The most common renovation horror story is a project that cost twice the estimate and took three times as long.
The 10-20-30-40 Rule
Allocate 10% for design fees and permits, 20% for mechanical systems, 30% for finishes, and 40% for casework and major furniture.
Contingency
Every renovation budget should include 15% to 20% contingency for mid-project changes and unforeseen conditions.