Almost anything can be altered. However, some alterations require so much work that it makes the alteration impractical. There is a direct correlation between the time it takes to make an alteration and the cost.
Here are some things that go into assessing the viability of an alteration.
1) Taking in versus letting out: It is easier to make things smaller than it is to make them bigger. Making things smaller involves using less material than is current employed. Making things bigger involves adding extra material.
2) Accessibility: Just like a motorcycle, the parts of a garment are assembled in a certain order. The difficulty of alteration is closely related to how many steps it takes to access the pieces that need to be altered. Things like side seams are relatively easy to access. A center front zipper often requires opening up many seams, making it difficult to access.
3) Leather: Leather alterations need to be more carefully considered because leather, unlike woven fabric, is compromised the more punctures are made in it. A seam is a line of small holes punched by a needle with a thread running through it. Sewing seams multiple times in the same place will make the leather like a perforated sheet of paper – easy to tear. This means the number of times a seam can be re-sewn are limited. A seam can also be re-sewn without adding new holes to preserve the integrity of the leather. This can be done quickly, albeit very carefully, by machine and slowly, but more precisely, by hand.