1) Poor irrigation plan. Water is one of the most damaging things to a home and landscape Be sure you have a plan for irrigation timing, runoff, rainwater and more.
2) Napkin plans. Without dimensions and drawings, even a good contractor can get 'lost' when hardscape and landscape is not to scale. You won't be disappointed or need to re-visit decisions and measurements when you have a plan.
3) Starting before having materials in hand. It often seems as though it would save time to start demo or partial build before you decide what materials you'l use. While occasionally that's true, more often you end up with huge delays and run into issues with one part of the project needing completion before another is begun. Best to wait!
4) Over planting. Landscapes do grow. It's nice to have a full yard right away but having to thin out an over planted yard in just a few years - or even in 10 - is challenging and costly.
5) Planting in the wrong place. Every specimen that you desire should be planted in the right location for sunlight, water, and space or it won't thrive. A quick consultation with a landscape designer or nursery is ideal.