Design development is the phase between concept design and construction documents. The big idea is already set. Now the team works out the details: room sizes, structural grid, mechanical systems, materials, and finishes. Drawings become more precise. Decisions become harder to reverse.

Architects coordinate with engineers and consultants during this phase. The structural engineer sizes the beams and columns. The mechanical engineer lays out ducts and pipes. The electrical engineer plans power and lighting. Conflicts are identified and resolved before they become expensive problems in the field.

Cost estimating becomes more reliable. The design is detailed enough to price with reasonable accuracy. If the estimate exceeds the budget, the team must adjust scope, quality, or both. This is often the most uncomfortable part of the process.

Design development ends when the design is sufficiently resolved to produce construction documents. It is a milestone, not a ceremony. The work continues.