A drop-in grid base for coil units is provided. A plurality of extruded plastic tubes, plastic-covered wire profiles, and cross wires are arranged in substantially the same plane to form a grid foundation. The cross wires intersect the extruded plastic tubes and wire profiles. Base wires having a series of shallow, v-shaped notches are interwoven in substantially the same plane as the grid foundation. The ends of the base wires are looped for attachment inside the seating cavity of a framed furniture product. A coil unit may be attached directly to the drop-in grid base. Alternatively, the drop-in grid base may be separately installed into a seating cavity frame, for later installation of a coil unit on top of the drop-in grid base.