Do extra empty columns affect sql table size significantly?

Question

I have a few columns that I know won't be used initially. Is it a good practice to add columns only when needed or just leave those extra columns there?

Is it just a little more space in the header or every row?

Answer 1

Depends on the column type to some extent.

Variable sized columns (such as varchar(n)) will only use 4 extra bytes (approx) per row.

I would add columns only when required, since it is less likely that redundant columns will be added and never removed.