The first thing I did was remove the dress zip and replace it with an invisible zip. Plus I released the seams to the max. But it had boning so there is only so much you can do there. But this client hates anything too tight, so after more discussion we went with adding to the side seams using the off cut from the hem.
Whenever you are increasing the size of something, AND it needs to be shortened, I do recommend adding the extensions BEFORE cutting the hem. However we didn't have that option now, so I had to have faith it would all be ok and shortened the hem using that piece for the sides. The reason I say that is that when a garment is tight is will get shorter, and when the garment is loose it will get longer.
Have you ever noticed how trousers get longer when you loose weight? Same thing applies to dresses and gowns.
Happy altering
Judith