Some garments cannot take the alteration being asked for, and we will say so at the fitting rather than charge you for one that will not hold. The most common case is a fused seam - a cheaper suit or dress with the hem or seam bonded rather than stitched. A fused seam frays and separates once it is cut and re-pressed, so letting it out or re-hemming it can leave a visible line or a seam that fails within a season.
The second common case is seam allowance - the extra fabric folded inside a seam beyond the stitch line. Letting out a waist or side seam only works if that fabric is actually there. Many ready-to-wear garments are cut with almost none, especially at the smallest sizes, so a size and a half of room asked for might only have a size half available. We measure the allowance at the fitting and tell you the real number before we pin anything.
When either of those is true, we say so before any cutting starts and before you are charged for the fitting itself. We would rather turn away work we cannot do well than take payment for a result that will not last.
