Simply no, not every dress can be altered to fit exactly the way you would like. Remember two rules of thumb:
It is easier to take something in than it is to let it out. If you are wearing your mother’s wedding gown it may be worth going to great lengths to get the gown to fit. But if you are buying a new gown, get the correct size. Making a bridal gown larger is difficult and you may not like the fit afterwards.
Don't buy a gown more than one size larger than your own size. Larger gowns are cut proportionally larger all over the gown. The neckline, the bodice, and shoulders will all be over sized. This means the gown may not fit quite right if only the side seams are taken in. However, some wedding dresses are designed to fit several sizes, with a corset back that ties instead of zips. To get the best fit, stay as close to your correct size as possible.
Could I do the alterations myself?
If you (or a friend or family member) are a competent seamstress, you could make many of your alterations easily and save quite a bit of money. For instance, a hem from a professional tailor might range from $55 for a very basic hem to $200 for multiple layers but might not be a difficult chore for someone who knows their way around a sewing room. It all depends on the construction of the gown. Hems can be very simple, but be aware that if your gown has lace at the hemline, it may require more extensive work.
Some alterations are more difficult and should be reserved for someone who really knows what they are doing. But wedding gowns are designed to be altered and many are constructed in such a way as to make adjustments easier. Take a good look at the interior construction of your gown to determine how difficult it will be to make the necessary adjustments before you take it on.
Alterations Timeline
If possible, order your dress at least 12 weeks before your wedding to allow enough time for the necessary adjustments. Plan your last fitting several weeks before your wedding so you have time to make last minute adjustments if needed. Weddings can be stressful and you may find that you either lose a little weight or gain a little weight during your engagement.
Alterations Tips
Try your wedding gowns on without any make-up. You want to be as clean as possible to avoid soiling your gown. And wear your nicest under clothes, as it's likely you may be only partially dressed during your fitting.
To get an accurate accessment of gown size, also be sure to wear the bra and shoes that you will be wearing for your wedding. Don’t try to find shoes with the right heels after your gown has been altered; buy your shoes prior to having it hemmed.