The Winter Palace's backup house - Sklyayev's house

The Winter Palace's Spare House is a mansion (as well as a palace) built at the end of the 19th century by architect Nikolai Becker on the site of older 18th-century buildings. It is located in Saint Petersburg at the addresses Palace Embankment, 30 and Millionnaya Street, 31. The wing of the spare house is located on Millionnaya Street.

In the 1710s, the house on the plot now occupied by the Winter Palace's spare house was two stories high, had a high porch, and a sloping roof. This house belonged to Feodosy Sklyaev, a companion of Peter I, bombardier of the Preobrazhensky Regiment, and shipbuilder. Sklyaev had no heirs, so after his death in 1728, the house was transferred to the treasury. Notably, Sklyaev's house was located to the right of Peter's Winter Palace.

In the 1730s, Anna Ioannovna gifted the house and plot to Gustav Biron, brother of the Russian regent Ernst Biron. In the 1740s, the house was rebuilt, presumably by architect Carlo Giuseppe Trezzini, for Juliana Mengden, a lady-in-waiting and close friend of the ruler Anna Leopoldovna. At the beginning of the 19th century, the house passed to Countess Vorontsova, who initiated another reconstruction of the house and the construction of stone wings in the courtyard. One of these wings was rented out by the countess to the palace company of the Preobrazhensky Regiment. The Vorontsovs owned the house with its wings for about half a century until, in 1860, the palace department purchased the entire complex. The mansion became the Winter Palace's spare house.

In 1877–1878, under the direction of architect Becker, the main building (along the embankment) was rebuilt for Grand Duke Alexei Alexandrovich and took on its modern appearance; the wing on Millionnaya Street became four stories high. In 1885, Alexei Alexandrovich acquired a palace on the Moika River, and the house on Palace Embankment passed into the hands of Count Georgy Fyodorovich Mengden. The rebuilding of the building into a revenue house (finishing of residential and ceremonial rooms, as well as reconstruction of the wing) was carried out by academician Tikhobrazov. In the 1890s, Mengden's revenue house became the property of the court again, restoring its status as the Winter Palace's spare house.

In the early years after the October Revolution, the building housed the Petrograd Department of the Main Directorate for Museums and the Protection of Art, Antiquities, Folk Life, and Nature Monuments. In the 1930s, the house was occupied by employees of the Academy of Sciences, the Hermitage, and university professors.

Currently, the house is under the jurisdiction of the State Hermitage Museum. As early as 1988, it was directly connected to the other Hermitage buildings through the Hermitage Theatre, in the courtyard of which architect V. P. Lukin created a new gallery-transition on an arch.

In 2017, the Hermitage's scientific complex with the latest equipment was established here. The laboratory of scientific restoration of Eastern painting, located in the Spare House, is unique not only in Russia but worldwide.

Sources:

https://ru.wikipedia.org/wiki/Запасной_дом_Зимнего_дворца