The short answer
Separating design from execution keeps specifications honest; a combined turnkey price is simpler but harder to audit.
When design is billed separately, the designer has no incentive to specify expensive materials to increase the execution value, and you can tender the work to more than one contractor.
A turnkey contract is easier to manage and fixes responsibility in one place, but insist on a detailed BOQ and a sample approval process so the finished work matches what was promised.
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