Facts

The Federal Office for Agriculture (FOAG, case 1C_389/2020) and the Federal Office for Spatial Development (ARE, case 1C_394/2020) appeal against the judgment of 2 June 2020 of the Administrative Law and Public Law Chamber of the Vaud Cantonal Tribunal (CDAP, AC.2018.0318). That judgment concerned the approval of the new general zoning plan (PGA) and its regulations of the Municipality of Valeyres-sous-Montagny, adopted by the General Council on 2 October 2017.

On 30 July 2018, the Department of Territory and Environment (DTE) partially approved the PGA. It notably refused the extension of the building zone onto farmland surfaces (SDA) south of the village, for fourteen parcels (nos. 54, 55, 58, 60, 61, 64, 65, 66, 74, 98, 362, 531, 532 and 542), on the ground that this contravened art. 30 SPO and cantonal plan measure F12. The cantonal service (SDT) had noted that these parcels were classified as SDA in the cantonal inventory and that there was no longer any cantonal margin of manoeuvre, SDA reserves having fallen sharply (61 ha at the end of 2016).

Seised by the municipality, the CDAP partially upheld the appeal. It reformed the DTE decision on two points: approval of the two sectors intended for a neighbourhood plan was suspended until measures guaranteeing their availability were adopted, and the zoning of the fourteen parcels as building land was approved. The CDAP held that these parcels, partly built on in the northern part, of an area less than one hectare and often on slopes exceeding 18%, were not materially SDA, that the geodata of the 1992 inventory were not reliable, and that other surfaces could be inventoried within the framework of the AggloY agglomeration project.

The two federal offices appealed to the Federal Supreme Court and seek refusal of the building zoning. The FOAG further requested suspensive effect, which was refused by order of 16 July 2020. The municipality seeks dismissal of the appeals; the cantonal court waives submissions; the Vaud Directorate General for Territory and Housing seeks admission.

Considerations

The cases are joined (art. 24 FCPA applicable by analogy via art. 71 FSCA). The appeals are admissible as appeals in public law matters against a final judgment approving a municipal zoning plan (art. 82 ff. FSCA and art. 34 para. 1 SPA). The federal offices have standing to appeal (art. 89 para. 2 FSCA, art. 48 para. 4 SPO and art. 34 para. 3 SPA).

The Federal Supreme Court recalls the legal framework for SDA: they must be preserved (art. 3 para. 2 let. a and art. 15 para. 3 SPA, art. 26 SPO). Under art. 30 para. 1bis SPO, SDA may be classified as building land only if an objective deemed important by the canton cannot reasonably be achieved without them (let. a) and if the surfaces will be used optimally (let. b). The Federal Supreme Court emphasises that the situation of very low SDA reserves in the canton of Vaud (measure F12) requires a rigorous application of these requirements.

On the preliminary review, the Federal Supreme Court recalls that the cantonal SDA inventory constitutes a basic study and a sectoral plan of a directive level, which is not directly open to challenge but may be subject to incidental review when a zoning plan is adopted (art. 21 para. 2 SPA, art. 9 para. 1 SPA; ATF 143 II 276, ATF 145 II 11). The conditions and scope of such review must be defined when it is a matter of zoning inventoried surfaces as…