Facts

[D. AG] is the owner of Parcel No. 761 in Frauenfeld, known as the "Stammerau". The property, measuring 18,208 m², is undeveloped and is included in the Federal Inventory of Swiss Heritage Sites of National Significance (ISOS). In 2005, the Municipality of Frauenfeld publicly presented the "Walzmühle Residential Development" zoning plan, which provided for the construction of 81 residential units on Parcel No. 761. Several neighbors, including [E. AG] , filed objections. During the subsequent appeal proceedings, [E. AG] and the [F.] condominium association filed a request for the protection of the Walzmühle and its surroundings. In 2012, the Municipality of Frauenfeld revoked the "Walzmühle Residential Development" zoning plan and rejected the request for protection.

Subsequently, [D. AG] prepared a new zoning plan (Stammerau Zoning Plan) for its Parcel No. 761. This plan was publicly presented by the Municipality of Frauenfeld in 2016. [G.] and [A.] filed objections and also requested the protection of the entire parcel. In 2018, the Municipality of Frauenfeld rejected both the request and the objections. An appeal filed against this decision was rejected by the Department of Construction and Environment (DBU) of the Canton of Thurgau in 2020. At the same time, the DBU approved the zoning plan (with the exception of Art. 22 para. 1 of the associated special building regulations) and the partial revocation of the "Walzmühlestrasse" building line plan.

[G.] and [A.] filed appeals against the two decisions of the DBU with the Administrative Court of Thurgau. In two separate decisions dated October 13, 2021, the Administrative Court dismissed the appeals.

Considerations

The Federal Supreme Court first notes that the contested decisions are closely related in content and that it is therefore justified to consolidate the appeal proceedings.

Regarding the request for the protection of the Stammerau, the Federal Supreme Court confirms the view of the Administrative Court that a final decision has already been made on this matter. The appellants have not substantially challenged the reasoning of the Administrative Court in this respect.

On the question of the consideration…