One Team, Accountable for the Whole Job
Running design and construction as one job means the same team that draws up the plan is also accountable for building it. There's no gap where a designer hands off to a builder who has to reinterpret someone else's drawings, and no argument later about whether the finished work matches what was planned.
A retaining wall that needs a different footing, or a pergola post that lands near a service line, gets sorted on the spot instead of triggering a round of calls between separate contractors. Structural work, decking, paving and planting all get scheduled and sequenced as part of one project, so the timeline holds together instead of stalling while one contractor waits on another to finish.