If you are going to get a contractor to construct a retaining wall for you, there are several factors that you will need to take into account when the two of you are determining what approach to take with the construction of this wall.
How often it rains and snows in your local area
When deciding how to design the wall with your contractor, you will need to factor in how often rains and snows in your local area. If you live somewhere where snowfall is extremely rare and rainfall is usually light and infrequent, then you probably won't need to integrate any drainage features into the wall.
However, if your property is in a region where the autumn and winter months are wet and snowy, then you must ensure that your contractor puts some type of drainage system into the wall that they build. The soil that retaining walls are built to support can become incredibly saturated if it's soaked with rain or snow and there are no drainage pipes integrated into the walls.
This type of excessive soil saturation can lead to several problems; the first is that any flowers planted in this soil might die as a result of being waterlogged. The second is that if the grassy area behind the wall is supposed to be used as a scenic area in which to walk or sit, the saturated grass may make it unusable as anyone who stands on it will find that their feet sink into the ground and end up drenched in mud. The third is that the soaking-wet, swollen soil against which the retaining wall is pressed could, over time, damage it and cause the stone that it is built from to crack.
What you plan to do with the grassy area that the wall will be supporting
You and the contractor should also discuss what it is that you intend to do with the grassy area that your new wall will be supporting. Your plans for this area will determine the design features of the wall.
For example, if the soil that the wall will be holding in place is not very fertile and would be difficult to grow flowers in, then you might want to get the contractor to create a trellis that lines the top of the retaining wall so that you can disguise this dry grassy area and instead add some plants such as sweet pea or honeysuckle to this trellis in order to make your garden look a bit nicer. If on the other hand, the grassy area is quite pretty and you want to set up a seating area on it, then you may want to get the contractor to create a set of steps that are integrated into the structure of the wall so that you can easily access the grassy area without having to jump onto it.
For more information about retaining walls, contact a local resource.