information:highlighting, stylistic:tone, structural:summarization
Basilica of San Lorenzo (1421–1442)

Brunelleschi undertook the major project of the Basilica of San Lorenzo soon after he had begun the Foundling Hospital. The Basilica was the largest church in Florence, sponsored by the Medici family (the church hosted the family's mausoleum). Numerous architects worked at the church, including, later, Michelangelo. Brunelleschi designed the central nave, with the two collateral naves on either side lined by small chapels, and the old sacristy.

The first stage of the project was the Old Sacristy, built between 1419 and 1429. It contains the tomb of Giovanni di Bicci de' Medici and his wife, beneath a central dome, simply decorated. The chapel is a cube of about 11 metres (36 feet) on each side, covered with a hemispheric dome. A level of ornamental entablements divides the vertical space into two parts, and pilasters support the dome. The altar is set into a recess at one end beneath a smaller dome. All of the arcs of the ceiling are supported by pilasters, like classical columns, set into the walls. This room, using classical elements in an innovative way, established itself as a standard in Renaissance interior architecture.

Along the nave, slender columns with Corinthian capitals replaced the massive pillars of Gothic architecture. A coffered ceiling of square compartments with delicately gilded trim replaced the traditional vaulted ceiling of the central nave. Circular windows above each chapel were introduced to bridge the difference in height between the low chapels and the much higher nave. The new interior projected an impression of harmony and balance.

Brunelleschi used white walls in the Old Sacristy, which later became a common element of Renaissance architecture. Leon Battista Alberti (who wrote in 1450 the standard text of Renaissance art) argued that, since classical times, according to prominent authors like Cicero and Plato, white was the only color suitable for a temple or church and praised "the purity and simplicity of the color, like that of life."