Exterior of single-family home A CalAtlantic Group property. The company delivered 14,602 single-family homes last year.

MIAMI—Lennar Corp.’s merger with CalAtlantic Group became effective at 11:59 pm on Monday, creating the nation’s largest homebuilder by revenue. The $9.3-billion deal, including the assumption of $3.6 billion of debt, was first announced this past October and was approved nearly unanimously by stockholders in both companies.