Avance successfully represented Honkajoen Tuulipuisto Oy, the owner of the operational onshore Kirkkokallio wind farm, in a landmark case before the Supreme Administrative Court of Finland (Korkein hallinto-oikeus, KHO) concerning wake effects between neighbouring wind farms. In its precedent ruling of 12 August 2026 (KHO:2026:59), the Supreme Administrative Court set aside both the city council’s plan approval and the Administrative Court's earlier ruling, finding that the planning authority had failed to adequately investigate how a new, larger wind farm would affect the production of the existing neighbouring wind farm.
The case concerned a partial master plan which allowed a developer to build new larger wind turbines right next to Honkajoen Tuulipuisto’s existing smaller wind turbines. The closest new turbines were sited only 2.6–3.5 rotor diameters from, and upwind of, the existing turbines. Honkajoen Tuulipuisto appealed the plan, arguing, among other things, that the new turbines' wake effect – the reduction in wind speed and increase in turbulence that a turbine causes downwind – would materially reduce its wind farm's electricity production and cause unreasonable harm to it within the meaning of the Finnish Land Use Act.
The Administrative Court of Turku dismissed the appeal in the first instance. The Supreme Administrative Court granted Honkajoen Tuulipuisto leave to appeal and, reaching a different conclusion than the lower court, set aside the plan approval. The Supreme Administrative Court held that a wind turbine’s reduction of wind speed and increase in turbulence (wake effect) directly and foreseeably affects the electricity production of other turbines sited too close to it downwind, and that it could not be exluded that the new wind farm would reduce the existing wind farm's electricity production so significantly as to cause it unreasonable harm. The Supreme Administrative Court noted that since these production-loss effects had not been investigated or assessed at all when the plan was prepared, the plan did not meet the Land Use Act's requirement that a plan be based on adequate studies and impact assessments and could not be shown to satisfy the Land Use Act's substantive requirement that a plan must not cause unreasonable harm to landowners or other rights holders.
As wind farms in Finland grow larger and sites become more contested, developers are proposing siting new turbines closer to existing wind farms. Until this ruling, Finnish law and jurisprudence offered no clear answer on how planning authorities should reconcile the competing interests of neighbouring wind power operators in these situations, or on the consequences of failing to do so. This precedent ruling will now guide Finnish authorities, courts, developers and their advisers when new wind farms are sited next to existing ones.
If you are interested in this topic or have other legal questions relating to the energy sector you would like to discuss, you can contact our experts Minna Katajoki, Sampsa Suominen and Antto Junkkari.