Facts

Parties and authorities: The appellant, [A.] , is the owner of plots nos. 1111 and 1112 in the cadastre of the commune of Pregny-Chambésy, forming part of the Perrot estate. The respondents, owners of plot no. 1101 (manor house), submit their observations and request dismissal of the appeal. The Conseil d'Etat of the Republic and Canton of Geneva requests dismissal. The owner of plot no. 1110 submits brief observations. Patrimoine Suisse Genève does not wish to be a party to the proceedings. Previous authority: Administrative Chamber of the Court of Justice of the Republic and Canton of Geneva, judgment of 8 January 2019 (ATA/7/2019).

Context: The Perrot estate comprises several plots (nos. 1101, 1103, 1110, 1111, 1112 and 1427). The red brick manor house, built at the end of the 1880s by the architect Jacques-Elysée Goss, has been listed in the inventory since 16 October 1987. A classification procedure was opened on 26 September 2005 at the request of the public art society, later renamed Patrimoine Suisse Genève. In a report of 31 March 2010, the Sub-commission on Monuments and Antiquities (SCMA) of the Cantonal Commission for Monuments, Nature and Sites (CMNS) issued a favourable opinion on the classification, including non-cadastral features (paths, walls, greenhouse, orangery, gardens, basin, grottoes), while recommending that the preservation of views be guaranteed.

Cantonal decision: By order of 21 June 2017, the Conseil d'Etat classified all the buildings and the plots or parts of plots delimited according to plan no. 29'888A-530. The appellant and the owners of plots nos. 1110 and 1427 appealed; the Administrative Chamber dismissed the appeals.

Requests before the Federal Supreme Court: The appellant requests annulment of the cantonal judgment and of the classification order insofar as it concerns plots nos. 1111 and 1112, with remittal for the drawing up of a new protection plan. In the alternative, he requests a finding that the classification order is not enforceable against him and recognition of his right to build on plot no. 1112 in accordance with the communal regulations and plan no. 29'888A-530.

Main question: Is the classification of the appellant's private plots, visually separated from the estate by a group of trees, within the protection perimeter of a landscape ensemble compatible with the guarantee of property, the principle of good faith and the prohibition of arbitrariness (art. 13 LPMNS)?

Considerations

Admissibility and establishment of the facts: The public law appeal is admissible (art. 82 ff. FSCA). The appellant, owner of two plots within the perimeter, has standing to sue (art. 89 para. 1 FSCA). His objections concerning the factual findings (independent island character, 5th villa zone, application for a building permit) are appellate in nature or are relevant only with regard to proportionality; they do not permit correction of the facts within the meaning of art. 97 para. 1 FSCA (art. 106 para. 2 FSCA).

Guarantee of property (art. 26 Cst., art. 36 Cst.): The restriction must have a legal basis, be justified by a public interest and respect proportionality. Arts. 4 and 10 LPMNS provide a legal basis of sufficient formal standing; the grounds for classification concern a landscape ensemble (a site worthy of protection within the meaning of art. 4 let. b LPMNS) and not merely buildings, so the argument based on art. 15 LPMNS regarding immediate surroundings is unfounded. The public interest in protecting the Perrot estate, recognised by the specialist commissions and the cantonal court, is established; the absence of reciprocal views and the wooded belt are not decisive, as the latter forms an integral part of the landscape park. That the cantonal and communal master plans treat the plots as subject to development is not decisive, as the plots remain buildable.

Proportionality: The classification is suitable and necessary to preserve the site, since neither the communal construction regulations nor the vegetation regulations allow influence over the architectural treatment of new constructions in the absence of the plots' inclusion in the protection perimeter. The plots remain buildable, and the specific restrictions may be reviewed when a building permit is granted or refused. There is no violation of proportionality.

Good faith (art. 5 para. 3 and art. 9 Cst.): On 28 April 2017, the Cantonal Service for Monuments and Sites sent the appellant's architect a plan dated 27 March 2016 showing building zones, whereas a more recent plan of 22 February 2017 showed none. The indication "for information only" does not automatically exclude the protection of good faith.…