The best time is in the spring. No matter how much top dressing you apply, you will be smothering grass plants that need oxygen and sunlight to survive. In the spring, conditions are usually cool and moist, so the lawn grows vigorously and is easily able to punch through the top dressing. Many homeowners also seed when they top dress the lawn.
Apply top dressing to a thickness that is equivalent to one third the height of the lawn. If your lawn is three inches tall, spread top dressing at a thickness of one inch.
If you are trying to raise your lawn significantly, keep in mind that once the top dressing settles and compacts, it will only raise your lawn by ten to twenty percent of the thickness you laid down.
Only apply another layer of top dressing once the lawn has completely filled in through the first layer.
If you are looking to raise your lawn by several inches, it will be necessary to resod or seed.