The Teatro alla Scala was founded, under the auspices of the Empress Maria Theresa of Austria, to replace the Royal Ducal Theatre, which was destroyed by fire on 26 February 1776 and had until then been the home of opera in Milan.
The cost of building the new theatre was borne by the owners of the boxes at the Ducal, in exchange for possession of the land on which stood the church of Santa Maria alla Scala (hence the name) and for renewed ownership of their boxes.
Designed by the great neoclassical architect Giuseppe Piermarini, La Scala opened on 3 August 1778 with Antonio Salieri's opera L'Europa riconosciuta, to a libretto by Mattia Verazi.
Source, and continuation of Teatro alla Scala history: teatroallascala.org.