What tolerance ranges are typically achievable in shop-fabricated millwork vs on-site installation?

Shop-fabricated millwork can usually be produced to much tighter tolerances than on-site installation because it is built in a controlled environment with consistent references. Dimensions within a few millimeters are often achievable in the shop, depending on scale and materials.

On site, tolerances are influenced by framing variation, substrate conditions, and sequencing with other trades. As a result, installations typically require allowances for adjustment through shims, scribes, and reveals. Effective detailing accounts for this difference rather than assuming shop-level precision can be maintained in the field.