The most common schedule drivers are:

• permitting and plan review (especially in HOA-heavy areas)
• long-lead items (windows/doors, cabinetry, specialty stone, certain fixtures)
• scope changes midstream
• selection delays (finishes, lighting, plumbing, hardware)
• unforeseen conditions in remodels (framing, electrical, water intrusion, structural)
• weather and access constraints in winter months

A well-run preconstruction phase is the best way to prevent timeline creep.