Pierre Lassurance (also Cailleteau), French architect, part of a family of architects. Several members of the family held important posts as architects during the 17th century and early 18th. The most prominent of these was Pierre Lassurance I; one of his sons, Jean Lassurance (1695-1755), worked almost exclusively for Mme de Pompadour, while another, Pierre Lassurance II (no dates), became contrôleur of the châteaux of Saint-Germain-en-Laye and Monceaux.

In 1679 he was employed as a mason under Jules Hardouin-Mansart at the Château de Clagny, near Versailles. Between 1684 and 1699 Lassurance served as Hardouin-Mansart's chief draughtsman in the Service des Bâtiments du Roi, providing designs for interiors at the château of Versailles and the Hôtel des Invalides, Paris. When Hardouin-Mansart became Surintendant des Bâtiments in 1699 Lassurance's salary was doubled; the following year he was promoted to architecte et dessinateur. In 1702 he took the post of contrôleur at the Hôtel des Invalides and, establishing an office in Paris at the same time, he was then able to pursue private commissions in response to the escalating demand for sumptuous urban mansions in the city.

Lassurance's first private building, the Hôtel de Rothelin, was begun c. 1700; this was followed in 1704 by the Hôtel Desmarets (destroyed 1800); both exemplify the desire for simplicity and convenience that characterizes Parisian architecture at the beginning of the 18th century. In each case, a single longitudinal axis divides the ground-plan into two symmetrical halves, starting at the entrance portal on the street and continuing across the courtyard into the corps de logis. In the main block, the paired vestibule and salon on this axis separate the principal apartments which lie to left and right. The court and garden elevations are typical of Lassurance's style, insofar as the classical orders are used only on the central pavilions, and the tall, narrow windows have become the salient motifs.

The plans of Lassurance's next two houses, the Hôtel d'Auvergne (1707-08; destroyed 1880) and the Hôtel de Maisons-Soyecourt (1708), differ from earlier designs as a result of the unusual breadth of the sites. In each case, Lassurance divided the plan in half, positioning the forecourt and corps de logis on one side and service elements on the other. In another pair of exceptional designs, the Hôtel de Neufchatel-Béthune (1706-07; destroyed after 1866) and the Hôtel de Rohan-Montbazon (1719; much altered), he gave a pavilion-like appearance to the single-storey corps de logis.

In 1722 Lassurance began the Hôtel de Roquelaure. Its unusual plan resulted from his decision to incorporate an older structure into the new hôtel and provide access to the various ground-floor apartments from both sides of the corps de logis. The concave wall of the street entrance conceals the awkward relationship of the axis of the plan to the road, while this concavity is repeated in reverse on the forecourt side to provide service areas. During its construction he was also involved in the initial planning of two important buildings located in the Faubourg Saint-Germain - the Palais-Bourbon and the Hôtel de Lassay, although after his death in 1724 Jean Aubert altered the plans.

Lassurance was involved also in the extensive remodeling of older structures - and modernized the Hôtel de Pussort for the Duc de Noailles in 1711 (altered 1830). His only country house was the Château d'Ivry-Petit-Bourg (c. 1724; destroyed 1750), commissioned by the Duc d'Antin, which he designed in a restrained style comparable to that of his hôtels. Lassurance's interest in producing simplicity and rigour in his buildings was based primarily on the mature works of Hardouin-Mansart, such as the Hôtel de Noailles (1679), Saint-Germain-en-Laye, and the principles discussed by C. A. Daviler in his Cours d'architecture (1691).

On the whole, Lassurance's work is more restricted in scope than that of his contemporary, Robert de Cotte, and in its conservatism resembles work by Jacques Gabriel V (1667-1742). Plans and elevations of Lassurance's hôtels particuliers were published by Jean Mariette in his L'Architecture française (1727) and Jacques-François Blondel in his work of the same name (1752-56), and the houses were recorded by Germain Brice in the Description de Paris (notably the editions of 1706, 1713 and 1717).