How Do You Plan Home Renovation Costs Without Breaking the Budget?

I feel like every time someone starts a home renovation the budget goes out of control so fast. Materials cost more than expected, labor charges keep changing and then if you change design in between it gets even more messy.

For those of you who have done a renovation or maybe even built a house, how did you plan your costs? Did you just keep a rough number in mind or did you use something like a construction cost estimator before starting?

From what I’ve seen, having a clear estimate in the start really helps to avoid big surprises later on. But I wanna know your side too.. Do you plan every detail from day one or just adjust things as they come up?