All plants need top soil for healthy and strong root systems. Top soil provides the nutrients and organic material needed for long-term health. You may get grass seed to sprout in the subsoil and start growing. However, when the blades become stressed due to heat, disease, insects, or improper watering they won't be strong enough. Contact a local garden shop that sells bulk top soil, compost and other organic matter. They will help you calculate how much top soil you'll need. And you will need it wherever or whatever you are going to plant.

Don't buy any topsoil until you evaluate what it will do to your drainage. I just bought a house that sits in a hole created by adding a little topsoil every year. Fortunately we don't have a basement, but I'm going to have to have many yards of material hauled away if I want this to be done right.

Before doing anything, I suggest getting a soil test to see what your soil is really like. You might think it is not top soil, but your local top soil might be worse than what you have. Only a soil test will tell. Furthermore, just because you get "new" topsoil, that does not mean you don't still need a soil test to see what the soil needs. Most poor soils can be improved considerably by allowing them to sit under several inches of mulch for 3-5 months. You can do that at any time.

There is nothing magical about topsoil. Technically, it is only the top 4-6 inches of the soil surface and depending on how it was sourced, it may or may not contain any organic matter or be very nutrient rich. And since there is nothing to regulate what is sold as topsoil - in bags or bulk - you pay your money and you take your chance of what you get in return.

I agree about getting a soil test before you add anything. You may not need to. Sandy and not rocky sounds like a not half bad seed bed, if one that just needs some grading or leveling to be properly functional. Additional nutrients can be supplied if necessary.

Very sandy soil often means it drains fast and your lawn will need a lot of watering in summers dryer than this one. You may find rich topsoil cheaper in the long run.

Get the soil test first before committing to anything. Let it guide you. Most people do a poor job of evaluating their soil. Let the professionals do it.