Approval of NESO's tCSNP2 Refresh Methodology

What changed

Ofgem has approved NESO's methodology for refreshing the second transitional Centralised Strategic Network Plan (tCSNP2 Refresh). The decision, published 15 April 2026, greenlights the analytical framework NESO will use to reassess the transmission network build programme through the 2030s. This is not a decision on which projects proceed — it is the approval of the process by which those decisions will be made.

The original tCSNP2, published in 2024, set out a recommended package of transmission reinforcements. Since then, project designs have matured, cost estimates have shifted, connections reform has progressed, and offshore network configurations have changed. The refresh will update the plan against current reality rather than 2024 assumptions.

What this means in practice

The tCSNP2 Refresh is the mechanism through which NESO decides what gets built on the onshore and offshore transmission network. Its recommendations flow directly into RIIO-ET3 — Ofgem's price control for transmission owners (National Grid ET, SP Transmission, SSEN Transmission). Whatever the refresh recommends, those companies build, and consumers pay for through Transmission Network Use of System (TNUoS) charges on every electricity bill.

Three things make this refresh consequential:

Updated cost estimates. The 2024 tCSNP2 was built on cost assumptions that predate detailed project design work. Transmission projects routinely see cost escalation between strategic planning and detailed design — the Eastern Green Link subsea cables, for example, saw costs rise from initial estimates as engineering progressed. The refresh will reprice the programme against current construction costs, supply chain capacity, and delivery schedules. If costs have risen — as they almost certainly have — the programme either gets more expensive or gets smaller. Both outcomes matter for TNUoS.

Connections reform integration. The government's connections reform programme is reshaping which projects hold valid grid connection agreements. Queue positions are being reassessed; speculative projects are being removed. The generation and demand assumptions underpinning the transmission plan depend on which connections actually proceed. If the queue shrinks significantly — as intended — some reinforcements designed to serve projects that no longer exist may no longer be needed. Conversely, strategic connections for data centres and industrial loads may require network capacity in locations the 2024 plan did not prioritise. The refresh methodology determines how NESO incorporates these changes.

Offshore network design. The shift toward coordinated offshore networks — connecting multiple wind farms to shared onshore landing points rather than individual radial connections — changes both the topology and cost allocation of offshore transmission. The refresh will update offshore assumptions, which affects both the scale of onshore reinforcement needed and how costs are split between onshore TNUoS payers and offshore generators.

For transmission owners, the refresh determines their workload and revenue for the remainder of RIIO-ET3 and into ET4. For developers, it signals which parts of the network will have capacity and when. For consumers, it sets the trajectory of TNUoS charges — already rising and projected to increase further as the transmission investment programme accelerates.

The underlying tension is structural. NESO plans the network; Ofgem funds it through price controls; transmission owners build it. NESO has no direct financial exposure to the costs it recommends. The methodology approval is Ofgem's main lever for ensuring the planning assumptions are defensible before they translate into consumer-funded capital expenditure. The 108-page methodology document sets out how NESO will model demand scenarios, generation backgrounds, and network options — but the real test is whether Ofgem's approval process imposed meaningful constraints on NESO's assumptions, or whether this is procedural sign-off on a framework NESO designed.

What happens next

NESO will now execute the approved methodology to produce the tCSNP2 Refresh itself — the updated list of recommended transmission projects, with revised costs, timelines, and designs. No publication date has been confirmed, but the refresh feeds into the pipeline for the full Centralised Strategic Network Plan (CSNP), which is due by December 2028. That suggests the refresh needs to be complete well before then — likely late 2026 or early 2027 — to inform the CSNP's starting position.

The CSNP, when it arrives, will be the first non-transitional strategic network plan under the new framework. It replaces the previous system where transmission owners proposed their own reinforcements and Ofgem assessed them individually. The shift to centralised planning by NESO is intended to produce a more coordinated, whole-system network design. Whether it also produces a more expensive one depends on the assumptions baked into exactly this kind of methodology.

Separately, the connections reform programme will continue to reshape the queue in parallel. The interaction between a shrinking connection queue and an expanding transmission plan is the key variable. If the refresh locks in reinforcement costs before connections reform has finished pruning the queue, consumers risk paying for network capacity that serves projects which never materialise. The sequencing matters, and Ofgem's decision to approve the methodology now — rather than waiting for connections reform outcomes — suggests they have accepted that risk.