That would depend.

If I'm developing something for a customer, before I start coding I try to define a rigorous spec that a) the customer can understand as defining their requirement, and b) contains enough detail to ensure the requirement is exactly specified. This can be a challenge with completely non-technical customers.

When coding less formally (writing support utilities when working under contract, writing my own modules etc) I tend to work out a fairly precise but very basic spec beforehand, write the first version; then immediately think of and start adding lots of new functionality.