What are standard alterations? The most commonly accepted industry definition includes the following for "standard alterations"
Hemming the length
Taking in the side seams
Adding a bustle
Another word I like to use for "standard alterations" is "minimum alterations". It includes those things that you absolutely need to have done that way your gown is not falling off of you, dragging on the ground, etc.
It is a common industry standard, that a gown can be taken in about 2 sized and still be considered standard alterations.
Keep in mind, you will likely have "non-standard" alterations that you will fall in love with and put on your need list. For example, most other seamstresses add bra cups or darts to fill or shape the bust of a gown. This would be considered "standard"...but the majority of my clients opt to have me re-shape the actual bust seams and provide my signature fit.