A reputable design-build firm should land within roughly 10% of the final number on the initial estimate, but accuracy depends entirely on how detailed the design is at the time you ask. As of April 2026, Kingdom & Co. uses real cost data from past projects to build estimates — the more developed the floor plan, material selections, and finish schedule, the tighter the estimate. A loose, half-baked design produces a fluffier number; a fully designed project produces a number you can plan against.