Be aware of the intended application shelf-life. A throw away agency campaign does not need the same level of craftsmanship as your companies flagship product which needs to support the company for the next decade.
If you pay for your poor design decisions for a decade without a major rewrite, the application will be riddled with complexities and repetition. If you over design a simple project, you may end up with either to much coffee to maintain, or be a little over budget.
What’s worse: 10-20% over budget now, or paying your development team staff an extra few years salary for maintenance, loss of morality, the inevitable churn and finally new staff training.