Facts
The appellants, [A.] , [B.] and other neighbours, own plots adjoining the project. Parcel no. 5954 of the commune of Lausanne, located at avenue [L.] , belongs to the association [J.] and was promised for sale to the company [I. SA] (the builder). It lies in an urban zone under the regulations on the general land-use plan and on constructions of the commune of Lausanne (RPGA), in a central sector.
The parcel carries a two-storey building of 440 m2 (ECA no. 12232), built in 1952 for the activities of [J.] , including a worship hall with 450 seats. Since 1997, this building has been rated 3F in the architectural survey; it is neither classified nor listed in the inventory within the meaning of the Vaud Act on the protection of nature, monuments and sites (LPNMS).
Building permit applications from 2008 (partial demolition) and the successive negative opinions of the Communal Delegate for the protection of the built heritage and of the Property, Heritage and Logistics Service (SIPAL) did not succeed. In 2015, the builder filed a new project providing for the demolition of building ECA no. 12232 and the construction of 40 dwellings with a place of worship, an underground car park of 22 spaces and four outdoor parking spaces, on a ground surface of 962 m2 (ground floor, four storeys and attic). The project, amended several times, gave rise to objections. SIPAL maintained a negative opinion, describing the new construction as a low-quality « bar ». The Delegate for the protection of the built heritage issued a favourable opinion on the demolition.
On 30 June 2017, the municipality of Lausanne dismissed the objections and issued the building permit. Seised by the neighbours, the Administrative and Public Law Court of the Vaud Cantonal Tribunal (CDAP) partially upheld the appeals by judgment of 10 December 2018, making the permit subject to the laying of a 0.30 m layer of topsoil on the 283 m2 green roof, and otherwise confirmed the communal decisions.
The appellants lodge a public law appeal with the Federal Supreme Court, seeking principally the annulment of the municipal decision of 30 June 2017 and, in the alternative, the annulment of the cantonal judgment and remittal of the case. The cantonal court waived submitting observations. The application for suspensive effect was granted by presidential order of 25 February 2019.
Main question: do the demolition of the existing building, the derogation granted as regards green spaces and playgrounds (art. 55 RPGA), and the aesthetics and integration of the project (art. 86 SPA, art. 69 et seq. RPGA, ISOS) withstand review for arbitrariness and the right to be heard?
Considerations
As to admissibility, the public law appeal is open against a final decision of last cantonal instance in the field of public building law (art. 82 let. a, 86 para. 1 let. d, 90 FSCA). The neighbouring owners, being particularly affected, have standing (art. 89 para. 1 FSCA). The summary of facts presented by the appellants, which departs from the cantonal findings without demonstrating that they are manifestly inaccurate, is inadmissible (ATF 140 III 115; 137 II 353). The requests for investigative measures concerning the report of the special commission on 20th-century heritage are dismissed, the complaints being capable of examination on the basis of the file, and facts arising after the contested judgment being inadmissible (art. 99 para. 1 FSCA).
As to the right to be heard (art. 29 para. 2 Cst.), the refusal to take evidence violates this right only if the anticipatory assessment of the evidence is arbitrary. In this case, the cantonal court found that no classification was envisaged in the short term and that no precautionary measure had been taken in respect of the building, although the project provided for its demolition. The statements attributed to SIPAL were not confirmed at the hearing, and the final report was not produced before the cantonal instance. The complaint is rejected.
As to the demolition of the building, the Federal Supreme Court recalls that the application of an aesthetics clause (art. 86 SPA, art. 69 and 73 RPGA) must not empty the zoning regulations of their substance; a prohibition on construction is justified only by a predominant public interest, where the protection concerns remarkable aesthetic qualities (ATF 115 Ia 114; 101 Ia 213). Review of cantonal and communal rules is limited to arbitrariness (ATF 144 I 170), and particular restraint is required in matters of protection of monuments and sites given the wide discretion of local authorities (ATF 142 I 162). The cantonal court considered that the building, although worthy of interest but lacking sufficient architectural value, was no longer suited to its original use, that no other viable use existed, and that restoration would have required major works. It based its assessment on the Delegate's opinion, her local inspection, the documents produced and SIPAL's opinions, without deciding the question of competence. The appellants' criticisms, which are of an appellate nature, do not demonstrate arbitrariness. As for the ISOS, it has no directly binding effect but constitutes an element of federal interest to be weighed; the cantonal court took it into account, and the protection objective A does not absolutely preclude demolition (judgment 1C_279/2017). The complaint is rejected.
As to the derogation from green spaces and playgrounds, art. 51 and 52 RPGA respectively require 20 m2 of green spaces and 1.30 m2 of playground per 100 m2 of floor area. The project, with a gross area of 5'044 m2, would require approximately 1'009 m2 of green spaces, whereas 649 m2 are planned, with no playground. Art. 55 RPGA allows the municipality to set particular conditions of application where the land is insufficient; the wording of art. 50 RPGA does not allow new constructions to be excluded from this…