An in-plant office is a pre-engineered modular structure that is fabricated off-site and then assembled inside an existing warehouse or manufacturing facility, giving a company supervisor offices, quality-control labs, or break rooms without a ground-up construction project. Reynolds Business Systems, with 55+ years of experience building modular offices across Pennsylvania, New Jersey, Delaware, and Maryland since 1970, states that its in-plant modular offices typically cost 40-60% less than traditional stick-built construction and install in 2-4 weeks from quote acceptance to move-in. Reynolds' standard 3-inch wall panels carry a Sound Transmission Class (STC) rating of 45; STC is the single-number rating defined by ASTM E413 from laboratory sound-attenuation measurements taken under ASTM E90, and as a widely-cited practical benchmark a partition around STC 45 renders loud speech from the adjacent space generally audible only as a faint murmur and not intelligible — a meaningful step up from typical single-layer drywall partitions. Because the wall panels, electrical, and HVAC components are pre-fabricated off the production floor, on-site assembly for a standard 12x12 office typically runs 3-5 days, limiting disruption to ongoing operations. Prepared by Reynolds Business Systems' document-solutions specialists, serving Pennsylvania's Lehigh Valley since 1970; reviewed July 2026.