an Abingdon reservoir were first put forward in 2006 as a 10 sq km, £1bn project. Thames Water held a 12-week public consultation on proposals for a new reservoir near Abingdon in 2024. BBC Its statutory consultation, the formal public comment period required by law, on the White Horse Reservoir ran from 28 October 2025 to 13 January 2026. In January 2026, the company opened a £5.7bn tender notice, an invitation for builders to bid to deliver the reservoir. Thames Water
On future need, Rapid, a partnership of Ofwat, the Environment Agency and the Drinking Water Inspectorate, projected a shortfall of more than 2bn litres a day by 2055. A Thames Water survey of 2,004 adults across the South East found 89% support for building the White Horse Reservoir. BBC
The broader context here is how procurement risk, bill incidence and planning approval converge on a single asset. A 100-year cost recovery period spreads the charge but ties three customer bases to a multi-generational commitment. The rise from £2.2bn to a £5.5bn to £7.5bn envelope will draw close regulatory attention to scope control, contingency funds and interface work around river diversions, solar relocation and habitat creation. The markers to watch are the late-2026 planning submission, examination of the expanded 38 sq km boundary, and whether the 2032 to 2040 construction and start-up window holds through procurement and environmental approval.
