Jacket – Don’t buy a jacket if the shoulders are not your size. Upper body is the most expensive part of a jacket to alter. Getting the shoulders right and altering the rest is less expensive.
Pants - the waist is the most important part that should fit you.
Don’t buy the pants if the legs are too tight for your liking, very often there is no fabric to let it out.
Dress – give the largest part of your body the most attention. If your upper part is bigger than below the waist, then get that area fitted first. And vice versa, If you have large hips, then fit that part first and reduce the top segment at alteration stage.
Check the length of a dress carefully, lengthening with false hem can be expensive.
Check if there are layers in skirt segment – the more the layers, the higher the price of shortening.
Skirt – Make sure to get your waistline correct. Letting out is not always possible due to the lack of fabric in the seam allowance. Extensions with gussets are costly.
Check the fabric – delicate materials are more difficult to work with, so the alterations will be more expensive.
Check if there is a lining - the cost of the modification will be 10-30% more If the garment is lined.
Look up for studs, zips, other ornamental details; excessive hardware might double up the costs.
10 things to consider when buying clothes online or off the rack